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Am J Physiol Heart Circ Physiol 282: H380-H388, 2002;
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Vol. 282, Issue 1, H380-H388, January 2002

Effects of age, gender, and blood pressure on myogenic responses of mesenteric arteries from C57BL/6 mice

Robert Gros1,2, Ryan Van Wert1,2, Xiaomang You1,2, Eric Thorin3, and Mansoor Husain1,2

1 Heart and Stroke Richard Lewar Center of Excellence, University of Toronto, Toronto, Ontario; 2 Division of Cellular & Molecular Biology, Toronto General Hospital Research Institute, Toronto, Ontario M5G 2C4; and 3 The Montreal Heart Institute, Montreal, Quebec H1T 1C8, Canada


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

The myogenic response (MR) may represent an important physiological parameter underlying arterial blood pressure (BP). We studied the effects of age, gender, and BP on the MR of mesenteric arteries from 8- to 52-wk-old mice. Increasing age and BP are associated with an increase in the perfusion pressure at which tone develops (myogenic set point). An inverse correlation exists between age and extent (magnitude) of the MR in male (r2 = 0.93, P = 0.0087) and female mice (r2 = 0.90, P = 0.013) as well as between BP and extent of the MR in male (r2 = 0.96, P = 0.0036) and female (r2 = 0.90, P = 0.014) mice. In contrast, the strength of the MR (slope of active diameter-pressure relationship) and phenylephrine-mediated constriction did not differ among these groups. Although gender had no effect on MR at any perfusion pressure or age, only male mice showed significant salt-induced hypertension and an associated increase in the set point and reduction in the extent of the MR. The set point and extent of the MR is linked to the in vivo pressure during development and experimental hypertension.

salt-induced hypertension


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

HYPERTENSION IS THE MOST COMMON cardiovascular disease and an important risk factor for myocardial infarction, stroke, and renal failure (28). Although the basic abnormality in hypertension is increased peripheral resistance (9), the mechanisms underlying this reflect a combination of structural and functional factors. On a functional level, peripheral resistance is a balance between vasodilator and vasoconstrictor mechanisms of the vascular smooth muscle. The myogenic response in resistance vessels is characterized by vasoconstriction in response to an increase in intraluminal pressure and vasodilation on pressure alleviation. This phenomenon is a component of the autoregulation of blood flow and the establishment of basal vascular tone (reviewed in Refs. 3 and 43). As such, the myogenic response may represent an important physiological parameter in determining arterial blood pressure (BP). Although both normal development (aging) and male gender are associated with a higher incidence of elevated BP (22, 23), the relationship of these factors with the myogenic response has not been well explored.

To date, several studies have examined vasomotor responses in the mouse (2, 4-6, 14, 24, 30, 31, 39, 41, 42, 47, 50, 51) (reviewed in Ref. 8). In some studies (4-6, 8, 24, 39, 42, 47, 50, 51), transgenic and knockout technologies were used to study pathways that may affect cardiovascular physiology. However, many of these studies (5, 8, 42, 50) focused on conduit vessels such as the aorta and carotid artery, and few of these studies (14, 17, 27, 30, 31, 39, 51) explored the myogenic response. Indeed, only three examined myogenic tone in the mesenteric arterial system (17, 30, 39) and none addressed the issue of age-, gender-, and BP-dependent effects on the myogenic responses of these vessels.

An age-dependent loss of the extent (magnitude of change from passive to active vessel diameter at a given experimental perfusion pressure) of the myogenic response has been reported when comparing mesenteric arteries from swine 1-3 and 35-40 days old (36, 40). However, as development of these swine was also associated with increases in arterial BP (40), it is unclear as to whether the observed reductions in this feature of the myogenic response can be attributed to age-related processes other than increasing BP.

Similarly, although female gender-dependent reductions in the extent of the myogenic response have been described in mouse and rat cerebral arteries (13-15), rat muscle arterioles (19), and rat coronary arteries (49), whether these differences are specifically related to the female gender or gender-related differences in BP has not been examined.

