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Am J Physiol Heart Circ Physiol 277: H1931-H1939, 1999;
0363-6135/99 $5.00
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Vol. 277, Issue 5, H1931-H1939, November 1999

Role of NO and K+ATP channels in adenosine-induced vasodilation on in vivo canine subendocardial arterioles

Toyotaka Yada, Osamu Hiramatsu, Hiroyuki Tachibana, Eiji Toyota, and Fumihiko Kajiya

Department of Medical Engineering and Systems Cardiology, Kawasaki Medical School, 577 Matsushima, Kurashiki, Okayama, 701-0192, Japan


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Adenosine (Ado) plays an important role in regulation of coronary vascular tone with nitric oxide (NO) and ATP-sensitive K+ (K+ATP) channels. In vitro, it was reported that subendocardial (Endo) arterioles are more sensitive to Ado than subepicardial (Epi) arterioles. The purpose of this study was to observe enhanced vasodilation of Endo arterioles directly and to evaluate possible roles of K+ATP channels and NO in the different responses of Endo and Epi arterioles to Ado-induced vasodilation. We evaluated dilation of Endo and Epi arterioles (<120 µm) of beating canine hearts (n = 19) by Ado (20 and 50 µg · kg-1 · min-1 ic) before and after K+ATP channel blockade (glibenclamide; 200 µg/kg ic), inhibition of NO synthase [NG-nitro-L-arginine methyl ester (L-NAME); 30 µg · kg-1 · min-1, 20 min ic], or glibenclamide + L-NAME using a novel needle-probe CCD intravital microscope. Ado induced dose-dependent vasodilation in both Epi and Endo arterioles, but vasodilation was greater in Endo arterioles, i.e., increase at 120 s (maximum dilation) after Ado (50 µg · kg-1 · min-1) was 17% in Endo and 13% in Epi arterioles (P < 0.01). Endo arteriole dilation was attenuated by blockade of K+ATP channels from 18% (Ado) to 9% (Ado+glibenclamide) increase (P < 0.001) and by inhibition of NO synthase from 17% (Ado) to 9% (Ado+L-NAME) (P < 0.005). Epi arteriole vasodilation was attenuated by blockade of K+ATP channels from 15 to 9% (P < 0.005) and inhibition of NO from 16 to 10% (P < 0.005). Suppression of vascular response was additive (Endo, 14 to -1%; Epi, 12 to 3%) with glibenclamide + L-NAME. We conclude that 1) the degree of Ado-induced vasodilation was greater in Endo than in Epi arterioles, with higher sensitivity of smaller arterioles in both layers and 2) transmural difference of arteriolar sensitivity to adenosine was abolished or reversed by K+ATP channel blockade and/or by NO synthase inhibition, indicating crucial involvement of K+ATP and NO in transmural sensitivity difference.

coronary circulation; adenosine 5'-trisphosphate-sensitive potassium ion channel


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

ADENOSINE IS AN IMPORTANT regulatory factor of myocardial flow. In vivo canine studies indicate that adenosine preferentially dilates coronary microvessels <150 µm and that the degree of dilation increases with decreasing vessel size (6, 18). In vivo study with radioactive microspheres has shown that adenosine redistributes myocardial blood flow to the endocardium (32), suggesting that the endocardial vessels may be more sensitive to adenosine. Quillen and Harrison (30) demonstrated that in vitro endocardial microvessels are also more sensitive to adenosine than epicardial microvessels. Extravascular compression (41) and the local metabolic environment (40) differ substantially between subendocardial (Endo) and subepicardial (Epi) vessels. However, there has been no direct observation on Endo arterioles of a beating canine heart during adenosine-induced vasodilation. Recently, it has been possible to evaluate Endo vessels using our novel charge-coupled device (CCD) intravital microscope (12, 41).

