AJP - Heart AJP: Lung Cellular and Molecular Physiology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Heart Circ Physiol 284: H1601-H1611, 2003. First published January 9, 2003; doi:10.1152/ajpheart.00578.2002
0363-6135/03 $5.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
284/5/H1601    most recent
00578.2002v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (8)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Maliszewska-Scislo, M.
Right arrow Articles by Rossi, N. F.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Maliszewska-Scislo, M.
Right arrow Articles by Rossi, N. F.
Vol. 284, Issue 5, H1601-H1611, May 2003

Effect of blockade of endogenous angiotensin II on baroreflex function in conscious diabetic rats

Maria Maliszewska-Scislo1,3, Tadeusz J. Scislo3, and Noreen F. Rossi1,2

1 Departments of Medicine and Physiology, Wayne State University and 2 John D. Dingell Veterans Affairs Medical Center, Detroit, Michigan 48201; and 3 Department of Physiology, University of Gdansk, 80-822 Gdansk, Poland


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Little is known about baroreflex control of renal nerve sympathetic activity (RSNA) or the effect of angiotensin II (ANG II) on the baroreflex in diabetes. We examined baroreflex control of RSNA and heart rate (HR) in conscious, chronically instrumented rats 2 wk after citrate vehicle (normal) or 55 mg/kg iv streptozotocin (diabetic) before and after losartan (5 mg/kg iv) or enalapril (2.5 mg/kg iv). Resting HR and RSNA were lower in diabetic versus normal rats. The range of baroreflex control of HR and the gain of baroreflex-mediated bradycardia were impaired in diabetic rats. Maximum gain was unchanged. The baroreflex control of RSNA was reset to lower pressures in the diabetic rats but remained otherwise unchanged. Losartan decreased mean arterial pressure (MAP) and increased HR and RSNA in both groups but had no influence on the baroreflex. Enalapril decreased MAP only in normal rats, yet the increase in HR and RSNA was similar in both groups. Thus in diabetic rats enalapril produced a pressure-independent increase in HR and RSNA. Enalapril exerted no effect on the baroreflex control of HR or RSNA in either group. These data indicate that in conscious rats resting RSNA is lower but baroreflex control of RSNA is preserved after 2 wk of diabetes. At this time, the baroreflex control of HR is already impaired and blockade of endogenous ANG II does not improve this dysfunction.

baroreceptors; enalapril; heart rate; losartan; renal sympathetic nerve activity


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

ALTERATIONS of the autonomic nervous control of cardiovascular function are a common finding in diabetes mellitus. Diabetic autonomic dysfunction and cardiovascular diseases are associated with high mortality (19), accounting for roughly 80% of all diabetic deaths (27). Parasympathetic control of the heart is diminished in diabetic patients as evidenced by the reduced beat-to-beat variation in heart rate (HR) (5, 33). In addition, a blunted chronotropic response to exercise and subnormal plasma catecholamine levels suggest an impairment of sympathetic activity in diabetic individuals (10, 17).

The arterial baroreceptor reflex plays a key role in regulating sympathetic and parasympathetic outflow, thus influencing systemic arterial pressure, HR, and regional blood flow. More importantly, the baroreflex is involved in the control of renal tubular sodium reabsorption, renin release, and renal hemodynamics, all of which play a key role in diabetic renal function. Numerous studies on baroreflex function in experimental diabetes mellitus have yielded conflicting results (12, 16, 26, 34-37). In streptozotocin (STZ)-diabetic rats, baroreflex activity changes as the disease progresses, thereby accounting for some of the variability in the findings (11). Most data on baroreflex function in diabetes are limited to the HR response. Only a few reports exist on baroreceptor control of renal sympathetic nerve activity (RSNA) in diabetes. McDowell et al. (37) reported no change in baroreflex control of RSNA in alloxan-diabetic rabbits. Patel and Zhang (40) obtained similar results in STZ-diabetic rats. However, both studies were done on anesthetized animals, and anesthesia is known to influence baroreflex function (43). To our knowledge, no data are available on baroreflex-mediated changes in RSNA in conscious diabetic rats.

Diabetes is also characterized by changes in plasma levels of several humoral factors. Extensive attention has focused on the renin-angiotensin system and the generation of ANG II. That ANG II plays an important role in the progression of diabetic renal disease is supported by the beneficial effect that blockade of this system [either by angiotensin-converting enzyme (ACE) inhibitors or AT1 receptor antagonists] exerts on the progression of renal failure (2, 31). In contrast to plasma renin activity, which is generally decreased (3, 44), renal renin content is typically increased in diabetes (3). Whether the renin-angiotensin system is, in fact, suppressed in diabetes is still debatable. In the early stages of diabetes, plasma renin activity is increased (8) and may result in augmented plasma ANG II concentration. Elevated plasma ANG II concentration (28) may also result from increased ACE activity often found in diabetic patients (46) or STZ-diabetic rats (23, 44, 45).

Several lines of evidence suggest that ANG II is involved in the regulation of sympathetic nerve activity and baroreflex function. For example, ANG II was shown to attenuate the baroreflex control of HR (7, 21, 22) and RSNA (4, 7, 22). In heart failure rats, blockade of AT1 receptors with losartan increased RSNA baroreflex gain, restoring it to normal values (14). Despite the widespread use of ACE inhibitors and AT1 blockers in diabetics, it is not known how endogenous ANG II affects baroreflex function of HR and RSNA in diabetes.