Finally, BP-dependent alterations in the strength [slope of the active pressure-vessel diameter relationship (7)] of the myogenic response were described in spontaneously hypertensive rats (SHR), where weaker myogenic responses were observed in cerebral arteries obtained from SHR compared with normotensive Wistar-Kyoto (WKY) rats (38). However, other reports have shown either no change or increased strength and/or extent of the myogenic response in SHR, depending on the age of the animals or arteries studied (7, 12, 18, 21, 44).

Thus the interrelationships among age, gender, and BP and their effects on fundamental features of the myogenic response have not been well studied in any animal preparation. To directly test the hypotheses that increasing age, female gender, and experimental elevations in BP increase the perfusion pressure at which myogenic tone develops (the set point of the myogenic response) and reduce its extent and strength, we have employed perfusion pressure myography in isolated mesenteric arteries from wild-type C57BL/6 mice. Our data show that neither development- nor salt-dependent elevations in BP affect the strength of observed myogenic responses or phenylephrine-mediated vasoconstriction. By contrast, we observed a direct relationship between the resting in vivo BP and the set point at which myogenic tone develops and an inverse relationship between in vivo BP and the magnitude of the myogenic response at defined experimental perfusion pressures. As such, we establish a normal standard against which molecular genetic models of hypertension and/or altered myogenic responsiveness may be compared.


    MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Animals. Male and female C57BL/6 mice were purchased from Charles River Laboratories (Montreal, Quebec, Canada) and housed under a 12:12-h light-dark cycle with normal chow and water ad libitum. In a separate series of experiments, 8-wk-old male and female mice were fed either a normal or a high-salt diet (8% NaCl). The Animal Care Committee of the Toronto General Hospital approved all animal procedures.

Systolic BP determination. Indirect tail-cuff measurements of systolic BP were obtained in anesthetized (100 mg/kg ip ketamine and 10 mg/kg ip xylazine) mice by use of a mouse tail monitor (Kent Scientific; Litchfield, CT). Mice were placed on a warming blanket (37°C), and 3-5 consistent measurements were obtained for each mouse. BP measurements were obtained weekly from 8 to 24 wk of age and then at 26, 28, 32, 36, and 52 wk of age.

Vessel harvest and determination of myogenic responses. Mice were killed by cervical dislocation. The entire intestine was grossly dissected from the mouse and placed in a petri dish bathed in ice-cold physiological saline solution (PSS). Vascular dissection was performed under a stereoscope. Mesenteric arteries (second order) were stored in ice-cold PSS for <6 h. A pressure myograph was used to determine vessel diameter (Living Systems Instrumentation; Burlington, VT). This device consists of a small PSS-filled chamber (5 ml) into which two fine, diametrically opposed, moveable glass pipettes protrude. Constant temperature (37°C) was afforded by a heating element and thermostat. Under a dissecting microscope, an artery was mounted onto the pipettes and secured with sutures. One pipette was fitted distally with a stopcock; the other was attached to a peristaltic pump. This pump pressurized the artery and allowed for real-time control of the luminal perfusion pressure. Measurements were performed with a video dimension analyzer (VDA) connected to a camera on an inverted microscope. This device furnished real-time diameter measurements of the blood vessel. A ruler calibrated to the television monitor was used to confirm measurements of the VDA. Once mounted, vessels were equilibrated at 20 mmHg for 30 min; the PSS was changed every 10 min. To determine myogenic responses, the luminal pressure was increased in 20-mmHg steps from 20 to 120 mmHg. At each pressure, a 5- to 10-min equilibration period was allowed, and the vessel diameter was noted after a steady state was reached. At the end of each experiment, the chamber medium was replaced with Ca2+-free PSS, and the passive diameter was recorded for each pressure between 20 and 120 mmHg in 20-mmHg steps.

The set point of the myogenic response was defined as the lowest perfusion pressure at which vasoconstriction was first observed. The strength of the myogenic response was defined as the slope of the active diameter-pressure relationship (7) from the set point to 120 mmHg of perfusion pressure. The extent of the myogenic response at a given perfusion pressure was defined as the magnitude of the percent myogenic tone (%MT) at that pressure. %MT was expressed by the active (AD) and passive vessel diameters (PD) such that %MT = [(PD - AD)/PD] · 100%.

