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J Thorac Cardiovasc Surg 1995;110:1073-1082
© 1995 Mosby, Inc.
CARDIOPULMONARY BYPASS, |
Boston, Mass.
Supported by National Heart, Lung, and Blood Institute grant HL 46716 and American Heart Association–Massachusetts Affiliate grant 13-501-912.
Received for publication Jan. 24, 1995. Accepted for publication April 12, 1995. Address for reprints: Frank W. Sellke, MD, Division of Cardiothoracic Surgery, Beth Israel Hospital, 330 Brookline Ave., Boston, MA 02215.
Abstract
The purpose of the present study was to examine the role of adenosine triphosphate-sensitive potassium channels in mediating the coronary hyperemic response after crystalloid cardioplegia. Thirteen pigs were placed on normothermic cardiopulmonary bypass support. Hearts were arrested with cold (4° C) crystalloid ([K+] 25 mmol/L) cardioplegic solution for 60 minutes. In seven of these pigs, hearts were then reperfused for 60 minutes with warm blood, and the animal was separated from cardiopulmonary bypass. The in vivo responses to the intracoronary administration of the K+adenosine triphosphate channel blocker glibenclamide (50 gm/kg per minute) or the K+adenosine triphosphate channel opener pinacidil (2 gm/kg per minute) were evaluated before cardiopulmonary bypass (baseline) and after 2 minutes and 60 minutes of reperfusion in the cardioplegia-reperfusion group. Under baseline conditions, glibenclamide and pinacidil induced a respective decrease and increase in coronary blood flow and an increase and a decrease in coronary vascular resistance. Coronary responses to glibenclamide and pinacidil were markedly enhanced after 2 minutes or 60 minutes of postcardioplegia reperfusion. In vitro responses of coronary arterioles (90 to 180µm) were examined in a pressurized, no-flow state with video microscopy. The contractile response of coronary arterioles to glibenclamide and the relaxation response to pinacidil were significantly enhanced 2 minutes or 60 minutes after reperfusion (all p<0.05 versus control). The response to pinacidil was markedly inhibited by glibenclamide, which confirms these antagonistic effects on K+adenosine triphosphate channels. Decreased tissue concentrations of adenosine triphosphate in the coronary arterial smooth muscle and myocardium were observed after cardioplegia and persisted for up to 60 minutes of reperfusion (both p<0.05 versus control). These results suggest that coronary hyperemia associated with postischemic cardioplegia is mediated in part by activation of K adenosine triphosphate channels in the coronary microcirculation. (J THORACCARDIOVASCSURG1995;110:1073-82)
A growing body of evidence suggests that adenosine triphosphate-sensitive potassium (K+ATP) channels contribute to the regulation of coronary vascular tone. The opening of K+ATP channels induces hyperpolarization and relaxes the vascular smooth muscle by a reduction in Ca++ influx. Daut and associates 
1 have demonstrated that blockade of K+ATP channels abolishes coronary vasodilation induced by hypoxia in the isolated heart of guinea pigs. The K+ATP channel blocker glibenclamide also has been shown to inhibit hyperemic response to brief coronary occlusion in the canine coronary circulation. 2 In addition, a number of investigations have showed that activation of K+ATP channels mediates dilation of cerebral arterioles 3 and renal afferent arterioles 4 during hypoxia. Thus K+ATP channels contribute importantly to the control of blood flow in hypoxia or in ischemic settings, or in both.
Reactive coronary hyperemia is a phenomenon frequently observed during the postcardioplegia period. The mechanism underlying this hyperemic response has not been defined, although it may involve the local release of metabolic vasodilators such as adenosine, prostaglandins, nitric oxide, and alterations in myogenic reactivity of coronary arterioles. 5-8 However, the role of K+ATP channels in mediating coronary hyperemic response after hyperkalemic crystalloid cardioplegia has not been characterized. Adenosine has been suggested to activate K+ATP channels 9 ; however, the roleof adenosine in the activation of K+ATP channels during coronary hyperemia associated with crystalloid cardioplegia is undefined. Consequently, the present study was designed to examine in vivo and in vitro coronary microvascular responses in a clinically applicable model of extracorporeal circulation and cold, hyperkalemic cardioplegia. We hypothesized that reactive coronary hyperemia associated with postischemic cardioplegia is mediated, at least in part, by activation of K+ATP channels in the coronary microcirculation.