The present studies were undertaken 1) to examine baroreflex control of HR and RSNA in STZ-treated conscious rats and 2) to test the hypothesis that blockade of endogenous ANG II action will improve baroreflex function.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Adult male Sprague-Dawley rats weighing ~275-300 g were obtained from Harlan Sprague Dawley (Indianapolis, IN). They were housed under controlled conditions (21-23°C; lights on, 07:00-19:00 h) and had free access to water and standard rat chow. The rats were cared for in accordance with the principles of the National Institutes of Health Guide for the Care and Use of Laboratory Animals. All protocols were reviewed and approved by our Institutional Animal Investigation Committee.

Induction of diabetes mellitus. Rats were randomly assigned to one of two groups: 1) normal group and 2) diabetic group. Diabetes was induced by a single injection of STZ (55 mg/kg iv, Sigma) dissolved in 0.01 M citrate buffer, pH 4.5, and administered 15 days before the experiment. Normal rats received a similar volume of vehicle alone.

Surgical procedures. Two days before surgery, each rat was conditioned to remain for 120 min within a custom-made Plexiglas study chamber that would be used during the experiment. The chamber allowed the rat to move forward and backward but not to turn around.

Rats were anesthetized with pentobarbital sodium (40 mg/kg ip). Catheters were inserted into the left carotid artery and jugular vein for arterial pressure monitoring and drug infusion, respectively. The catheters were filled with heparinized saline (100 U/ml), secured, tunneled subcutaneously, and exteriorized at the base of the neck.

For recording RSNA, the left kidney was exposed through a retroperitoneal approach and the renal nerve branch isolated and carefully dissected free. The nerve was placed on electrodes constructed of Teflon-coated silver wire (0.0055-in. diameter, A-M Systems) with the exposed ends wound into single loops. The nerve and electrodes were covered with silicone gel (Wacker Sil-Gel 601 A and B), which was allowed to harden before closure. A ground wire was sewn into the surrounding tissue. The electrodes and ground wire were tunneled subcutaneously and exteriorized at the base of the neck.

After animals had recovered from anesthesia, they were returned to their individual cages for a ~24-h recovery period before the start of any experimental procedures. By the end of the recovery period, the animals were grooming themselves normally and displayed normal cage activity.

Methods of measurement. Systemic arterial pressure and HR were measured on a beat-by-beat basis via a Gould P23 XL pressure transducer equipped with an analog-to-digital converter board (Biotech Products) and recorded on computer hard disk for off-line analysis. Data were sampled continuously at 6 Hz by using a DAP 3216a/415 data acquisition processor as the hardware platform.

Arterial pressure was measured by connecting the arterial catheter with a pressure transducer, which was coupled to an amplifier (Digi-Med BPA-200). The arterial pressure was digitized and recorded with a hemodynamic and neural data analyzer (Biotech Products). Mean arterial pressure (MAP) and HR were determined on-line from pulsatile pressure using the Biotech software and averaged over 1-s intervals. All data were stored on hard disk for subsequent analysis.

Renal nerve activity was amplified (5,000-20,000 times) and filtered (100-1,000 Hz) with a Grass P511 differential preamplifier and a high-impedance probe (HIP511GB). The probe and animals were located inside a shielded Faraday cage (Harvard). The amplified and filtered neurogram signal was channeled to an oscilloscope (Hameg Instruments, HM407) and Grass AM8 audiomonitor for visual and auditory evaluation, respectively. The amplified nerve activity was digitized, rectified, integrated, and averaged over 1-s intervals by the computer data acquisition system (Biotech Products). Background noise was determined at the end of experiment after administration of a bolus dose of trimetaphan camsylate, 15 mg/kg iv (Hoffman-La Roche). RSNA was defined as the amount of recorded nerve activity after subtraction of background noise.

Experimental protocol. In normal and diabetic rats, baroreflex curves were generated before and after intravenous injection of losartan or enalapril. The rats were studied at least 24 h after completion of surgery. On the day of the experiment, each rat was placed into the study chamber and attached to the recording equipment. MAP, HR, and RSNA were monitored continuously. The rat was allowed to stabilize for at least 30 min before the experiment started. Several minutes of baseline data were recorded. Control arterial baroreflex curves were then generated by producing ramp changes in arterial pressure over ~2 min. MAP was increased to ~180 mmHg by intravenous infusion of phenylephrine hydrochloride (200 µg/ml; RBI) and decreased to ~50 mmHg by intravenous infusion of sodium nitroprusside (200 µg/ml; Ohmeda). Phenylephrine was infused in increasing rates of 5-50 µg · kg-1 · min-1 and nitroprusside in rates of 7.5-100 µg · kg-1 · min-1. Fifteen to twenty minutes were allowed between the infusions to permit all parameters to return to baseline values. Once the parameters had returned to baseline following the control baroreflex measurements, the rats were given an intravenous injection of either the AT1 receptor antagonist losartan (5 mg/kg; Merck) or enalapril (2.5 mg/kg; Sigma). Rats with functioning electrodes were studied again the subsequent day. Injections of losartan or enalapril were randomly assigned to one of the experimental days. In preliminary experiments, this dose of losartan completely abolished the pressor response (~45 mmHg) to a 100-ng iv injection of ANG II over a 1-h period. The same was shown by Petersen and DiBona (41). The dose of enalapril was chosen to match the change in MAP evoked by losartan and has been shown to be sufficient to block ANG II formation (1). Twenty minutes after the drug injection, the baroreflex curve protocol was repeated.