The vascular responses to adenosine have been shown to be attenuated after inhibition of endothelial nitric oxide (NO) synthesis (39) or after endothelium is removed (1, 33). ATP-sensitive potassium (K+ATP) channel blockade also reduced the coronary vasoactivity of adenosine (7), indicating the involvement of activation of K+ATP channels in vasodilation in addition to activation of vascular smooth muscle adenylyl cyclase. In the present study, we evaluate whether the endothelial releases of NO and activating K+ATP channels play an important role in adenosine-induced vasodilation, especially in the higher sensitivity of Endo arterioles to adenosine. To address these questions, arteriolar images after adenosine administration in both Endo and Epi were assessed in vivo before and after inhibition of NO by L-NAME or blockade of K+ATP channels using our needle-probe microscope.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Animal preparation. These experiments were approved by the Animal Research Committee of Kawasaki Medical School and conducted according to the "Guide for the Care and Use of Laboratory Animals" of Kawasaki Medical School. Nineteen adult mongrel dogs (15-25 kg) of either sex were anesthetized with ketamine (10 mg/kg im) and pentobarbital sodium (25 mg/kg iv). Anesthesia was maintained with 50- to 100-mg supplements of pentobarbital as needed. After intubation, each animal was artificially ventilated with room air supplemented by 100% oxygen at a rate sufficient to maintain arterial PO2 and PCO2 in the physiological range (pH 7.35-7.45, PCO2 25-40 mmHg, PO2 >70 mmHg). A high-frequency jet ventilator (model VS600, IDC, Pittsburgh, PA) was used to minimize the effect of lung motion on the visualization of arteriolar image. Each animal was placed on a heating blanket to maintain body temperature at 37°C. The right carotid artery and the right jugular vein were catheterized for hemodynamic and arterial blood gas measurements and for fluid administration. Heparin sodium (750 U/kg iv) was given as an anticoagulant. Aortic pressures (AoP) and left ventricular pressures (LVP) were measured with an 8-Fr pigtail double-manometer catheter (Millar model SPC-784A, Millar) inserted via the right carotid artery. After a median sternotomy and a left thoracotomy through the fifth intercostal space, the heart was exposed and suspended in a pericardial cradle. The proximal portion of the left anterior descending coronary artery (LAD) was isolated near its origin, and a transonic flow probe (T206, Transonic Systems) was placed around the vessel. A 24-gauge catheter was introduced into a diagonal branch of the LAD for intracoronary injection of indocyanine green and other drugs.

An electrocardiogram (ECG) was recorded by standard leads. Heart rate was kept constant at 100 beats/min during the experiment by right ventricular pacing after atrioventricular node activity was suppressed with injection of 37% formaldehyde into the region of the atrioventricular node.

Needle-probe intravital microscope with CCD camera. Details of the needle-probe CCD intravital microscope were described previously (12, 41). Briefly, this system (VMS 1210, Nihon Kohden, Tokyo, Japan) consists of a needle probe, a camera body containing a CCD camera, a lens system with light guides, a control unit, a light source, a monitor, and a videocassette recorder (VCR, EVO-9850, Sony, Tokyo, Japan). The camera body has a 1/2-in. CCD image sensor, ~250,000 pixels, and 330-line horizontal resolution. The needle probe (diameter 4.5 mm, length 180 mm) contains a gradient index (GRIN, 2 pitch) lens surrounded by 18 annularly arranged light guide fibers. The image passes through the GRIN lens and is focused on the CCD image sensor. Images on the CCD are monitored and recorded on a videotape every 33 ms. The tissue is illuminated through the light guide fibers by a halogen lamp (150 W). A green filter is used to accentuate the contrast between the vessels and surrounding tissue. The spatial resolution is ~5 µm (motion resolution = 2.5 µm) for ×200 (41). The maximum depth of field is about 250 µm.

The needle probe is enclosed in a Silastic 14-Fr double-lumen sheath. A doughnut-shaped balloon on the tip of the sheath avoided direct compression of the vessels by the needle tip. To obtain a clear image of vessels, blood between the tip of the needle probe and endocardial surface inside the doughnut was flushed away with warm (37°C) Krebs-Henseleit buffer solution injected through a microtube of the sheath.