METHODS
Animal preparation
Yorkshire pigs (18 to 22 kg) of either sex were premedicated with ketamine (10 mg/kg, intramuscularly) and anesthetized with 
-chloralose and urethane (respectively 60 mg/kg and 300 mg/kg intravenously initially and 15 mg/kg and 60 mg/kg every 60 minutes as needed). Pigs were tracheally intubated and the lungs mechanically ventilated. In the control group (n = 7), a sternotomy was performed and the animal was heparinized (500 units/kg). The heart was rapidly excised and immediately placed in cold (5° to 10° C) 3-[N-morpholino]propanesulfonic acid (MOPS) buffer solution of the following composition (in millimoles per liter): NaCL 145.0, KCL 4.7, CaCL2 2.0, MgSO4 1.2, glucose 5.0, pyruvate 2.0, ethylenediaminetetraacetic acid 0.02, NaH2PO4 1.2, and MOPS 3.0.
Cardioplegia group
In six pigs, after approximately 15 minutes of cardiopulmonary bypass (CPB), an aortic crossclamp was placed and 300 ml of cold (4°C) crystalloid, hyperkalemic ([K+] 25 mmol/L) cardioplegic solution was infused into the aortic root at a pressure of 60 mm Hg. The composition of the crystalloid cardioplegic solution was (in millimoles) NaCL 121, KCL 25, NaHCO312, and glucose 11.1; pH 7.6, oxygen tension 160 to 260 mm Hg. Saline slush was placed on the surface of the heart to provide topical hypothermia during the crossclamp period. At no time did the heart fibrillate during the ischemic cardioplegic period. Myocardial temperature of the anterior left ventricular wall was measured with a probe and ranged from 6° C to 14° C during the period of cardioplegia. Infusion of the cardioplegic solution (150 ml) was repeated at 20-minute intervals for 60 minutes (two additional doses) during which time CPB was continued. After 60 minutes of cardioplegic arrest, the heart was rapidly excised and immediately placed in cold (1° to 4° C) MOPS buffer solution.
Reperfusion group
In seven pigs, the same procedure was performed as in the cardioplegia group. However, after 60 minutes of cardioplegic arrest, the aortic crossclamp was removed and the heart was reperfused with normothermic blood from the bypass circuit. The heart was kept decompressed with a left ventricular vent placed through the left ventricular apex until a stable rhythm was obtained. In the case of ventricular fibrillation, the heart was defibrillated with 10 joules after the myocardial temperature rose to greater than 30° C. Pigs were weaned from CPB shortly after release of the aortic crossclamp when arterial pressure and cardiac rhythm became stable, and the pigs were then decannulated. During the reperfusion period, mean arterial pressure was maintained between 50 and 70 mm Hg. After 60 minutes of reperfusion, the heart was rapidly excised and immediately placed in cold MOPS buffer solution.
In vivo studies
In vivo responses to glibenclamide were studied in six pigs from the cardioplegia-reperfusion group. An 8F micromanometer-tipped catheter (Millar Instruments, Inc., Houston, Tex.) was inserted from the left ventricular apex into the left ventricular cavity. Left ventricular rate of pressure rise (dP/dt) was derived by differentiating left ventricular pressure signal over time. For intracoronary isoproterenol administration, a silicone rubber catheter (inner diameter 0.3 mm, outer diameter 0.5 mm) was introduced into the proximal left anterior descending (LAD) coronary artery according to a method similar to that described by Hard and Barger. 10 Coronary blood flow was measured with an ultrasonic flow probe (Transonic System Inc., Ithaca, N.Y.) that was placed around the LAD coronary artery distal to the point of the cannulation for intracoronary drug administration. A pair of ultrasonic crystals was implanted in the left ventricular subendocardium perfused by the LAD branches. The crystals were placed parallel to the short axis of the left ventricle. Percent systolic shortening was calculated as follows: (end-diastolic segment length – end-systolic segment length/end-diastolic segment length) x 100.