Data analysis. RSNA measured in microvolts varies considerably between animals depending on nonphysiological factors such as the condition of the electrode, nerve-electrode contact, and size of nerve bundle; therefore, direct comparison of the absolute level of nerve activity is not possible. Thus RSNA was normalized using the maximum nerve activity as the 100% value (maximum nerve activity equals the voltage of the upper plateau of the control baroreflex curve). MAP-HR and MAP-RSNA curves were constructed for each rat by fitting all individual data points (MAP, HR, and RSNA averaged over 1-s intervals) to a four-parameter sigmoid logistic function using a standard software package (SigmaPlot, Jandel Scientific). The equation used for this mathematical model is
HR or RSNA<IT>=</IT>(<IT>P</IT><SUB>1</SUB><IT>−P</IT><SUB>2</SUB>)<IT>/</IT>{1<IT>+</IT>exp[<IT>P</IT><SUB>3</SUB>(MAP<IT>−P</IT><SUB>4</SUB>)]}<IT>+P</IT><SUB>4</SUB>
where P1 is the upper plateau of the curve, P2 is the lower plateau, P3 is the slope coefficient (a coefficient describing the distribution of gain along the curve), and P4 is MAP at the midpoint of the curve (BP50). The range of baroreflex was calculated as the difference between the upper and lower plateaus (P1 - P2). The maximum gain (Gmax) was calculated as -(P1 - P2) · P3/4. In addition, the instantaneous gain over the full pressure range was determined by taking the first derivative of the baroreflex curve equation. The equation describing the derivative is
gain<IT>=</IT>

<IT>−</IT>(<IT>P</IT><SUB>1</SUB><IT>−P</IT><SUB>2</SUB>)<IT>P</IT><SUB>3</SUB>exp[<IT>P</IT><SUB>3</SUB>(MAP<IT>−P</IT><SUB>4</SUB>)]<IT>/</IT>{1<IT>+</IT>exp[<IT>P</IT><SUB>3</SUB>(MAP<IT>−P</IT><SUB>4</SUB>)]}<SUP>2</SUP>
For each individual animal's curve, the four parameters (P1 to P4) were derived and then averaged across animals. The mean parameters were used to generate an average baroreflex curve or average instantaneous gain curve for each group and for each treatment within each group. Resting values recorded before phenylephrine or nitroprusside infusions were averaged for each curve.

Baseline values before losartan or enalapril administration were calculated as the mean of a 5-min period before the injection. Changes in MAP, HR, and RSNA were then averaged across 1-min intervals at time 0 (immediately before the injection) and at 5, 10, 20, and 40 min postinjection.

Statistical analysis. All data are presented as means ± SE. Time-course data were analyzed by using two-way ANOVA with repeated measures across time. The modified Bonferroni test was used to compare individual means. For comparisons of baseline and baroreflex curve parameters, Student's t-test was used, paired or unpaired, when appropriate. A significance level of P < 0.05 was accepted.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The baseline characteristics of diabetic and control rats are shown in Table 1. Two weeks after STZ injection, diabetic rats had significantly higher glucose levels compared with normal rats (P < 0.001). Resting HR and RSNA (expressed as %max) were significantly lower in diabetic rats (P < 0.001 and P < 0.05, respectively). Absolute value of integrated RSNA was consistently lower in the diabetic group (4.9 ± 0.8 µV) versus vehicle-treated normal rats (9.3 ± 1.7 µV; P < 0.03). Body weight and MAP tended to be lower in the diabetic group, but the differences were not statistically different from normals (P > 0.05).

                              
View this table:
[in this window]
[in a new window]
  		Table 1.   Baseline values in normal and diabetic rats

Baroreflex control of HR and RSNA in STZ-induced diabetes. Diabetic rats exhibited markedly altered baroreflex regulation of HR. Not only was resting HR lower in diabetic rats compared with normoglycemic rats (Table 1, Figs. 1A and 2A), but HR was also reduced over the entire range of arterial pressure, resulting in a downward shift in HR baroreflex curve (Figs. 1A and 2A). As shown in Table 2, both the upper and the lower plateaus of the MAP-HR relationship were significantly reduced in diabetic rats. The upper plateau decreased more than the lower plateau (-88.5 vs. -44.9 beats/min), resulting in a significant decrease in the range of the HR baroreflex (Table 2). The BP50 of the MAP-HR curve and the Gmax did not differ between diabetic and normal rats (Table 2). However, diabetic rats had significantly reduced HR baroreflex gain compared with normal rats for pressures >120 mmHg (P < 0.05, Fig. 2C).


View larger version (17K):
[in this window]
[in a new window]
  		Fig. 1.   Representative baroreflex curves for heart rate (HR, A) and renal sympathetic nerve activity (RSNA, B) in normal () and diabetic rats (open circle ). triangle , Resting values. MAP, mean arterial pressure.



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 2.   Mean baroreflex curves describing reflex control of HR (A) and RSNA (B) in normal and diabetic rats. C and D: absolute value of instantaneous gain (first derivative) of the baroreflex curves in A and B, respectively. Circles on curves represent resting values (means ± SE). , Normal rats; open circle , diabetic rats. Shaded areas in C and D indicate significantly different baroreflex gain in diabetic vs. normal rats (P < 0.05).


                              
View this table:
[in this window]
[in a new window]
  		Table 2.   HR and RSNA baroreflex curve parameters in normal and diabetic rats

In contrast, RSNA baroreflex regulation remained less affected by the induction of diabetes (Table 2, Fig. 2B). The only significant change in the MAP-RSNA relationship in diabetic rats was a parallel leftward shift of the baroreflex curve, which was indicated by the significantly lower BP50 in this group of animals (Table 2). Because of the shift of the MAP-RSNA curve toward lower pressures, RSNA baroreflex gain in diabetic rats was significantly higher over the pressure range of 73-93 mmHg and was significantly lower through pressures of 122-142 mmHg, compared with normal rats (P < 0.05, Fig. 2D). The range, lower plateau, and Gmax of the MAP-RSNA relationship were similar in diabetic and normal rats (Table 2, Fig. 2B).