Measurements of arteriolar diameters. To visualize Endo arterioles, the sheathed needle probe was introduced into the left ventricle through an incision in the left atrial appendage via the mitral valve. After the surgical procedure and instrumentation, at least 30 min were allowed for stabilization of the monitored variables. The doughnut-shaped balloon was inflated and then gently placed on the endocardial surface of the septum between the anterior and posterior papillary muscles. The position of the needle probe was moved slowly and carefully to search for arterioles while the intervening blood was flushed away with buffer solution. Once a clear arteriolar image was obtained, the operator kept the probe position on the vessel manually. The vascular image was then stored on the VCR. Arterioles were differentiated from venules by an injection of indocyanine green (5-10 mg/ml) from a diagonal branch of the LAD. Vascular images with indocyanine green were analyzed without a green filter. After dye injection, the image of arterioles appeared before that of venules. Vascular imaging by indocyanine green was also used to identify the arteriole as a tributary of the LAD.

At the end of the experiment, the sequential images were transferred to a Power Macintosh 8500/120 computer (Apple Computer, Cupertino, CA) for diameter measurements using appropriate software (Image 1.49, National Institutes of Health Research Services Branch, Bethesda, MD). The end-diastolic diameters of arterioles were defined as the diameters in the image appearing just before the onset of the R wave of the ECG. The vascular images at end diastole were analyzed with a freeze-frame modality with a time resolution of ~30 pictures/s (41). The diameters of five scan lines neighboring each other were averaged, and then the vascular diameters at end diastole for five consecutive heartbeats were ensemble averaged. We recorded AoP, LVP, and ECG simultaneously with the arteriolar images.

We also measured the diameter change in Epi arterioles with the same instrument (41). The camera body with the needle probe was steadied by hand. After inflation of the doughnut-shaped balloon, we placed the needle probe gently on the Epi arteriole. Krebs-Henseleit buffer solution was dripped continuously on the epicardial surface. Measurements of Endo and Epi arterioles were performed separately because it was practically difficult to measure them simultaneously.

Experimental protocol. After the surgical procedure and instrumentation, at least 30 min were allowed for stabilization of the monitored variables. Ibuprofen (12.5 mg/kg iv) was injected ~20-30 min before the beginning of the protocol to inhibit the formation of cyclooxygenase products that might be released by complement activation induced by blood-tubing interactions. The vascular responses of Endo (or Epi) arterioles after administration of adenosine (20 µg · kg-1 · min-1 ic, SUN Y4001, Suntory, Tokyo, Japan) were analyzed for 3 min by measuring end-diastolic vascular diameters with the LAD flows. After returning to the conditions before adenosine administration, we repeated the experiment by increasing the adenosine dose (50 µg · kg-1 · min-1 ic). We also examined the reproducibility of the adenosine-induced vasodilation in both Endo (3 vessels from 3 dogs) and Epi (3 vessels from 3 dogs) arterioles in two consecutive administrations of adenosine (50 µg · kg-1 · min-1 ic) with a 30-min interval.

To evaluate the role of the K+ATP channel in adenosine-induced vasodilation, glibenclamide (200 µg · kg-1 · min-1 ic) was slowly infused into the diagonal branch of the LAD using a syringe pump (STC 525, Terumo, Tokyo, Japan). Four minutes after the start of glibenclamide infusion, the vascular responses to adenosine (50 µg · kg-1 · min-1) in Endo (or Epi) arterioles were observed for three minutes with the LAD flow responses. To examine the role of NO in adenosine-induced vasodilation, NO synthesis was inhibited using L-NAME solution that was continuously infused into the diagonal branch of the LAD at a rate of 0.5 ml/min (30 µg · kg-1 · min-1 ic) for 20 min using the syringe pump. Vascular responses to adenosine (50 µg · kg-1 · min-1) in Endo (or Epi) arterioles were evaluated for 3 min immediately after L-NAME infusion was stopped. The combined effects of glibenclamide and L-NAME on adenosine-induced vascular response were examined. Glibenclamide was infused into the diagonal branch of the LAD (16 min after initiation of L-NAME infusion). Vascular responses to adenosine (50 µg · kg-1 · min-1) in Endo (or Epi) arterioles were evaluated for 3 min immediately after glibenclamide and L-NAME infusion was stopped.