In vivo responses to the intracoronary administration of glibenclamide and pinacidil were studied before initiation of CPB and after 2 minutes and 60 minutes of postcardioplegia reperfusion. Intracoronary administration of pinacidil was given at 2 µg/kg per minute (volume 0.6 ml/min) with the use of a miniature pump (Living System Instrumentation, Burlington, Vt.). Changes in coronary blood flow in response to pinacidil administration usually returned to the baseline level within 5 minutes after termination of the drug administration. An additional 15 minutes was then allowed for stabilization and washout. Glibenclamide at 50 µg/kg per minute was then administered by the intracoronary route. All pressure and flow signals were recorded on an eight-channel recorder (Honeywell-Electronics for Medicine Research, Natick, Mass.).
In two pigs, vehicle without glibenclamide or pinacidil was administered at the same rate (0.6 ml/min) as that during drug administration. The vehicle caused a minimal increase (approximately 4%) in coronary blood flow and had no effects on cardiac functional parameters.
In vitro coronary microvessel studies
Subepicardial coronary arterioles were dissected from the left ventricular myocardium perfused by LAD branches with use of a x10 to x60 dissecting microscope (Olympus Optical Co., Ltd., Tokyo, Japan). Microvessels were placed in an isolated Plexiglas acrylic plastic organ chamber, cannulated with dual glass micropipettes measuring 30 to 80 µm in diameter and secured with 10-0 nylon monofilament suture (Ethicon, Inc., Somerville, N.J.). Both ends of micropipettes were connected to a pressure reservoir so that intraluminal pressure could be varied by adjusting the height of the reservoir. The pressure was measured by a burette manometer connected to the micropipettes. MOPS buffer solution (pH 7.4) equilibrated with room air was continuously circulated through the organ chamber and a reservoir (total volume 100 ml). The solution was warmed to 37° C by an external heat exchanger. With an inverted microscope (x40 to x200 magnification, Olympus Optical Co., Ltd., Tokyo, Japan) connected to a video camera, the vessel image was projected onto a black-and-white television monitor (Hitachi Denshi Ltd., Taiwan). An electronic dimension analyzer (Living System Instrumentation) was used to measure internal lumen diameter. Measurements were recorded with a strip chart recorder (Graphtec, Irvine, Calif.).
In all experimental groups, relaxation responses to pinacidil, adenosine, and sodium nitroprusside were examined after precontraction of microvessels. Acetylcholine was used to precontract the microvessels because it induces the contractile response in the porcine coronary microvessel without releasing endothelium-derived relaxation factor. 11 Once the steady-state tone was reached, responses to pinacidil (10-9 to 10-4 mol/L), adenosine (10-9 to 10-4 mol/L), or sodiumnitroprusside (10-9 to 10-4 mol/L) were examined. The contractile response to glibenclamide (10-5 mol/L) was determined in nonprecontracted vessels. In addition, in noninstrumented control animals, relaxation responses to pinacidil and adenosine were examined in vessels pretreated with glibenclamide (10-5 mol/L) at least 15 minutes before a dose-response intervention was done. One to three interventions were done on each vessel. The order of drug administration was random. The dose response to pinacidil or adenosine was examined only once in each vessel to avoid tachyphylaxis. All drugs were applied extraluminally. Measurements were taken 2 to 3 minutes after the drug was administered, when the response had stabilized. The vessels were washed three times with MOPS buffer solution and allowed to equilibrate in a drug-free MOPS buffer solution for 10 to 15 minutes between experimental interventions.