Time course of changes in MAP, HR, and RSNA after losartan and enalapril. Figure 3 shows that after losartan administration, MAP decreased significantly (time effect P < 0.001) and similarly in both groups (group effect P = 0.747). HR increased significantly after losartan in both groups (time effect P < 0.001). The effect of losartan on HR was not statistically different between normal and diabetic rats (group effect P = 0.172). Losartan significantly increased RSNA (time effect P < 0.001), and the increases in RSNA did not differ between groups (group effect P = 0.847). At the time when the baroreflex curves were run (20 and 40 min after losartan), MAP, HR, and RSNA had stabilized in each group of rats. The changes in MAP, HR, and RSNA at 40 min did not differ from the respective changes at 20 min after losartan administration (P > 0.05).


View larger version (25K):
[in this window]
[in a new window]
  		Fig. 3.   MAP, HR, and RSNA responses to administration of losartan (A) or enalapril (B) in normal () and diabetic (open circle ) rats over time. Values are means ± SE. * P < 0.05 vs. baseline (at 0 min); # P < 0.05 vs. normal rats.

In contrast to losartan, enalapril evoked a significantly smaller decrease in MAP in diabetic compared with normal rats (group effect P = 0.007, interaction P = 0.022). Whereas in normal rats enalapril evoked a depressor response similar to that of losartan (maximum change -10.3 ± 1.5 mmHg at 20 min after enalapril), the change in MAP in diabetic rats in response to enalapril was very small (maximum change -3.8 ± 1.6 mmHg at 20 min). Changes in MAP in the diabetic group were not statistically significant at any time point after enalapril. Enalapril significantly increased HR and RSNA (time effects P < 0.001). There were no differences in the effect of enalapril on HR and RSNA between diabetic and normal rats (group effects P = 0.947 and P = 0.482, respectively), despite the difference in the effect of enalapril on MAP between these groups. Comparable to losartan administration, the changes in MAP, HR, and RSNA 40 min after enalapril did not differ from the respective changes at 20 min (P > 0.05).

Comparisons between treatments (losartan and enalapril) within groups revealed that in normal rats both treatments had a similar influence on MAP, HR, and RSNA (treatment effects P = 0.516, P = 0.892, and P = 0.152, respectively). In diabetic rats, on the other hand, the MAP depressor response was significantly smaller after enalapril than after losartan (treatment effect P = 0.008, interaction P = 0.027). Nonetheless, the increases in HR and RSNA were similar after both treatments in this group of animals (treatment effect P = 0.156 and P = 0.225, respectively).

Baroreflex control of HR and RSNA after losartan and enalapril. Neither losartan nor enalapril changed the MAP-HR relationship in both groups of animals (Table 3, Fig. 4). Losartan lowered BP50 in diabetic rats but not in the normal group. After enalapril, the BP50 was unchanged in the diabetic group and was elevated in the normal rats. The attenuated gain of reflex bradycardia over higher pressures in diabetic rats was not improved by either losartan or enalapril (data not shown).

                              
View this table:
[in this window]
[in a new window]
  		Table 3.   HR baroreflex curve parameters in normal and diabetic rats before and after administration of losartan and enalapril



View larger version (15K):
[in this window]
[in a new window]
  		Fig. 4.   Effect of losartan or enalapril administration on mean baroreflex control of HR in normal (A) and diabetic rats (B). Circles on curves represent mean resting values (means ± SE). , Before drug administration; open circle , after drug administration.

Administration of losartan shifted the MAP-RSNA curves to lower values of MAP (Fig. 5), which was reflected in significantly lower BP50 in each group of animals (Table 4). Losartan did not significantly alter any other RSNA baroreflex parameters in either normal or in diabetic rats. Enalapril exerted no effect on baroreflex control of RSNA in normal animals (Table 4 and Fig. 5). In contrast, the upper plateau was consistently higher after enalapril in diabetic rats (in 7 of 8 rats), but its magnitude was highly variable (range: 5.6-140.1%max) and did not reach statistical significance (P = 0.095).


View larger version (15K):
[in this window]
[in a new window]
  		Fig. 5.   Effect of losartan or enalapril administration on mean baroreflex control of RSNA in normal (A) and diabetic rats (B). Circles on curves represent mean resting values (means ± SE). , Before drug administration; open circle , after drug administration.


                              
View this table:
[in this window]
[in a new window]
  		Table 4.   RSNA baroreflex curve parameters in normal and diabetic rats before and after administration of losartan and enalapril


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

This is the first study in which baroreflex control of RSNA was assessed in conscious diabetic rats. The study examines the role of endogenous ANG II in the baroreflex function in diabetic rats by blocking either the production of endogenous ANG II or antagonizing its action at the AT1 receptor. The major findings after 2 wk of STZ-induced diabetes are that 1) baroreflex control of HR is impaired in its range and gain for bradycardia, while at the same time the range and gain of RSNA baroreflex are preserved but the reflex is reset to lower pressures; 2) the impairment of baroreflex control of HR is not dependent on ANG II because neither AT1 receptor blockade nor ACE inhibition restored the range or bradycardia gain of the HR baroreflex to normal values; and 3) enalapril produced a pressure-independent increase in HR and RSNA in diabetic rats.

Baroreflex control of HR and RSNA in conscious diabetic rats. Diabetic autonomic dysfunction is associated with a high risk of mortality, which makes its early detection clinically important. The present study was therefore undertaken specifically to investigate early changes in arterial baroreflex regulation of HR and RSNA.