To observe contractile responses to phenylephrine in the absence of myogenic constriction, vessels were pressurized to 60 mmHg and allowed to equilibrate for 30 min. At 60 mmHg, myogenic constriction did not play a significant role in any age group examined (see Figs. 3 and 4). After the basal diameter (BD) was recorded, a phenylephrine (1 nmol/l-30 µmol/l) concentration-response curve was constructed. A 3- to 5-min equilibration period was allowed with each dose. The percent constriction (%C) was calculated from the BD and the constricted diameter (CD) at a given phenylephrine concentration, and the maximal CD (MD) was observed such that %C = [(BD - CD)/(BD - MD)] · 100%.

Data analysis. All data are expressed as means ± SE. Statistical analysis was done using either one-way or two-way ANOVA with Bonferroni post hoc tests and Student's t-test where appropriate.

Solutions and chemicals. PSS was of the following composition (in mmol/l): 130 NaCl, 14.9 NaHCO3, 10.0 glucose, 4.70 KCl, 1.17 MgSO4, 1.18 KH2PO4, 1.60 CaCl2, and 0.027 EDTA. Ca2+-free solution was of identical composition with CaCl2 omitted and 1.00 mmol/l EGTA added. Both solutions were brought to pH 7.4 and aerated continuously with 12% O2-5% CO2-83% N2. Phenylephrine was made up daily in PSS and stored on ice.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Body weight, age, and BP. Body weight was significantly correlated with age in both male (r2 = 0.84, P < 0.001) and female (r2 = 0.70, P < 0.001) mice (Fig. 1A). To examine the relationship between BP and development, we assessed systolic BP in both male and female mice ranging from 8 to 52 wk of age. Systolic BP was significantly correlated with age in both male (r2 = 0.38, P < 0.001) and female (r2 = 0.26, P < 0.001) mice (Fig. 1B). Systolic BP was significantly increased in both male and female mice at 24, 36, and 52 wk of age compared with 8-wk-old mice. No significant difference in systolic BP was observed between 36 and 52 wk of age in either male or female mice (Fig. 1B). In addition, a significant positive correlation was observed between systolic BP and body weight for both male (r2 = 0.38, P < 0.001) and female (r2 = 0.21, P < 0.001) mice (Fig. 1C).


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  		Fig. 1.   Positive correlation between body weight and age (A), systolic blood pressure (BP) and age (B), and systolic BP and body weight (C) in male () and female () C57BL/6 mice. Linear regression lines are shown for male and female mice.

Myogenic responses as a function of age, gender, and BP. To examine the relationship among age, gender, and myogenic responses, we assessed active and passive mesenteric artery diameters from male and female mice at different ages and experimental perfusion pressures.

Although passive vessel diameters appeared to increase with age, no consistent development- or gender-dependent differences were observed (Fig. 2). However, an age-dependent increase in the set point of the myogenic response (the lowest perfusion pressure at which myogenic tone develops) was observed in both male and female mice (Fig. 3). No significant myogenic tone was observed in 36- or 52-wk-old mice of either sex (Fig. 3).


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  		Fig. 2.   Passive diameter-experimental perfusion pressure relationships of mesenteric arteries at various ages. Mesenteric arteries were obtained from male (A) and female (B) mice. Data are expressed as means ± SE. *P < 0.05 for 52- vs. 8-wk-old male mice. #P < 0.05 for 52- and 36- vs. 8- and 24-wk-old female mice. Passive diameters in 24-wk-old female mice were significantly smaller than in age-matched male mice.



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  		Fig. 3.   Age-dependent increases in the set point of the myogenic response. Normalized passive and active diameter-experimental perfusion pressure relationships of mesenteric arteries obtained from male and female mice at 8, 12, 24, 36, and 52 wk of age. Data are expressed as a percentage of the passive diameter at 100 mmHg for each individual age and gender group and represent the means ± SE. *P < 0.05 between active and passive diameters for male mice. #P < 0.05 between active and passive diameters for female mice.