Biochemical assay
In control animals (n = 4), and in animals after 2 minutes (n = 4) or 60 minutes (n = 4) of postcardioplegia reperfusion, the hearts were removed en bloc and placed in cold physiologic saline solution (4° C). The myocardial specimen was obtained from the subepicardial myocardium of the left ventricle perfused by the LAD branches. The vascular specimen was obtained from the distal LAD coronary artery, which was dissected free and was cut into small segments (approximately 1 cm in length). The endothelium was deliberately removed by gently rubbing the luminal surface with the tip of the forceps. The specimens were immediately placed in liquid nitrogen and maintained at -80° C until assay. The specimens were weighed and pulverized in a pestle kept in liquid nitrogen. The fragments were quickly dropped into a test tube filled with chloroform (2 ml) and methanol (1 ml) solution and kept in an ice bath. 12 After addition of 1 ml mobile phase, the sample was homogenized at 2400 rpm for 1 minute and was then centrifuged at 3000 rpm for 5 minutes to separate the layers. The top aqueous layer was transferred to another test tube and the extraction was repeated using mobile phase (1 ml). The sample was then combined with the water layer and stored at -80° C until analysis. The high-performance liquid chromatography method was used to measure ATP according to the method similar to that described by Stocchi and colleagues. 13 The blank, standard, sample, and recovery spikes were all analyzed under the same conditions. The recovery of the nucleotide was between 88% and 96%. The analytic column was 25 cm in length (Bondapak, C-18, Water, Milford, Mass.). The mobile phase consisted of potassium dihydrogen phosphate (0.07 mol/L), tetrabutylammonium (0.004 mol/L), methanol (13%), and water (87%) (pH 5.65). The results were expressed as micrograms per gram wet weight of tissue.
Drugs
Glibenclamide, adenosine, sodium nitroprusside, acetylcholine, and sodium nitroprusside were obtained from Sigma Chemical Co. (St. Louis, Mo.). Pinacidil was obtained from Research Biochemical International (Natick, Mass.). Glibenclamide was dissolved in dimethyl sulfoxide (2%) and saline to which sodium bicarbonate was added to adjust the pH to 8.5. Pinacidil was dissolved in dimethyl sulfoxide (2%) and saline solution. Other drugs were dissolved in ultrapure distilled water. All solutions were prepared on the day of the study.
Data analysis
The response of microvessels to each agent was examined only once in each animal. Therefore each animal served as one sample. The data were pooled from each dose response in each experimental group and an average was calculated. Relaxation responses were expressed as percent relaxation of the acetylcholine-induced precontraction of the vessel diameter. Values were expressed as mean plus or minus the standard error of mean. The paired Student's t test was used to compare changes in hemodynamic variables in response to in vivo glibenclamide or pinacidil administration. The in vitro contractile responses to glibenclamide and ATP concentrations in all experimental groups were compared by two-way analysis of variance with factorial design and Scheffe's post hoc test for multiple comparisons. Comparisons of in vitro dose responses of all experimental groups were done with two-way analysis of variance with repeated-measure design and Scheffe's post hoc test. A p value of less than 0.05 was considered to be significant.
RESULTS
Vessel characteristics
At an intraluminal pressure of 50 mm Hg, coronary microvessels ranged 90 to 180 µm in internal diameter, averaging 143 ± 3 µm, 152 ± 10 µm, and 135 ± 5 µm in the control, cardioplegia, and cardioplegia-reperfusion groups, respectively. Percent precontraction after application of acetylcholine was 36% ± 2%, 37% ± 2%, and 30% ± 1% in the control, cardioplegia, and cardioplegia-reperfusion groups, respectively. Mean concentrations of acetylcholine required to obtain these percent contractions were 10-6 mol/L,10-6 mol/L, and 10-6 mol/L in the control, cardioplegia, and cardioplegia-reperfusion groups, respectively.
Tissue ATP measurements
In control animals, ATP concentrations in subepicardial coronary arteries and myocardial tissues were 87 ± 23 and 481 ± 95 µg per gram wet tissue, respectively. ATP concentrations in coronary arteries decreased to 25 ± 2 µg per gram wet tissue after cardioplegia and to 29 ± 4 µg per gram wet tissue after reperfusion (both p < 0.05 versus control). Myocardial concentrations of ATP were reduced to 184 ± 54 and 216 ± 10 µg per gram wet tissue after cardioplegia and reperfusion, respectively (both p < 0.05 versus control) (Table I).