In accordance with other studies in rats (11, 25), STZ-induced diabetes was associated with a marked reduction in HR, although in human diabetes resting tachycardia is frequently reported (5, 6, 17). Although several studies have reported hypotension in STZ-diabetic rats (11, 12, 25, 34, 35), blood pressure in the diabetic rats in the present study was lower but not statistically different from the normal group. Our data indicate that RSNA is decreased in conscious diabetic rats. Baseline RSNA expressed as a percentage of maximum activity, as well as the absolute value of integrated renal nerve activity, was significantly lower in the diabetic group. Decreased RSNA was not the result of peripheral neuropathy because the response of the renal nerve to nitroprusside-induced hypotension was not suppressed in diabetic rats. In fact, it tended to be augmented. Our results, showing decreased renal nerve activity in conscious STZ-diabetic rats, are consistent with the well-documented subnormal plasma norepinephrine levels in diabetic patients (10, 17) and STZ-diabetic rats (39, 47), which imply diminished overall sympathetic tone in diabetes. Few studies have directly measured sympathetic nerve activity in diabetes, and all were done in anesthetized animals. Bunag et al. (9) found no difference between normal and STZ-treated rats in splanchnic nerve firing rate. McDowell et al. (37) reported a small (although statistically not significant) increase in RSNA (expressed as spikes/s) in alloxan-diabetic rabbits. Our study shows that resting renal sympathetic tone is lower after 2 wk of diabetes.

Baroreflex control of HR was markedly impaired in the diabetic group. The range was reduced by ~23%, which was due to a greater reduction of the upper plateau of the curve than the lower plateau. A reduction in the range of baroreflex control of HR was also found in alloxan-diabetic rabbits but only after 12 wk (36). As evidenced by analysis of the changes in the instantaneous gain over the full range of pressures, diabetic rats had unchanged gain for reflex tachycardia but showed attenuated gain for reflex bradycardia at pressures over 120 mmHg. This result is consistent with studies in diabetic animals (12, 36) as well as clinical studies (5, 6, 17) where impaired reflex bradycardia was shown.

Baroreflex control of HR in diabetes has been the subject of numerous investigations, in both humans and experimental animals. In diabetic patients there is a well-documented attenuation of reflex bradycardia in response to increases in arterial pressure (5, 6, 17). Reflex tachycardia in response to decrease in arterial pressure is also impaired (5, 38). In STZ-treated rats, baroreflex-mediated bradycardia has been reported to be reduced (12), normal (34, 35), or even enhanced (26). Baroreflex tachycardia was attenuated (12, 34, 35) or enhanced (16). On the other hand, in alloxan-diabetic rabbits baroreflex bradycardia was attenuated and reflex tachycardia was unchanged (36). These conflicting data may be attributed to differences in experimental and analytic approaches. The time after diabetes induction (11) and the method of analysis of baroreceptor sensitivity (20) as well as the animal model used can change the interpretation of the data.

In the studies cited above, baroreflex sensitivity was assessed either as the slope of the linear part of the MAP-HR relationship (26) or as the slopes for the increases and decreases of MAP separately (12, 35, 36). Moreover, several authors evaluated baroreflex sensitivity only based on the HR response to phenylephrine-induced increases in MAP (11, 34, 40). Obviously, these analytic methods have some limitations, especially because they permit evaluation of baroreflex sensitivity only over a narrow range of blood pressures where the MAP-HR relationship is linear. In our study we compared whole sigmoidal baroreflex curves for both HR and RSNA, thereby analyzing baroreflex function over a broad range of arterial pressures. This method has advantages over linear fitting in that it also gives an estimate of the plateau values at the extremes of blood pressures; i.e., the range of HR or RSNA over which the baroreflex operates. It also allows computation and comparison of the instantaneous baroreflex gain (as the first derivative of the sigmoidal curve) at each pressure level between groups. To our knowledge, this is the first study to evaluate baroreflex control of HR and RSNA over the full range of pressures in diabetic rats.

Our results showed that 2-wk STZ-induced diabetes differentially influences baroreflex control of HR and RSNA. Diabetic rats exhibited blunted baroreflex control of HR, whereas at the same time the baroreflex regulation of RSNA was preserved. Neither maximum gain nor the range of baroreflex control of RSNA changed at this stage of diabetes. The MAP-RSNA curve was shifted significantly to the left, indicating that in early diabetes the baroreflex control of renal nerve activity is reset to lower arterial pressures. In contrast, resting bradycardia in diabetic rats was accompanied by a downward shift of the MAP-HR curve with no change in the midpoint of the curve, indicating that there was no significant pressure resetting of the HR baroreflex. Our results confirmed and extended previous findings in anesthetized STZ-diabetic rats where the baroreflex response of RSNA was evaluated only to increases of arterial pressure and was found to be unchanged (40). Also, no change in baroreflex control of RSNA occurred in rabbits after 24 wk of diabetes (37). Although the latter results are difficult to compare with ours because of the different time duration of diabetes, different diabetogen, and different animal model, the results of both studies together with our results strongly suggest that baroreflex control of RSNA is not impaired in either the early or in later stage of diabetes.

Effect of ANG II blockade on baroreflex control of HR and RSNA in diabetic rats. In the present study, antagonism of AT1 receptors or blockade of the major pathway for ANG II synthesis by ACE inhibition increased HR and RSNA in both control and diabetic rats, consistent with the increase of RSNA observed after acute intravenous (30) or after intracerebroventricular administration of losartan in rabbits (4). However, DiBona et al. (15) showed that intravenous losartan decreased RSNA in rats after the blood pressure had been restored to normal values with methoxamine. This may indicate that the increase in RSNA after losartan, observed in our study, together with increased HR, may be a reflex response to the losartan-induced fall in MAP, although other possibilities cannot be excluded. Extensive studies conducted in Head's laboratory (4, 21, 24) on conscious rabbits on the effect of central ANG II on renal nerve activity and its baroreflex regulation lead to the conclusion that endogenous ANG II can act centrally either to stimulate or to inhibit sympathetic activity. Losartan crosses the blood-brain barrier (32), therefore, our findings with losartan to increase RSNA are consistent with a central sympathoinhibitory action by ANG II (4, 21, 24).