Age- and gender-dependent alterations in the extent of the myogenic response (magnitude of %MT development) at given experimental perfusion pressures are detailed in Fig. 4. In 8-wk-old male mice, myogenic tone is first observed at 80 mmHg, and the extent of the response at this experimental perfusion pressure (13.5 ± 2.7%) is significantly greater than in 24-, 36-, and 52-wk-old mice (Fig. 4A). At 120 mmHg of perfusion pressure, 8-, 12-, and 24-wk-old male mice show a maximum %MT of 19.1 ± 2.2% (n = 12), 18.1 ± 3.9% (n = 4), and 17.7 ± 2.5% (n = 9), respectively. By contrast, the extent of the myogenic response at this pressure was reduced in 36- (10.0 ± 3.8%, n = 5) and 52-wk-old mice (1.7 ± 1.3%, n = 4, P < 0.05) (Fig. 4A). In female mice, the %MT (at 120 mmHg of perfusion pressure) was similar in 8-, 12-, and 24-wk-old mice, namely, 20.2 ± 4.5% (n = 6), 21.6 ± 2.0% (n = 5), and 21.1 ± 2.1% (n = 5), respectively, and reduced in 36- (11.4 ± 2.4%, n = 4) and 52-wk-old mice (3.3 ± 2.4%, n = 4, P < 0.05) (Fig. 4B). In female mice, onset of a significant myogenic response was not observed until an experimental perfusion pressure of 100 mmHg, which is in contrast to the observation made in male mice. However, the %MT in 8-wk-old mice at 80 mmHg of perfusion pressure was not significantly different between male (13.5 ± 2.7%) and female (8.2 ± 4.1%) mice [P = not significant (NS)]. Interestingly, the onset of the myogenic response in female mice appears to be less influenced by age (Fig. 4B) compared with male mice (Fig. 4A, graded myogenic response with age at 100 mmHg of pressure). However, similar to observations made in male mice, no myogenic tone was observed in mesenteric arteries obtained from 52-wk-old female mice at any of the perfusion pressures tested (Fig. 4B).


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  		Fig. 4.   Age-dependent decline in the extent of the myogenic response. Mesenteric arteries were obtained from male (A) and female (B) C57BL/6 mice. Data are expressed as means ± SE. *P < 0.05 for 8 vs. 24, 36, and 52 wk at 80 mmHg; **P < 0.05 for 8, 12, and 24 vs. 36 and 52 wk at 100 mmHg; ***P < 0.05 for mice 8, 12, 24, and 36 vs. 52 wk old at 120 mmHg.

To further explore the apparent age-dependent decrease in the extent of the myogenic response, we examined the relationship between age and %MT development at 100 and 120 mmHg of experimental perfusion pressure. These pressures were chosen for analysis because of the significant myogenic tone observed at these perfusion pressures for both male and female mice. A highly significant inverse correlation was observed between age and %MT at 100 mmHg in both male (r2 = 0.93, P = 0.0087) and female (r2 = 0.90, P = 0.013) mice (Fig. 5A). A similar significant inverse correlation was also observed at 120 mmHg of experimental perfusion pressure for both male (r2 = 0.92, P = 0.0096) and female (r2 = 0.88, P = 0.02) mice (data not shown). Because a significant correlation was observed between systolic BP and age (Fig. 1A), we wondered whether there was a relationship between systolic BP and the extent of myogenic tone development at 100 mmHg of experimental perfusion pressure. A significant inverse correlation was observed between in vivo systolic BP and %MT at 100 mmHg of experimental perfusion pressure in both male (r2 = 0.96, P = 0.0036) and female (r2 = 0.90, P = 0.014) mice (Fig. 5B). This inverse relationship suggests that higher systolic BPs are associated with a decrease in the extent of myogenic tone development at this perfusion pressure.


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  		Fig. 5.   Inverse correlation between the extent of myogenic tone development at 100 mmHg of perfusion pressure and age (A) and systolic BP (B) in male and female mice of 8, 12, 24, 36, and 52 wk of age. Data are expressed as means ± SE. Linear regression lines are shown for male and female mice. The equation of the line expressing percent myogenic tone (%MT) as a function of systolic BP is %MT= (119.2 - BP)/1.95 for male mice and %MT = (105.6 - systolic BP)/1.26 for female mice.

