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J Appl Physiol 89: 529-536, 2000;
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Vol. 89, Issue 2, 529-536, August 2000

Role of KATP+ channels and adenosine in the control of coronary blood flow during exercise

Keith Neu Richmond, Johnathan D. Tune, Mark W. Gorman, and Eric O. Feigl

Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, Washington 98195-7290


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

The present study was designed to examine the role of ATP-sensitive potassium (KATP+) channels during exercise and to test the hypothesis that adenosine increases to compensate for the loss of KATP+ channel function and adenosine inhibition produced by glibenclamide. Graded treadmill exercise was used to increase myocardial O2 consumption in dogs before and during KATP+ channel blockade with glibenclamide (1 mg/kg iv), which also blocks adenosine mediated coronary vasodilation. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous values by using a previously tested mathematical model (Kroll K and Stepp DW. Am J Physiol Heart Circ Physiol 270: H1469-H1483, 1996). Coronary venous O2 tension was used as an index of the balance between O2 delivery and myocardial O2 consumption. During control exercise, myocardial O2 consumption increased ~4-fold, and coronary venous O2 tension fell from 19 to 14 Torr. After KATP+ channel blockade, coronary venous O2 tension was decreased below control vehicle values at rest and during exercise. However, during exercise with glibenclamide, the slope of the line of coronary venous O2 tension vs. myocardial O2 consumption was the same as during control exercise. Estimated interstitial adenosine concentration with glibenclamide was not different from control vehicle and was well below the level necessary to overcome the 10-fold shift in the adenosine dose-response curve due to glibenclamide. In conclusion, KATP+ channel blockade decreases the balance between resting coronary O2 delivery and myocardial O2 consumption, but KATP+ channels are not required for the increase in coronary blood flow during exercise. Furthermore, interstitial adenosine concentration does not increase to compensate for the loss of KATP+ channel function.

canine; glibenclamide; ATP-sensitive potassium channels


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

THE ROLE OF ATP-SENSITIVE potassium (KATP+) channels in coupling coronary blood flow to myocardial metabolic activity in coronary vascular smooth muscle has been examined by numerous investigators for various physiological and pathophysiological conditions. There is considerable evidence that KATP+ channels play a role in regulating basal coronary blood flow (9, 15, 29, 30), although this has not been observed in every case (17). KATP+ channels also play a role during hypoxic coronary vasodilation (6, 23) and coronary reactive hyperemia (1, 4, 9) but not during coronary autoregulation (31).

There are contradictory reports in the literature regarding the role of KATP+ channels in regulating coronary blood flow when myocardial metabolism is increased. Katsuda et al. (17) reported attenuated coronary vasodilation during pacing-induced tachycardia with glibenclamide, a KATP+ channel blocker, suggesting a role for these channels in mediating local metabolic coronary vasodilation. However, Richmond et al. (28) found the increase in flow was not altered during cardiac pacing, suggesting no requisite role for KATP+ channels in local metabolic vasodilation. In 1993, Duncker et al. (7) reported that flow was not attenuated by glibenclamide during exercise and concluded that KATP+ channels are not essential for coronary vasodilation during exercise. More recently, Duncker et al. (9) reported that both coronary blood flow and coronary venous oxygen tension decreased with glibenclamide and that coronary venous oxygen tension was further decreased with the addition of the adenosine-receptor antagonist 8-phenyltheophylline. The interpretation by Dunker et al. (9) was that adenosine increased when KATP+ channels were blocked, thus compensating for the loss of KATP+ channel function. However, glibenclamide has been shown to be an effective inhibitor of adenosine-induced coronary vasodilation (2-4, 6, 8, 9, 25, 27, 31). Therefore, adenosine would have to increase substantially (as much as 10-fold) to overcome the inhibition. The interpretation of these results is made difficult by the absence of plasma adenosine measurements during exercise with KATP+ channel blockade. In a recent study in which these measurements were made in closed-chest anesthetized dogs during cardiac paired pacing (28), this laboratory found that interstitial adenosine did not increase to overcome the inhibition by glibenclamide when oxygen consumption was increased ~90%. However, this does not exclude the possibility that adenosine might increase to overcome the inhibition by glibenclamide when myocardial oxygen consumption is further increased by exercise.