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Tables II and III. Before CPB and cardioplegic arrest, glibenclamide decreased the coronary blood flow rate from 7 ± 1 to 6 ± 1 ml/min (p = 0.076) and increased coronary vascular resistance from 7 ± 1 to 13 ± 2 mm Hg/ml per minute (p = 0.268), whereas pinacidil increased the coronary blood flow rate from 9 ± 2 to 27 ± 5 ml/min (p = 0.005) and decreased coronary vascular resistance from 12 ± 1 to 3 ± 1 mm Hg/ml per minute (p = 0.001). Coronary blood flow increased by 192% of the precardioplegia level and coronary vascular resistance decreased to 4 ± 1 mm Hg/ml per minute (both p < 0.01 versus baseline) 2 minutes after release of the aortic crossclamp. At the same time, glibenclamide decreased the coronary blood flow rate from 20 ± 3 to 7 ± 2 ml/min (p = 0.006) and increased coronary vascular resistance from 4 ± 1 to 24 ± 13 mm Hg/ml per minute (p = 0.049) and pinacidil increased the coronary blood flow rate from 20 ±3 to 40 ±7 ml/min (p= 0.002) and decreased coronary vascular resistance from 4 ± 1 to 2 ± 1 mm Hg/ml per minute (p = 0.006). After 60 minutes of postcardioplegia reperfusion, while coronary blood flow was still 166% of the precardioplegia level (p < 0.05 versus baseline value), glibenclamide and pinacidil caused a respective decrease from 19 ± 4 to 5 ± 1 ml/min (p = 0.009) and an increase from 19 ± 4 to 47 ± 11 ml/min (p = 0.047) in coronary blood flow and an increase from 4 ± 1 to 19 ± 6 mm Hg/ml per minute (p = 0.042) and a decrease from 4 ± 1 to 2 ± 1 mm Hg/ml per minute (p = 0.006) in coronary vascular resistance.
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Tables II and III), except in two animals in which glibenclamide administration after 2 minutes of reperfusion decreased left ventricular systolic pressure, fractional shortening, and dP/dt. However, changes in these cardiac functional parameters were preceded by a significant reduction in coronary blood flow.
In vitro response to pinacidil and glibenclamide
Pinacidil induced a significant relaxation response in control vessels. The response to pinacidil was enhanced after 2 minutes or 60 minutes of reperfusion (both p <0.05 versus control, Fig. 3). In control vessels, glibenclamide induced a 3% ± 1% contraction. The contractile response to glibenclamide was significantly increased to 5% ± 1% in the cardioplegia group and to 6% ± 1% in the cardioplegia-reperfusion group (both p < 0.05 versus control), indicating an increased open-state probability of K+ATP channels.
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In vitro effects of glibenclamide on relaxation responses to pinacidil and adenosine
In control vessels, pretreatment with glibenclamide significantly attenuated the relaxation response to pinacidil (p < 0.01 versus vessels without glibenclamide, Fig. 3). In control vessels pretreated with glibenclamide (10-5 mol/L), the relaxation response to adenosine was diminished (p < 0.01 versus that in vessels without glibenclamide, Fig. 4) but to a lesser degree compared with the response to pinacidil. This indicates that activation of K+ATP channels was involved only partially in the adenosine-induced relaxation of coronary microvessels.