We then examined the effect of development, gender, and systolic BP on the strength of the myogenic response. Although the complete absence of a myogenic response in 52-wk-old animals precluded analysis of the active diameter-perfusion pressure relationship at this age, all other groups of both genders showed similar slopes of the active diameter-pressure relationship for perfusion pressures from the myogenic set point to 120 mmHg (data not shown). Neither development nor the in vivo BP exerted any effect on the strength of the myogenic response (Fig. 6, A and B).


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  		Fig. 6.   Absence of age- and BP-dependent differences in the strength of myogenic tone development. Slopes (linear regression lines) of the active diameter-perfusion pressure relationship were obtained from mice that develop myogenic tone (8-36 wk). A plot of these slopes vs. age (A) and systolic BP (B) show no relationship [P = not significant (NS)]. Data are expressed as the means ± SE.

To examine more specifically the role of elevated BP on the myogenic response, we employed a salt-induced model of hypertension. Systolic BP was significantly increased in male mice on a high-salt (8% NaCl) diet for 3 wk compared with age-matched control mice on a normal diet (128 ± 9 mmHg, n = 5 vs. 94 ± 3 mmHg, n = 5, P < 0.01). Passive diameters were not significantly different between male mice fed a high-salt diet compared with mice fed a normal salt diet (data not shown). Furthermore, salt-induced hypertension was associated with an increase in the set point of the myogenic response (from 80 to 100 mmHg) and a reduction in the extent of myogenic tone development at 80, 100, and 120 mmHg of perfusion pressure (Fig. 7A). The strength of the myogenic response in male mice fed a high-salt diet versus those on a normal diet was not different (high salt: -0.22 ± 0.06; normal: -0.26 ± 0.03, P = NS). In contrast to male mice, female mice fed a high-salt diet for 7 wk did not develop hypertension compared with age-matched female mice on a normal diet (high salt: systolic BP 99.8 ± 10.1 mmHg, n = 4 vs. normal: systolic BP 101.3 ± 8.7 mmHg, n = 5; P = NS). The set point, extent, and strength of myogenic tone development was not significantly different between female mice fed a high-salt vs. normal diet (Fig. 7B and data not shown).


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  		Fig. 7.   High-salt (8.0% NaCl) diet-induced hypertension of male mice (see text for details) was associated with a significant reduction in the extent (%MT) of the myogenic response (A). No significant differences in %MT were observed between female mice (B) on a high-salt vs. normal diet. *P < 0.05 vs. normal salt diet group by two-way ANOVA.

Phenylephrine responsiveness in mesenteric arteries. To examine whether the decline in myogenic tone with age was a generalized defect in smooth muscle contractility, mesenteric arteries were subjected to cumulative phenylephrine concentrations of 1 nmol/l to 30 µmol/l. In male mice of all age groups, progressive vasoconstriction with phenylephrine followed a very similar course, yielding nearly identical dose response curves (data not shown). In mice 8, 24, 36, and 52 wk old, the EC50 values for phenylephrine were 79 ± 17, 81 ± 22, 88 ± 15, and 111 ± 14 nmol/l, respectively, and were not significantly different at any age group. The EC50 values for female mice were not significantly different at any age or from age-matched male values (data not shown). Similarly, in salt-induced hypertensive male mice, the reduction in extent of %MT was not associated with altered phenylephrine-mediated contractions (high-salt diet: EC50 88 ± 11 nmol/l, n = 5 vs. normal diet: EC50 61 ± 7 nmol/l, n = 4, P = NS).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

In the present study, we studied the effects of development, gender, and BP on the set point, extent, and strength of the myogenic response in mesenteric arteries obtained from male and female C57BL/6 mice. We have demonstrated an age-dependent increase in the myogenic set point in mesenteric arteries. In both male and female mice, the minimum perfusion pressure at which significant myogenic vasoconstriction was observed increases with age (80 mmHg at 8 wk, 100 mmHg at 12 and 24 wk, and 120 mmHg at 36 wk). This increase in the myogenic set point with age (Figs. 3 and 4) also parallels a significant increase in systolic BP. Although this may in part reflect a maturation of the cardiovascular system, such that increases in BP during development are associated with elevations in the myogenic set point, it remains unclear as to whether this is mediated solely by increases in BP, an alternate age-dependent mechanism, or both.