Because of these conflicting findings and interpretations and because of the lack of information on the change in adenosine concentration during exercise, the present study was designed to examine the role of KATP+ channels during exercise and to determine whether adenosine increases sufficiently to compensate for the loss of KATP+ channel function and adenosine inhibition produced by glibenclamide. Experiments were conducted at rest and during graded treadmill exercise in chronically instrumented dogs. Coronary sinus and arterial plasma adenosine were measured and interstitial adenosine levels were estimated with a distributed mathematical model.

Inhibition of KATP+ channels decreased the balance between resting coronary oxygen delivery and myocardial oxygen consumption but did not further compromise coronary blood flow during exercise. No evidence was found for increased adenosine levels compensating for the loss of KATP+ channel function during exercise.


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

Surgical preparation. Experiments were performed on 10 adult male mongrel dogs weighing 23-37 kg that were taught to run on a motorized treadmill. The surgical procedures performed in the present study were previously described by Tune et al. (33). Briefly, a splenectomy was performed through a midline abdominal incision to minimize changes in hematocrit during exercise. After this procedure, a left lateral thoractomy was performed in the fifth intercostal space. With use of a modified Seldinger technique, a polyurethane catheter was implanted into the descending thoracic aorta to measure aortic blood pressure and obtain arterial blood samples. A second polyurethane catheter was placed in the coronary sinus via a purse-string suture in the right atrial appendage for coronary venous blood sampling. The circumflex coronary artery was dissected free, and a flow transducer (see Pressure and flow measurement) was placed around the artery. No instruments were implanted in the myocardium and no surgical stitches were placed in the ventricles to avoid injury to the tissue, which would release adenosine. The animals were allowed at least 10 days for recovery before experiments were conducted.

Pressure and flow measurement. The coextruded polyurethane catheter was used in the aorta so that a high-fidelity Mikro-tip catheter pressure transducer (3F, SPR-524, Millar Instruments, Houston, TX) could be inserted at the time of the experiment to measure aortic blood pressure (10, 12). The pressure transducer was introduced into the aortic catheter through a hemostatic control valve (Tuohy-Borst adapter, Mallinckrodt Medical, St. Louis, MO), which allowed arterial blood samples to be withdrawn while a fluid-tight seal was maintained.

Coronary blood flow was continuously measured throughout the experimental protocol (see Experimental protocols) with an ultrasonic, perivascular flow transducer (Transonics, Ithaca NY). The flow transducers were calibrated before and after chronic implantation. The average difference between the before and after slopes for the flow calibrations was 6 ± 1% (SE; n = 7). After all experiments were completed, the animals were euthanized with pentobarbital sodium, and the circumflex perfusion territory was dyed with india ink. The weight of the dyed tissue was used to calculate coronary blood flow per gram of perfused myocardium.

Blood sampling. Arterial and coronary venous blood samples were collected simultaneously in heparinized glass syringes that were immediately sealed and placed on ice. The samples were analyzed for hydrogen ion concentration, carbon dioxide tension, and oxygen tension with an Instrumentation Laboratories 1306 pH/blood-gas analyzer (Waltham, MA). Oxygen content was determined by using the fuel-cell method (Total O2X, Hospex, Chestnut Hill, MA). In addition, a portion of the arterial and coronary venous blood samples were transferred into NaF-coated vials to prevent glycolysis and lactate concentration, which was determined with a model 1500 lactate analyzer from Yellow Springs Instruments (Yellow Springs, OH). Myocardial oxygen consumption (µl O2 · min-1 · g-1) was calculated by multiplying coronary blood flow per gram of perfused tissue by the arterial-coronary venous difference in oxygen content. Percent myocardial lactate extraction was calculated as the difference in arterial and coronary venous lactate concentration divided by the arterial lactate concentration.