DISCUSSION
The present study provides evidence that reactive coronary hyperemia associated with postischemic cardioplegia is mediated, at least in part, by activation of K+ATP channels. Under baseline conditions, blockade of K+ATP channels with glibenclamide decreased coronary blood flow by only 9%. Immediately after removal of the aortic crossclamp and initiation of myocardial perfusion, there was a prompt and sustained (up to 60 minutes) increase in coronary blood flow. Subsequent administrations of glibenclamide resulted in respective 66% and 72% decreases in coronary blood flow 2 minutes and 60 minutes after reperfusion. These in vivo findings were consistent with in vitro observations that showed that the relaxation response to the K+ATP channel opener pinacidil and the contractile response to the K+ATP channel blocker glibenclamide were markedly enhanced after cardioplegia, which suggests an increased open-state probability of the K+ATP channel. Thus the present study demonstrates that activation of K+ATP channels is a mechanism underlying the coronary hyperemia. It was noteworthy that glibenclamide significantly attenuated but did not completely abolish coronary hyperemia and that pinacidil increased established coronary hyperemia. Thus mechanisms other than activation of K+ATP channels are also involved in postcardioplegic hyperemia. It is possible that the primary mediator of the hyperemic response may be activation of K+ATP channels because of reduced tissue concentration of ATP. However, impaired myogenic contraction 5,6,14 and the flow-mediated release of nitric oxide undoubtedly contribute significantly to an additional increase in coronary vasodilation. 7
Since the K+ATP channel was first described by Noma 15 in cardiac myocytes, this channel has beenidentified in the pancreatic ß cell, 16 central nervous system, 17 skeletal muscle, 18 and vascularsmooth muscle. 19 A previous study has shown that K+ATP channels mediate coronary hyperemia after brief coronary occlusion. 8 Furthermore, a substantial base of evidence has suggested that coronary and cerebral vasodilation elicited by hypoxia is significantly attenuated by glibenclamide. 1,3,20 In canine hearts, increases in coronary blood flow in response to a reduction in perfusion pressure or during exercise were prevented after blockade of K+ATP channels. 21-23 Similarly,activation of K+ATP channels on exposure to hypoxia was shown in the isolated canine diaphragm, 24 renal afferent arterioles of rats, 4 and the isolated ferret lung. 25 The opening of these channels is likely caused by reduction in the cytosolic ATP concentration because K+ATP channels are inactivated in the presence of physiologic concentrations of intracellular ATP. In the isolated heart of guinea pigs, Duat and associates 1 compared the effects of hypoxia with application of the mitochondrial uncoupler, 2,4-dinitrophenol, and observed similar coronary responses to both interventions. Electrophysiologic evidence obtained with the use of the patch-clamp technique has also demonstrated that these channels can be directly activated in response to a reduction in intracellular ATP. 26 Of related interest, hyperglycemia is reported to diminish the K+ATP channel-associated vasodilation in the pulmonary circulation. 25
Previous studies have suggested the correlation between activation of K+ATP channels and hypoxia or ischemia, or both; however, effects of ischemic crystalloid cardioplegia on K+ATP channels, especially in the coronary microcirculation, have not been elucidated. During cardioplegic arrest, the myocardium is deprived of its native blood flow and energy is derived primarily from the anaerobic metabolism. Despite a low basal oxygen consumption in vascular smooth muscle, its intracellular concentration of ATP was markedly decreased after brief coronary occlusion likely because of the lack of ATP and phosphocreatinine in this tissue. 27 Moreover, because intracellular ATP can be compartmentalized, a local decrease in the ATP production near the inner surface of the plasma membrane may also regulate these channels. 28 In the present study, decreases in ATP concentrations in the coronary artery wall and in the myocardial tissue were observed 2 minutes after reperfusion and persisted for up to 60 minutes of reperfusion. We obtained vessel specimens from the distal LAD coronary artery. It was unlikely, however, that the arteriolar smooth muscle metabolism would differ significantly from that of the large distal branches of the epicardial arteries. A previous finding showed a decreased ATP concentration in human myocardial tissues harvested before termination of crystalloid cardioplegia and 30 minutes after postcardioplegia reperfusion. 29 It has been suggested that metabolic changes in cardiac myocytes could alter the open-state probability of K+ATP channels in vascular smooth muscles because the oxygen concentration in the myocardium may function as a signal regulating the generation of ATP in vascular smooth muscle. 9 In addition, some metabolites released from ischemic tissues may contribute to the regulation of K+ATP channels. Indeed, a recent study has indicated a role of lactate in activation of K+ATP channels in cardiac myocytes of guinea pigs. 30
In the present study, glibenclamide at the dosage of 50 µg/kg per minute caused a 9% decrease in basal coronary blood flow, suggesting a role of K+ATP channels in the maintenance of basal coronary vascular tone. The coronary response to glibenclamide contrasts with that in reports that found 20% to 50% decreases in coronary blood flow in response to similar concentrations of glibenclamide in canine hearts. 23,31,32 Results of other studies were, however, consistent with our findings that K+ATP channels contributed minimally to basal vascular tone. 2,3,9 A previous in vitro study demonstrated that K+ATP channels are inactivated when the intracellular concentration of ATP is more than 1 mmol/L. Therefore, under physiologic conditions, most K+ATP channels appear to have already been inhibited, and glibenclamide would not be expected to reduce basal blood flow. The reason for the discrepancy in the glibenclamide-induced change in basal blood flow obtained from different studies remains unclear. It was possible that a higher level of intracellular ATP might be required in the in vivo setting to achieve an inhibitory effect on K+ATP channels compared with that under in vitro conditions. 31 Furthermore, factors (i.e., lactate) other than ATP may also contribute to the regulation of opening of K+ATP channels. 30 In addition, it is also plausible that the difference in the basal vascular tone may account for the different magnitude of changes in coronary blood flow in response to glibenclamide administration.