An increase in RSNA as well as HR also occurred after enalapril in both groups, even though enalapril did not decrease MAP in the diabetic animals. Thus the increase in HR and RSNA in diabetic rats after ACE inhibition is pressure independent and not a reflex response. It remains unclear why diabetic rats showed reduced responsiveness to the hypotensive action of enalapril, whereas losartan reduced MAP to the same extent in both groups. One possible explanation is that in diabetes plasma ACE concentration is elevated (23, 44-46), thus the same dose of enalapril may be less effective in inhibiting the transformation of ANG I to ANG II. We may also speculate that diabetic rats have a greater capacity for ANG II synthesis via alternative pathways or that the inhibitory effect of enalapril is blunted in diabetic animals, as has been shown in normal rats on high-sodium diet (29). The other question is what is the mechanism of pressure-independent increase in HR and RSNA after enalapril in diabetic animals. Among possible explanations, the role of enalapril-induced increased level of bradykinin (18) and its relation to diabetes should be considered and investigated further. It is also worth noting that a similar effect was observed in healthy humans wherein acute enalapril treatment increased muscle sympathetic nerve activity but did not change MAP (13).

Neither AT1 receptor blockade nor ACE inhibition had a significant effect on baroreflex control of HR and RSNA in control or diabetic rats. Similar results were obtained with enalapril in anesthetized STZ-diabetic rats, where baroreflex was assessed by using phenylephrine alone (40). In the present study the attenuated range of baroreflex control of HR and attenuated gain for baroreflex bradycardia in diabetic rats did not improve after acute losartan or enalapril administration. These observations indicate that impaired baroreflex control of HR in diabetic rats was not ANG II dependent. The mechanism of this impairment cannot be ascertained from the present study. However, experimental findings by others suggest that destructive changes in the myocardium in STZ-induced diabetes may account for the impaired cardiac ability to compensate for changes in arterial pressure (25, 42).

It is important to note that the present study was aimed only at short-term effects of ANG II blockade on baroreflex control in diabetes, and it is possible that long-term administration of losartan or enalapril would have different effects. Gaudet et al. (21) showed that there is a difference between the effects of acute and chronic intracerebroventricular administration of ANG II or losartan in rabbits.

In summary, the present study indicates that after 2-wk STZ-induced diabetes baroreflex control of HR is impaired but the baroreflex control of RSNA is preserved. The impairment of baroreflex control of HR is not dependent on endogenous ANG II, because neither AT1 receptor blockade nor ACE inhibition restored the baroreflex control of HR. ACE inhibition with enalapril produces a pressure-independent increase in HR and RSNA in diabetic rats.


    ACKNOWLEDGEMENTS

We appreciate the gracious help of Robert J. Pawlowicz for invaluable computer expertise.


    FOOTNOTES

This work was supported by a Merit Award and Research Enhancement Award Program of the Department of Veterans Affairs to N. F. Rossi.

Address for reprint requests and other correspondence: N. F. Rossi, Depts. of Medicine and Physiology, Wayne State Univ., 4160 John R St., #908, Detroit, MI 48201 (E-mail: nrossi{at}intmed.wayne.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

First published January 9, 2003;10.1152/ajpheart.00578.2002

Received 12 July 2002; accepted in final form 7 January 2003.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Allan, DR, McKnight JA, Kifor I, Coletti CM, and Hollenberg NK. Converting enzyme inhibition and renal tissue angiotensin II in the rat. Hypertension 24: 516-522, 1994[Abstract/Free Full Text].

2.   Andersen, S, Tarnow L, Rossing P, Hansen BV, and Parving HH. Renoprotective effects of angiotensin II blockade in type I diabetic patients with diabetic nephropathy. Kidney Int 57: 601-606, 2000[Web of Science][Medline].

3.   Anderson, S. Role of local and systemic angiotensin in diabetic renal disease. Kidney Int 52: S107-S110, 1997.

4.   Bendle, DR, Malpas SC, and Head GA. Role of endogenous angiotensin II on sympathetic reflexes in conscious rabbits. Am J Physiol Regul Integr Comp Physiol 272: R1816-R1825, 1997[Abstract/Free Full Text].

5.   Bennett, T, Farquhar IK, Hosking DJ, and Hampton JR. Assessment of methods for estimating autonomic nervous control of the heart in patients with diabetes mellitus. Diabetes 27: 1167-1174, 1978[Abstract].

6.   Bennett, T, Hosking DJ, and Hampton JR. Baroreflex sensitivity and responses to the Valsalva manoeuvre in subjects with diabetes mellitus. J Neurol Neurosurg Psychiatry 39: 178-183, 1976[Abstract/Free Full Text].

7.   Bishop, VS, and Sanderford MG. Angiotensin II modulation of the arterial baroreflex: role of the area postrema. Clin Exp Pharmacol Physiol 27: 428-431, 2000[Web of Science][Medline].

8.   Brands, MW, and Fitzgerald SM. Arterial pressure control at the onset of type I diabetes: the role of nitric oxide and the renin-angiotensin system. Am J Hypertens 14: 126S-131S, 2001[Web of Science][Medline].

9.   Bunag, RD, Tomita T, and Sasaki S. Streptozotocin diabetic rats are hypertensive despite reduced hypothalamic responsiveness. Hypertension 4: 556-565, 1982[Abstract/Free Full Text].