To evaluate the inhibiting efficacy of glibenclamide against a K+ATP channel opener (nicorandil, 0.1 µmol/min for 3 min ic) or L-NAME against an endothelium-dependent vasodilator (acetylcholine, 1.0 µg · kg-1 · min-1 for 3 min ic) in Endo (5 vessels from 2 dogs) and Epi (5 vessels from 2 dogs) arterioles, the vasodilatory responses before and after each inhibitor were compared.

Although the control experiments before each pharmacological intervention were performed in the same dogs, the experiments with different interventions usually were performed in different animals because of the difficulty of holding the probe near the same place for a long time. After completion of an experiment, the anesthetized animal was killed by a lethal intravenous dose of KCl.

Statistical analysis. Data are reported as means ± SE. The differences in time-dependent changes between two groups and the differences in percent diameter changes from control diameters between two groups were assessed by a multiple-regression analysis. The relationship between two variables was also expressed by a single regression line. Student's t-test was used for both paired and unpaired comparisons. The criterion for statistical significance was P < 0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Baseline characteristics. Table 1 lists the data of systemic and coronary hemodynamics. LVP and mean blood pressure were not significantly different between control conditions and during adenosine administration and between before and after inhibitor(s). After intracoronary administration of 20 µg · kg-1 · min-1 adenosine, the LAD flow during the evaluation of Endo and Epi arterioles increased 2.4- and 1.8-fold at 15 s and 4.6- and 3.5-fold at 120 s, respectively (Fig. 1, A and B). The LAD flow increment caused by a dose of 50 µg · kg-1 · min-1 during the observation of Endo and Epi arterioles showed almost the same pattern, i.e., 3.0- and 2.9-fold increase at 15 s and 5.6- and 5.7-fold increase at 120 s, respectively. These flow responses exhibited steady peak flow after 90 s from the beginning of the adenosine administration. The time-sequential diameter change in adenosine-induced vasodilation of Endo or Epi arterioles was not significantly different between the consecutive first and second experiments (data not shown), indicating the reproducibility of adenosine effects.

                              
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  		Table 1.   Systemic hemodynamic values during control conditions and after adenosine administration with or without glibenclamide and/or L-NAME



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  		Fig. 1.   Time course of coronary blood flow (A and B) and arteriolar diameter responses in subendocardium (Endo, C) and subepicardium (Epi, D) after adenosine administration of 2 different doses. A and B, coronary flows during the evaluation of Endo (C) and Epi (D) arterioles, respectively. Coronary flows and arteriolar diameters increased rapidly and then gradually before reaching steady peak values after 90 s from beginning of adenosine administration. See Table 1 for resting Endo and Epi arteriolar diameters. C, Control.

Vascular sensitivity to adenosine. Figure 2 shows representative images of Endo arterioles before and at 120 s after adenosine administration. Endo arterioles dilated remarkably after adenosine administration. Figure 1, C and D, shows the time course of arteriolar diameter responses in both Endo and Epi after administrations of two different doses. The arteriolar diameters in both layers rapidly increased within 15 s, and they continued to increase slowly and attained their maximum values after ~90-120 s. The pattern of time courses was apparently similar in both Endo and Epi, but the magnitude of the responses was greater in Endo than in Epi arterioles in both the early responses at 15 s and in peak response at 120 s.


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  		Fig. 2.   Images of Endo arterioles before (A) and at 120 s after (B) adenosine administration. Note that Endo arterioles dilated remarkably after adenosine administration (~10% increase).