We observed that the relaxation response to adenosine was, somewhat unexpectedly, blunted immediately after cardioplegic arrest but was enhanced to higher than the control level after reperfusion. Adenosine is thought to be a mediator of coronary hyperemia because it is released during hypoxia or ischemia, or both. However, its significant role in the genesis of hyperemic response was challenged by a study of Saito and colleagues 33 that showed that adenosine accounted for no more than 30% of the coronary hyperemic response. Adenosine is known as a coronary vasodilator that may act through several different mechanisms. It was suggested that adenosine might induce an endothelium-dependent relaxation. 34 Because nitric oxide, an endothelium-derived relaxing factor, has been shown to regulate coronary blood flow during reactive hyperemia, 7 adenosine might induce coronary hyperemia by the release of the endothelium-derived relaxing factor. However, previous investigations have demonstrated that N G -monomethyl-L-arginine (a nitric oxide synthase inhibitor) does not significantly alter the relaxation response of coronary microvessels to adenosine. 35,36 In addition, our previous studies have shown a progressive deterioration of the endothelial function in the coronary microcirculation after postcardioplegia reperfusion, 37,38 whereas the relaxation response to adenosine was completely recovered after 60 minutes of reperfusion. Thus it was unlikely that adenosine induced coronary vasodilation primarily through the endothelium-dependent mechanism. Recently, adenosine has been shown to relax vascular smooth muscles by activating K+ATP channels. However, we and others 35,36 showed that the relaxation response to adenosine was only partially inhibited by glibenclamide. Similarly, in cerebral arterioles of rabbits the adenosine-induced vasodilation was only slightly affected by glibenclamide. 3 In the present study, although responses to glibenclamide and pinacidil were increased immediately after cardioplegia, the relaxation to adenosine was decreased. This suggests that factors other than K+ATP channels contribute to the vasodilatory effect of adenosine. It is important to note that adenosine may increase intracellular cyclic adenosine monophosphate concentration by stimulating adenylate cyclase in the coronary vascular smooth muscle. 39 We have previously demonstrated that the relaxation response of coronary microvessels to forskolin (an adenylate cyclase activator) is reduced 2 minutes after reperfusion but is increased above the control level 60 minutes after reperfusion. 6 Thus the down- and up-regulation of the adenylate cyclase activity induced by respective ischemia-cardioplegia and reperfusion was comparable with characteristic changes in the response to adenosine observed in the present study. This suggests that adenosine appears to relax porcine coronary microvessels primarily by activation of adenylate cyclase. Because the relaxation response to adenosine was increased 60 minutes after reperfusion, adenosine might play in part a role in the late phase of postcardioplegia hyperemia. This assumption appears to support the hypothesis that adenosine released from ischemic tissues may be involved in determining the duration of reactive hyperemic response. 40
A decreased cardiac systolic function was observed in two animals during intracoronary administration of glibenclamide after cardioplegic arrest. It was unlikely that alteration in cardiac systolic function was caused by a direct effect of glibenclamide because this negative inotropic effect was preceded by a significant reduction in coronary blood flow. This observation was in agreement with the finding of Duncker and associates, 23 which showed that the glibenclamide-induced myocardial dysfunction was restored when the rate of coronary blood flow was returned to the baseline level with sodium nitroprusside. Thus glibenclamide may elicit coronary vasoconstriction and lead secondarily to ischemic myocardial dysfunction. This glibenclamide-induced effect may be particularly important in the heart subjected to hypoxia or ischemia, or both, in that under such conditions coronary vessels may be hyperactive to K+ATP blockers because of an increased probability of opening of these channels.