10.   Caviezel, F, Picotti GB, Margonato A, Slaviero G, Galva MD, Camagna P, Bondiolotti GP, Carruba MO, and Pozza G. Plasma adrenaline and noradrenaline concentrations in diabetic patients with and without neuropathy at rest and during sympathetic stimulation. Diabetologia 23: 19-23, 1982[Web of Science][Medline].

11.   Chang, KSK, and Lund DD. Alterations in the baroreceptor reflex control of heart rate in streptozotocin diabetic rats. J Mol Cell Cardiol 18: 617-624, 1986[Web of Science][Medline].

12.   Dall'Ago, P, Fernandes TG, Machado UF, Bello AA, and Irigoyen MC. Baroreflex and chemoreflex dysfunction in streptozotocin-diabetic rats. Braz J Med Biol Res 30: 119-124, 1997[Web of Science][Medline].

13.   Dibner-Dunlap, M, Smith Ml Kinugawa T, and Thames MD. Enalaprilat augments arterial and cardiopulmonary baroreflex control of sympathetic nerve activity in patients with heart failure. J Am Coll Cardiol 27: 358-364, 1996[Abstract].

14.   DiBona, G, Jones SY, and Brooks VL. ANG II receptor blockade and arterial baroreflex regulation of renal nerve activity in cardiac failure. Am J Physiol Regul Integr Comp Physiol 269: R1189-R1196, 1995[Abstract/Free Full Text].

15.   DiBona, GF, Jones SY, and Sawin LL. Effect of endogenous angiotensin II on renal nerve activity and its arterial baroreflex regulation. Am J Physiol Regul Integr Comp Physiol 271: R361-R367, 1996[Abstract/Free Full Text].

16.   Dowell, RT, Atkins FL, and Love S. Integrative nature and time course of cardiovascular alterations in the diabetic rat. J Cardiovasc Pharmacol 8: 406-413, 1986[Web of Science][Medline].

17.   Eckberg, DL, Harkins SW, Fritsch JM, Musgrave GE, and Gardner DF. Baroreflex control of plasma norepinephrine and heart period in healthy subjects and diabetic patients. J Clin Invest 78: 366-374, 1986[Web of Science][Medline].

18.   Erdos, EG, and Skidgel RA. The angiotensin I converting enzyme. Lab Invest 56: 345-348, 1987[Web of Science][Medline].

19.   Ewing, DJ, Campbell IW, and Clarke BF. The natural history of diabetic autonomic neuropathy. QJM 49: 95-108, 1980[Abstract/Free Full Text].

20.   Farah, VM, Moreira ED, Pires MD, Irigoyen MC, and Krieger EM. Comparison of three methods for the determination of baroreflex sensitivity in conscious rats. Braz J Med Biol Res 32: 361-369, 1999[Web of Science][Medline].

21.   Gaudet, E, Godwin SJ, and Head GA. Effects of central infusion of angiotensin II and losartan on baroreflex control of heart rate in rabbits. Am J Physiol Heart Circ Physiol 278: H558-H566, 2000[Abstract/Free Full Text].

22.   Guo, GB, and Abboud FM. Angiotensin II attenuates baroreflex control of heart rate and sympathetic activity. Am J Physiol Heart Circ Physiol 246: H80-H89, 1984[Abstract/Free Full Text].

23.   Hartmann, JF, Szemplinski M, Hayes NS, Keegan ME, and Slater EE. Effects of the angiotensin converting enzyme inhibitor, lisinopril, on normal and diabetic rats. J Hypertens 6: 677-683, 1988[Web of Science][Medline].

24.   Head, GA, and Mayorov DN. Central angiotensin and baroreceptor control of circulation. Ann NY Acad Sci 940: 361-390, 2001[Web of Science][Medline].

25.   Hicks, KK, Seifen E, Stimers JR, and Kennedy RH. Effects of streptozotocin-induced diabetes on heart rate, blood pressure and cardiac autonomic nervous control. J Auton Nerv Syst 69: 21-30, 1998[Web of Science][Medline].

26.   Jackson, CV, and Carrier GO. Influence of short-term experimental diabetes on blood pressure and heart rate in response to norepinephrine and angiotensin II in the conscious rat. J Cardiovasc Pharmacol 5: 260-265, 1983[Web of Science][Medline].

27.   Kessler, I. Mortality experience of diabetic patients. A twenty-six-year follow-up study. Am J Med 51: 715-724, 1971[Web of Science][Medline].

28.   Kikkawa, R, Kitamura E, Fujiwara Y, Haneda M, and Shigeta Y. Biphasic alteration of renin-angiotensin-aldosterone system in streptozotocin-diabetic rats. Renal Physiol 9: 187-192, 1986[Web of Science][Medline].

29.   Kocks, M, Buikema H, de Zeeuw D, and Navis G. Effect of sodium intake on vascular tissue ACE function during ACEi may explain sodium-induced therapy resistance to ACEi (Abstract). J Am Soc Nephrol 13: 334A, 2002.

30.   Kumagai, K, and Reid IA. Angiotensin II exerts differential actions on renal nerve activity and heart rate. Hypertension 24: 451-456, 1994[Abstract/Free Full Text].

31.   Lewis, EJ, Hunsicker LG, Bain RP, and Rohde RD. The effect of angiotensin-converting enzyme inhibition on diabetic nephropathy. N Engl J Med 329: 1456-1462, 1993[Abstract/Free Full Text].

32.   Li, Z, Bains JS, and Ferguson AV. Functional evidence that the angiotensin antagonist losartan crosses the blood-brain barrier in the rat. Brain Res Bull 30: 33-39, 1993[Web of Science][Medline].