Figure 3 shows the percent diameter increases in Endo and Epi arterioles at 120 s during adenosine administration of the higher dose (50 µg · kg-1 · min-1) plotted against the control diameters, i.e., those before adenosine administration. The magnitude of vasodilation in Endo arterioles was greater than in Epi arterioles irrespective of the control diameter values [average vasodilation of Endo, 17% (18 vessels from 10 dogs); Epi, 13% (22 vessels from 10 dogs); P < 0.01], but the magnitude of the vasodilatory responses was higher for smaller vessels in both Endo and Epi (P < 0.05).


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  		Fig. 3.   Percent diameter increases (%CD) in Endo and Epi arterioles (EndoA and EpiA, respectively) at 120 s during administration of higher adenosine dose (50 µg · kg-1min-1) plotted against control diameters (CD). Magnitude of vasodilation in EndoA was greater than in EpiA, irrespective of CD, and %CD was higher for smaller CD. Two lines indicate single regression in EndoA and EpiA.

Effects of K+ATP channels and NO on transmural responses to adenosine. Figure 4 shows the efficacy of K+ATP channel blockade and NO synthase inhibition during the evaluation of Endo and Epi arterioles. There was significant attenuation of arteriolar dilation by glibenclamide against nicorandil-induced vasodilation and also by L-NAME against acetylcholine-induced vasodilation in both Endo and Epi arterioles with no transmural difference in their inhibitory effects. With glibenclamide, L-NAME, or combined treatments, there was ~5% reduction in basal coronary arteriolar diameter, whereas these did not achieve statistical significance in both Endo and Epi arterioles (Table 1).


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  		Fig. 4.   Efficacy of each inhibitor compared between EndoA and EpiA. Vascular responses of EndoA and EpiA during administration of K+ATP channel opener (nicorandil, A) and endothelium-dependent vasodilator (acetylcholine, B) were significantly attenuated after glibenclamide (G) and NG-nitro-L-arginine methyl ester (L-NAME, L), respectively.

Figure 5 shows the comparison of the Endo vascular responses at 120 s after adenosine administration before and after K+ATP channel blockade or NO synthase inhibitor plotted against control diameters, i.e., those before adenosine or adenosine plus K+ATP channel blockade (or plus NO synthase inhibitor). The arteriolar dilation to adenosine was attenuated by both glibenclamide (P < 0.001) and L-NAME (P < 0.005). However, it is noteworthy that glibenclamide suppressed relatively smaller arteriolar responses, whereas L-NAME suppressed the relatively larger arteriolar vasodilation. Suppression of vascular response was additive by combined administration of glibenclamide and L-NAME, resulting in small dilation or even slight constriction. Figure 6 shows the effects of glibenclamide or L-NAME on the vascular responses of Epi arterioles at 120 s after adenosine administration. Arteriolar dilation to adenosine was attenuated by both glibenclamide and L-NAME. The degree of the vasodilatory attenuation was smaller in Epi than in Endo. Similar to findings in Endo, glibenclamide suppressed relatively smaller, and L-NAME relatively larger, Epi arteriolar vasodilation. Suppression of vascular response was additive by combined administration of glibenclamide and L-NAME, almost abolishing the vasodilation by adenosine.


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  		Fig. 5.   Comparison of Endo vascular responses at 120 s after adenosine administration between before and after ATP-sensitive K+ (K+ATP) channel blockade and/or NO synthase inhibition plotted against control diameters (CD). Arteriolar dilation to adenosine (%CD) was attenuated by both glibenclamide (G, A) and L-NAME (L, B) (by 50% on average for adenosine with glibenclamide and by 47% on average for adenosine with L-NAME; vs. adenosine only). Suppression of vascular response was additive (rather reversed) by combined administration of glibenclamide and L-NAME (C). Regression lines are as described in Fig. 3.



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  		Fig. 6.   Effects of glibenclamide (G) and/or L-NAME (L) on vascular responses of EpiA at 120 s after adenosine administration plotted against control diameters (CD). Arteriolar dilation to adenosine (%CD) was attenuated by both glibenclamide (A) and L-NAME (B). Suppression of vascular response was additive by combined administration of glibenclamide and L-NAME (C). Regression lines are as described in Fig. 3.