To address this, we utilized an acquired (salt induced) form of hypertension. Although previous studies have demonstrated increased BP in SHR (33, 34) and C3HeB/FeJ and C57BL/6 mice (1, 46) after salt loading, others have shown a lack of salt-induced hypertension in normotensive rats (10, 37) and wild-type C57BL/6 mice (25, 29). In our study, 3 wk of a high-salt (8% NaCl) diet significantly increased systolic BP in male mice. As observed with developmental elevations in BP, salt-induced hypertension was associated with a significant increase in the myogenic set point and a reduction in the extent of the myogenic response (Fig. 7A). Previous studies (33) in mesenteric arteries of SHR also showed that after 8 wk of a high-salt diet, a significant increase in mean arterial pressure was associated with a near complete loss of myogenic tone. In contrast to the results we obtained in male mice, female mice did not develop elevated BP during an even longer duration (7 wk) of a high-salt diet, and maximal myogenic tone development was not significantly altered (Fig. 7B). The lack of salt-induced hypertension in female mice may in part be explained by increased nitric oxide production. Kiraku et al. (26) observed that feeding female mice a high-salt diet for 12 wk resulted in increased nitric oxide production and no change in systolic BP. Taken together, our examination of both developmental increases in BP and salt-induced hypertension strongly suggests that the basal in vivo BP may regulate myogenic responsiveness. Indeed, the antihypertensive effect of the female gender is also consistent with the premise that BP is the dominant factor regulating myogenic responses.

A recent study (40) examining the extent of myogenic tone development in 3- and 35-day-old mesenteric arteries from swine also observed an age-dependent decrease in this parameter. This age-dependent decrease in the extent of the myogenic response was associated with a modest increase (11 mmHg) in mean systemic arterial BP that was, however, not statistically significant in the small numbers of animals tested (n = 5) (40). Osol and Halpern (38) observed "weaker" (reduced strength) myogenic responses in cerebral arteries obtained from SHR compared with normotensive WKY rats. However, other reports have shown either no change or increased extent and/or strength of myogenic responses in SHR, depending on the age of the animals or arteries studied (7, 12, 18, 21, 44). Although a detailed comparison of these studies reveals some inconsistencies, together they appear to indicate that during the early stages of hypertension development (4-12 wk of age in SHR), myogenic responsiveness is enhanced in hypertensive animals but is "reset" to normal levels with established hypertension (~20 wk of age in SHR). If "resetting" of the myogenic response to elevations in BP is a physiological occurrence, then failure of this adaptation may lead to enhanced myogenic tone and further increases in BP. Therefore, genetic mouse models displaying enhanced myogenic tone could represent models of systemic hypertension. To this end, our study establishes a quantitative age- and gender-dependent relationship between BP and myogenic tone, to which mouse models of altered BP or vasomotor function can now be compared.

In our study, no myogenic tone was observed in mesenteric arteries from 52-wk-old mice at any perfusion pressure tested (20-120 mmHg). Aging is associated with functional and structural changes in both conduit and resistance arteries (11, 35) (reviewed in Ref. 32). If mouse mesenteric arteries become "stiffer" or less compliant during development, this may result in a failure to detect changes in intraluminal pressures. As such, the maximal perfusion pressure (120 mmHg) applied to mesenteric arteries from 52-wk-old mice may have been below the set point for the myogenic response. To address this, we performed a small series of experiments (n = 3) to assess the effect of higher perfusion pressures. It is important to note that higher perfusion pressures (140-200 mmHg) were not associated with any significant myogenic tone development in mesenteric arteries obtained from 52-wk-old mice (data not shown). These data suggest that 52-wk-old arteries may have a generalized defect in "sensing" intraluminal pressure in addition to other possible functional and structural changes that may occur with development.