33.   Lishner, M, Akserrod S, Avi VM, Oz O, Divon M, and Ravid M. Spectral analysis of HR fluctuation: a non-invasive, sensitive method for the early diagnosis of autonomic neuropathy in diabetes mellitus. J Auton Nerv Syst 19: 119-125, 1987[Web of Science][Medline].

34.   Maeda, CY, Fernandes TG, Lulhier F, and Irigoyen MC. Streptozotocin diabetes modifies arterial pressure and baroreflex sensitivity in rats. Braz J Med Biol Res 28: 497-501, 1995[Web of Science][Medline].

35.   Maeda, CY, Fernandes TG, Timm HB, and Irigoyen MC. Autonomic dysfunction in short-term experimental diabetes. Hypertension 26: 1100-1104, 1995[Abstract/Free Full Text].

36.   McDowell, TS, Chapleau MW, Hajduczok G, and Abboud FM. Baroreflex dysfunction in diabetes mellitus. I. Selective impairment of parasympathetic control of heart rate. Am J Physiol Heart Circ Physiol 266: H235-H243, 1994[Abstract/Free Full Text].

37.   McDowell, TS, Hajduczok G, Abboud FM, and Chapleau MW. Baroreflex dysfunction in diabetes mellitus. II. Site of baroreflex impairment in diabetic rabbits. Am J Physiol Heart Circ Physiol 266: H244-H249, 1994[Abstract/Free Full Text].

38.   Navarro, X, Kennedy WR, and Ferrer MT. Cardiovascular responses to tilting in healthy and diabetic subjects. J Neurol Sci 104: 39-45, 1991[Web of Science][Medline].

39.   Ohtani, N, Ohta M, and Sugano T. Microdialysis study of modification of hypothalamic neurotransmitters in streptozotocin-diabetic rats. J Neurochem 69: 1622-1628, 1997[Web of Science][Medline].

40.   Patel, KP, and Zhang PL. Baroreflex function in streptozotocin (STZ) induced diabetic rats. Diabetes Res Clin Pract 27: 1-9, 1995[Web of Science][Medline].

41.   Petersen, JS, and DiBona GF. Furosemide elicits immediate sympathoexcitation via a renal mechanism independent of angiotensin II. Pharmacol Toxicol 76: 106-113, 1995.

42.   Ramanadham, S, and Tenner TE, Jr. Alterations in cardiac performance in experimentally-induced diabetes. Pharmacology 27: 130-139, 1983[Web of Science][Medline].

43.   Stornetta, RL, Guyenet PG, and McCarty RC. Autonomic nervous system control of heart rate during baroreceptor activation in conscious and anesthetized rats. J Auton Nerv Syst 20: 121-127, 1987[Web of Science][Medline].

44.   Ustundag, B, Cay M, Naziroglu M, Dilsiz N, Crabbe MJC, and Ilhan N. The study of renin-angiotensin-aldosterone in experimental diabetes mellitus. Cell Biochem Funct 17: 193-198, 1999[Web of Science][Medline].

45.   Valentovic, M, Elliott CW, and Bell JG. The effect of streptozotocin-induced diabetes and insulin treatment on angiotensin converting enzyme activity. Res Commun Chem Pathol Pharmacol 58: 27-39, 1987[Web of Science][Medline].

46.   Van Dyk, DJ, Erman A, Erman T, Chen-Gal B, Sulkes J, and Boner G. Increased serum angiotensin converting enzyme activity in type I insulin-dependent diabetes mellitus: its relation to metabolic control and diabetic complications. Eur J Clin Invest 24: 463-467, 1994[Web of Science][Medline].

47.   Yoshida, T, Nishioka H, Nakamura Y, and Kondo M. Reduced noradrenaline turnover in streptozotocin-induced diabetic rats. Diabetologia 28: 692-696, 1985[Web of Science][Medline].

Am J Physiol Heart Circ Physiol 284(5):H1601-H1611


This article has been cited by other articles:


Home page
 Am. J. Physiol. Regul. Integr. Comp. Physiol.Home page
M. Maliszewska-Scislo, H. Chen, R. A. Augustyniak, D. Seth, and N. F. Rossi
Subfornical organ differentially modulates baroreflex function in normotensive and two-kidney, one-clip hypertensive rats
Am J Physiol Regulatory Integrative Comp Physiol, September 1, 2008; 295(3): R741 - R750.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Regul. Integr. Comp. Physiol.Home page
R. A. Augustyniak, M. Maliszewska-Scislo, H. Chen, J. Fallucca, and N. F. Rossi
Acute angiotensin-converting enzyme inhibition evokes bradykinin-induced sympathetic activation in diabetic rats
Am J Physiol Regulatory Integrative Comp Physiol, December 1, 2007; 293(6): R2260 - R2266.
[Abstract] [Full Text] [PDF]

Home page
 J. Appl. Physiol.Home page
K.-J. Bar, M. K. Boettger, S. Berger, V. Baier, H. Sauer, V. K. Yeragani, and A. Voss
Decreased baroreflex sensitivity in acute schizophrenia
J Appl Physiol, March 1, 2007; 102(3): 1051 - 1056.
[Abstract] [Full Text] [PDF]

Home page
 HypertensionHome page
J. A. Joles and H. A. Koomans
Causes and Consequences of Increased Sympathetic Activity in Renal Disease
Hypertension, April 1, 2004; 43(4): 699 - 706.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
284/5/H1601    most recent
00578.2002v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (8)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Maliszewska-Scislo, M.
Right arrow Articles by Rossi, N. F.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Maliszewska-Scislo, M.
Right arrow Articles by Rossi, N. F.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online