Figure 7 shows the comparison of vasodilation after glibenclamide or L-NAME in Endo and Epi. Unlike the results of adenosine administration shown in Fig. 4, the difference of vascular responses between Epi and Endo was not observed after glibenclamide or L-NAME, although the size-dependent attenuation seems to be slightly different between two treatments. Furthermore, the attenuation of vascular response by combined administration of glibenclamide and L-NAME was greater in Endo than in Epi (P < 0.05).


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  		Fig. 7.   Comparison of vasodilation after glibenclamide (G) and/or L-NAME (L) between EndoA and EpiA plotted against control diameters. Difference of vascular responses between EndoA and EpiA disappeared after glibenclamide (A) or L-NAME (B). However, there was greater attenuation by combined administration of glibenclamide and L-NAME (C) in EndoA compared with EpiA (P < 0.05).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The major findings of the present study were that 1) the sensitivity of Endo arterioles to adenosine is greater than that of Epi arterioles, with higher sensitivities of smaller arterioles in both layers, 2) the transmural difference of the adenosine-induced vasodilation disappeared after either K+ATP channel blockade or NO synthase inhibition, and 3) the effect of combined treatments was additive, with greater suppression of Endo arterioles. The interpretations of our results critically depend on several factors: 1) critique of experimental methodology, 2) sensitivity of Endo and Epi arterioles to adenosine, 3) interaction of adenosine with NO and K+ATP channels, and 4) pathophysiological implications.

Critique of experimental methodology. A previous study in our laboratory has shown that the resolution of the imaging system is ~5 µm for an absolute static measurement (~2.5 µm for a measure of moving object) This is a small fraction of most of the differences seen in our study. Because holding the needle probe on a specific location on the endocardium for a long time was technically difficult, the observation of arteriolar responses during adenosine administration under control conditions and that after K+ATP channel blockade or NO synthase inhibition was usually performed on different arterioles in different dogs. We attempted to match the arteriolar size for the comparison as much as possible, although it was not easy to find the appropriate size of arterioles in Endo. We used a dose of 200 µg/kg glibenclamide or 30 µg/kg L-NAME (20 min) by intracoronary infusion, following the reports of Kanatsuka et al. (19) and Jones et al. (17). Infusion of these drugs had no significant effects on the hemodynamics under control conditions (Table 1). With glibenclamide, L-NAME, or combined treatments, there was ~5% reduction in basal coronary arteriolar diameter; however, this did not achieve statistical significance. It was reported that venous administration of glibenclamide (14) and L-NAME (37) affected the basal coronary tone significantly, whereas direct coronary administration [both in vivo (17, 19) and in vitro (15)] did not. The difference may be caused by the degree of involvement of systemic hemodynamic changes.

We confirmed that the LAD flow responses to acetylcholine were significantly attenuated after L-NAME administration. This substance potently inhibits endothelium-dependent dilation in response to agonists in the coronary circulation of different species (5, 13, 16, 23, 38). The flow-suppression effect of L-NAME was quickly reversed by the administration of L-arginine. The specificity of the K+ATP channel blocker was also validated, because no influence of glibenclamide on the vasodilation by nitroglycerin was found (data not shown). Furthermore, there was no significant difference between Endo and Epi arterioles in the efficacy of glibenclamide and L-NAME (Fig. 4).

Sensitivity of Endo and Epi arterioles to adenosine. In isolated coronary arteries, adenosine has a more potent effect in relaxing smaller arteries, e.g., Schnaar and Sparks (35) and Harder et al. (10) showed higher sensitivity of smaller arteries (<0.5 mm) to adenosine. Several studies have examined in vivo dilation of arterial microvessels in Epi to adenosine or its potentiator and arrived at the result that smaller arteries relaxed more potently to adenosine. We observed directly the response of Endo arterioles to adenosine for the first time and found that their size-dependent responses were similar to those of Epi arterioles. Interestingly, the magnitude of vasodilation was greater in Endo than in Epi irrespective of arteriolar size (see Fig. 3). Earlier in vivo studies with radioactive microspheres have shown a greater increase in endocardial versus epicardial blood flow to submaximal infusions of adenosine (32). Quillen and Harrison (30) found that endocardial arterioles in vitro are also more sensitive than epicardial arterioles to the effects of adenosine, although their vascular diameters were larger than the most effective size for adenosine (smaller arterioles). These studies are consistent with the present direct in vivo observation, although the earlier studies did not demostrate the size-dependent dilatory response to adenosine.