To our knowledge, no studies have been performed that examined age-related changes in mouse mesenteric artery stiffness or compliance. In addition, no significant age-related aortic stiffening was observed when comparing 4- and 13-mo-old C57BL/6 mice (48). However, an age-related decreased in mesenteric artery compliance has been reported in rats when comparing 3- and 10-mo-old animals (52). Because we did not measure wall thickness in this study, we are unable to examine age-, gender-, or systolic BP-associated differences in wall thickness-to-lumen ratios as they may pertain to myogenic responses. However, other studies (20) have shown that increased wall thickness-to-lumen ratios of mesenteric arteries from SHR were not associated with altered myogenic tone development.

Because phenylephrine-mediated constriction did not change with age, the loss of myogenic responsiveness in older animals is not due to a generalized loss of smooth muscle contractility. The discrepancy between age-related effects on myogenic- versus phenylephrine-mediated vasoconstriction may in part be explained by the absence in electromechanical coupling (pressure-mediated vasoconstriction, e.g., myogenic tone) of factors known to enhance smooth muscle cell contractility in pharmacomechanical coupling (agonist-mediated vasoconstriction, i.e., phenylephrine) (45). Of note, phenylephrine-mediated vasoconstriction was examined only at a perfusion pressure of 60 mmHg (the highest pressure at which no myogenic tone was observed in any age group tested). Accordingly, an interaction between agonist-mediated constriction and %MT at higher perfusion pressures cannot be ruled out.

Gender- and/or hormonal-dependent differences in the myogenic response have been described in both mouse and rat blood vessels (13-15, 19, 49). The decrease in myogenic tone development in female mice is in part mediated by estrogen-induced nitric oxide release (14, 15, 19, 49). We did not observe significant gender-mediated differences in myogenic tone development in isolated mesenteric arteries. Although 8-wk-old female mice did demonstrate reduced %MT at 80 mmHg of perfusion pressure compared with age-matched male mice, this was not statistically significant. In addition, comparable %MT was observed in both male and female mice at 100 and 120 mmHg of perfusion pressure. The lack of significant gender-dependent differences in myogenic tone development in our study may depend on the vascular preparation examined. Although Pearce et al. (39) have shown that nitric oxide is important in the counteraction of myogenic tone in mouse mesenteric arteries, perhaps the contribution of estrogen-induced nitric oxide release is significantly less in the mouse mesenteric artery than in other vascular beds.

The C57BL/6 strain is a common genetic background widely employed as a wild-type control (16, 17, 24). We report here that in examining the myogenic response, age, BP, and, to a lesser extent, gender are important factors that need to be carefully evaluated. Such consideration will be of particular relevance to the study of vascular responses in transgenic and knockout models of hypertension.


    ACKNOWLEDGEMENTS

M. Husain is a clinician-scientist of the Canadian Institutes of Health Research (CIHR) and was a recipient of infrastructure awards from the Canada Foundation for Innovation and the Ontario Research and Development Challenge Fund. These studies were supported by CIHR Grants CL42617 and MT14648 and Heart & Stroke Foundation of Ontario (HSFO) Grants NA3636 and NA4389. R. Gros was supported by a HSFO Fellowship and was a recipient of the Edward Christie Stevens and the Evelyn McGloin Fellowship Awards. R. Van Wert was supported by a Summer Studentship from the Canadian Hypertension Society and the John D. Schultz Science Student Scholarship from the HSFO. X. You was supported by a Fellowship from the Canadian Hypertension Society/Medical Research Council of Canada.


    FOOTNOTES

Address for reprint requests and other correspondence: M. Husain, Toronto General Hospital, 200 Elizabeth St., EN12-221, Toronto, Ontario, Canada M5G 2C4 (E-mail: mansoor.husain{at}utoronto.ca).

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.

Received 11 May 2001; accepted in final form 11 September 2001.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

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Am J Physiol Heart Circ Physiol 282(1):H380-H388
0363-6135/02 $5.00 Copyright © 2002 the American Physiological Society


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