In the present study, hyperkalemic ischemia-cardioplegia was associated with a 192% increase in coronary blood flow. Coronary reactive hyperemia can be elicited by a coronary occlusion as brief as 200 msec and the coronary blood flow may increase to 300% to 600% of the occlusion debt. 41 The role of coronary hyperemia in the preservation of cardiac function has been suggested in previous investigations. Although the possible explanation for this phenomenon remains unclear, it may include the improved removal of metabolites (e.g., H+ or lactate) as result of an increased coronary blood flow. Schwartz and colleagues 42 showed that limitation of peak reactive hyperemic coronary blood flow by epicardial coronary constriction delayed metabolic and functional recovery of the myocardium after brief myocardial ischemia. Furthermore, the correlations between the extent of hyperemia and recovery of left ventricular function and myocardial necrosis were postulated by Heyndrickx and associates 43 and Cobb and associates. 44 In addition, brief coronary occlusion has been occasionally done intraoperatively to assess coronary reserve capacity. The presence of the viable myocardium was indicated by a normal hyperemic response. Thus it appears that coronary hyperemia associated with activation of K+ATP channels may be of positive predictive value with respect to recovery of cardiac function after cardiac operation. However, excessive and persistent coronary hyperemia could result in detrimental effects (for example, tissue edema) on the heart. Moreover, a previous study reported an increased risk of cardiovascular mortality in diabetes mellitus by administration of K+ATP channel blockers. 45 Thus it remains to be determined whether in patients with diabetes the administration of antidiabetic sulfonylureas (e.g., glibenclamide) should be continued or discontinued before cardiac operations.
References
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  M. Ruel, T. A. Khan, P. Voisine, C. Bianchi, and F. W. Sellke Vasomotor dysfunction after cardiac surgery Eur J Cardiothorac Surg, November 1, 2004; 26(5): 1002 - 1014. [Abstract] [Full Text] [PDF]
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 M. Tofukuji, N. Matsuda, C. Dessy, K. G. Morgan, and F. W. Sellke INTRACELLULAR FREE CALCIUM ACCUMULATION IN FERRET VASCULAR SMOOTH MUSCLE DURING CRYSTALLOID AND BLOOD CARDIOPLEGIC INFUSIONS J. Thorac. Cardiovasc. Surg., July 1, 1999; 118(1): 163 - 172. [Abstract] [Full Text] [PDF]
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 J. S. Lawton, P.-W. Hsia, and R. J. Damiano Jr The adenosine-triphosphate-sensitive potassium-channel opener pinacidil is effective in blood cardioplegia Ann. Thorac. Surg., September 1, 1998; 66(3): 768 - 773. [Abstract] [Full Text] [PDF]
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A. Budrikis, R. Bolys, Q. Liao, R. Ingemansson, T. Sjoberg, and S. Steen Function of adult pig hearts after 2 and 12 hours of cold cardioplegic preservation Ann. Thorac. Surg., July 1, 1998; 66(1): 73 - 78. [Abstract] [Full Text] [PDF]
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M. Tofukuji, A. Stamler, J. Li, M. D. Hariawala, A. Franklin, and F. W. Sellke Comparative Effects of Continuous Warm Blood and Intermittent Cold Blood Cardioplegia on Coronary Reactivity Ann. Thorac. Surg., November 1, 1997; 64(5): 1360 - 1367. [Abstract] [Full Text]
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J. S. Lawton, J. D. Sepic, C. T. Allen, P.-W. Hsia, and R. J. Damiano Jr Myocardial Protection With Potassium-Channel Openers Is as Effective as St. Thomas' Solution in the Rabbit Heart Ann. Thorac. Surg., July 1, 1996; 62(1): 31 - 38. [Abstract] [Full Text]
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p < 0.05 versus baseline at PRE-BYPASS. Values shown as mean plus or minus standard error of mean.