A possible explanation for the difference between epicardium and endocardium in arteriolar response to adenosine may be the difference in the behaviors of NO synthesis and/or K+ATP channels between Endo and Epi (Figs. 5-7). Although the basic mechanism of the difference is unclear, the magnitude of oscillatory shear stress and/or hoop stress is higher in Endo. In our earlier study, we observed that adenosine enhanced coronary flow oscillation (oscillatory stress) in the deeper myocardium by measuring intramyocardial coronary flow (21). This may be caused mainly by external forces imposed on vessels with increased vascular volume. Furthermore, we showed that the pulsation amplitude during reactive hyperemic response increased much more in Endo than in Epi arterioles (42), indicating an increase in vascular pulsation (oscillatory hoop stress) in the Endo when blood volume was increased by vasodilation. Oscillation in shear stress enhances endothelium-derived NO release in vitro (27), and pulsatile perfusion increases NO-mediated coronary flow mediated in the canine heart (31). Thus the higher oscillatory mechanical stress in deeper layers by adenosine may induce more potent involvement of these vasodilatory mechanisms. The disappearance of the transmural difference in vascular response to adenosine after NO synthase inhibition and/or K+ATP channel blockade may indicate the crucial involvement of these factors in the sensitivity difference to adenosine between the two myocardial layers.

Interaction of adenosine with NO and K+ATP channel. Although the role of NO in the relaxation produced by adenosine is still controversial (22), most reports including the present study support the idea that the vascular response to adenosine is mediated to a greater or lesser degree by NO. The vasodilatory response of coronary resistance vessels to adenosine in Langendorff guinea pig heart was significantly reduced by L-NAME (39). In conscious dogs, intracoronary infusion of NG-nitro-L-arginine reduced the flow response to adenosine by ~50% (29) and, similarly, L-NAME (50 µg/kg) blocked nearly one-half of the flow increase by the dilation of coronary resistance vessels by adenosine (28). Thus adenosine could mediate the endothelial release of NO in coronary resistance vessels.

Adenosine-induced coronary vasodilation is partly inhibited by glibenclamide (1, 2, 7), which is consistent with our present study in both Endo and Epi arterioles. Depolarization of the endothelium with a high luminal K+ concentration completely blocked the endothelium component of vasodilation to adenosine (24). In addition, the increase in coronary flow caused by a low dose of adenosine was inhibited by glibenclamide and also by an L-arginine analog (26). It is believed that one of the important pathways of NO synthesis is electrochemical gradient-dependent Ca2+ influx into endothelial cells secondary to membrane hyperpolarization (8). Membrane hyperpolarization by adenosine may significantly contribute to the increase in cellular free Ca2+ and thus to the stimulation of NO synthesis, because adenosine has been shown to produce a sustained hyperpolarization of cultured coronary endothelial cells (36). All of these results may support the role of endothelial K+ATP channels (hyperpolarization of cell membrane) in the pathway of NO release by adenosine. Furthermore, after combined administration of glibenclamide and L-NAME, the dilation of arterioles and the degree of the vascular diameter increment were suppressed in the present study. These findings suggest that adenosine-induced vasodilation is modulated additively by the combined effects of K+ATP channels and NO. However, because mean coronary flow during combined administration was still two- or threefold higher than that before adenosine, smaller arterioles (<40 µm, resolution limitation of our microscope) with higher sensitivity to adenosine may continue to dilate considerably.

In the isolated canine epicardial artery, adenosine-induced vasodilation has been reported to be partially endothelium dependent (34). In conscious dogs, the increase in epicardial coronary arterial diameter by adenosine was completely prevented by L-NAME (28). This flow-dependent, indirect adenosine-mediated NO release may affect mainly the later phase of vasodilator response. In fact, we observed that L-NAME greatly attenuated the later-phase vasodilation in both Endo and Epi (Table 1), although this cannot exactly dissociate the direct and indirect roles of NO in adenosine-induced response.

Adenosine may also cause release of a vasodilatory prostanoid such as prostacyclin (20). However, because we used ibuprofen, an inhibitor of prostanoids, the release of a dilatory prostanoid such as prostacyclin does not seem to be involved in the endothelium-dependent relaxation in our study.

Pathophysiological implications. Under physiological conditions, the high sensitivity of Endo arterioles to adenosine via activation of K+ATP channels and NO release may optimize the flow to match the greater myocardial metabolic demands in the deeper myocardium. Hypoxia significantly potentiates responses to adenosine in some studies (9, 25). Tissue PO2 may be an important factor affecting vascular smooth muscle relaxation in response to endogenous adenosine, because endogenous adenosine is released during conditions of decreased tissue PO2 in vivo (3). This may resist ischemia to some extent, especially in Endo, which is more susceptible to ischemia. Functional impairment of K+ATP channels and/or NO synthesis, however, has been reported under pathophysiological conditions such as hypertension, atherosclerosis, and diabetes (4). Although the vasodilatory response to adenosine became similar between Endo and Epi arterioles after individual deletion of K+ATP channels and NO function, the degree of attenuation was larger in Endo than Epi arterioles. Furthermore, the combined administration of glibenclamide and L-NAME induced greater attenuation in the Endo vascular response, probably by reducing both relatively smaller (mainly by glibenclamide) and larger (mainly by L-NAME) arteriolar responses additively. Thus the impairment of K+ATP channels and/or NO function may lead to the higher susceptibility of Endo with greater mechanical stress and oxygen demands. We expect that our results will create the foundation for future studies on the interaction among the release of endogenous adenosine, NO synthesis, and action of K+ATP channel in several pathophysiological conditions.

In conclusion, 1) the degree of adenosine-induced vasodilation in Endo arterioles was greater than that in Epi arterioles, with higher sensitivity of smaller arterioles in both layers, and 2) the transmural difference of the arteriolar sensitivity to adenosine was abolished by K+ATP channel blockade and/or by NO synthase inhibition, indicating the crucial involvement of K+ATP and NO in the transmural sensitivity difference.


    ACKNOWLEDGEMENTS

The authors thank Kiyoshi Ichihara for statistical advice and Chikako Tokuda for expert technical assistance.


    FOOTNOTES

This study was supported by Grant-in-Aid 05454278 for General Scientific Research (B), Grant-in-Aid 05557043 for Developmental Scientific Research (B) from the Ministry of Education, Science, Sports, and Culture, Japan, and Research Project Grant 4-107 from Kawasaki Medical School.

This study was presented in part at the 69th Scientific Sessions of the American Heart Association, November 1996, New Orleans, LA.

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. §1734 solely to indicate this fact.

Address for reprint requests: F. Kajiya, Dept. of Medical Engineering and Systems Cardiology, Kawasaki Medical School, 577 Matsushima, Kurashiki, Okayama, 701-0192, Japan (E-mail:kajiya{at}med.kawasaki-m.ac.jp).

Address for correspondence: T. Yada, Dept. of Medical Engineering and Systems Cardiology, Kawasaki Medical School, 577 Matsushima, Kurashiki, Okayama, 701-0192, Japan (E-mail: yada{at}med.kawasaki-m.ac.jp).

Received 26 October 1998; accepted in final form 23 June 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Am J Physiol Heart Circ Physiol 277(5):H1931-H1939
0002-9513/99 $5.00 Copyright © 1999 the American Physiological Society


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