Plasma adenosine measurement. Arterial and coronary venous adenosine measurements were made at rest and during steady-state conditions at each exercise level, and plasma adenosine concentration was measured as previously described by Tune et al. (33). Briefly, blood samples (3.7 ml) were collected and simultaneously mixed with an ice-cold enzymatic stop solution (5.0 ml) to prevent any further metabolism of adenosine (26). The stop solution contained dipyridamole (32 µM), iodotubercin (1 µM), and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA; 10 µM) dissolved in cold 0.9% saline. Dipyridamole was used to inhibit cellular adenosine uptake. Iodotubercin was used to inhibit adenosine kinase, preventing incorporation of adenosine into AMP. EHNA inhibits adenosine deaminase, preventing degradation of adenosine to inosine. Theophylline (20 µM) was included in the stop solution as an internal recovery standard. Blood samples were immediately centrifuged at 15,000 rpm and 0°C for 2 min. Then, 5 ml of the plasma-stop solution supernatant were rapidly added to 1.8 ml of 4 N perchloric acid to precipitate plasma proteins. The samples were then purified by applying the neutralized supernatant to C18 Sep-Pak cartridges. Each sample was divided into two 100-µl aliquots, and adenosine deaminase (0.1 unit, Boehringer) was added to one of the aliquots, which was used as a paired blank.

The adenosine in each sample was separated on a Hewlett-Packard 1100 HPLC with a C-18 column (5 µm, 220 × 2.1 mm, Perkin-Elmer, Norwalk, CT). The adenosine peak was identified by comparison with plasma samples spiked with adenosine and with adenosine standards and by spectral analysis. The paired chromatograms were superimposed by using Hewlett-Packard Chemstation software, and the blank was subtracted from the unknown. The chromatogram peaks were integrated, and adenosine content was determined by comparison with known adenosine standards. Plasma adenosine concentration was calculated by accounting for dilution steps in sample handling and for hematocrit and was normalized for recovery with the theophylline standard in each sample. The detection limit of the assay is 1.5 pmol of adenosine, which is equivalent to approximately a 5.5 nM concentration in plasma. The recovery of 100 pmol of adenosine added to an initial blood sample and carried through the entire assay was 86 ± 10% (SD; n = 20).

Estimation of cardiac interstitial adenosine concentration. Cardiac interstitial adenosine concentration was estimated by using a four-region (plasma, endothelial cell, interstitial space, parenchymal cell) axially distributed mathematical model (18, 19, 32). The model describes the effects of blood flow, adenosine transport, and exchange between tissue regions, as well as cellular production and consumption on the relationship between arterial, venous, and interstitial adenosine concentrations. This model has been used previously to estimate interstitial adenosine concentrations in vivo and the constraints and assumptions have been described extensively (19, 32). The model accounts for myocardial blood flow heterogeneity and the change in heterogeneity that occurs with changes in blood flow. In addition, the model is constrained by previous estimates of capillary adenosine transport and metabolism adjusted for the level of coronary blood flow. Interstitial adenosine concentration was estimated by using the measured values of coronary plasma flow and arterial plasma adenosine concentration by adjusting cellular adenosine production in the model to fit the measured coronary venous plasma adenosine concentration. Interstitial adenosine concentration was calculated at rest and during steady-state exercise conditions with and without glibenclamide.

Experimental protocol. The hypothesis that interstitial adenosine concentration increases to compensate for the loss of KATP+ channel and adenosine-mediated coronary vasodilation with glibenclamide was examined at rest and during graded treadmill exercise. Each animal served as its own control. The dose of glibenclamide (1 mg/kg iv) used in this investigation was previously found to effectively block the vasodilating action of the KATP+ channel opener cromakalim and to shift the coronary blood flow dose-response curve to intracoronary infusions of adenosine to the right 10-fold (31). Intravenous administration of glibenclamide was chosen in preference to intracoronary infusion for several reasons. Intravenous infusion avoids direct intracoronary injection of the harsh alkaline vehicle that is required to get glibenclamide into solution. A 10-min intravenous infusion provides time for equilibration of the blocking agent and results in a steady-state concentration. A continuous intracoronary infusion of glibenclamide is not suitable for steady-state measurements because recirculation of the blocking agent would result in ever-increasing coronary concentrations. The dose of glibenclamide used in the present study (1 mg/kg iv) has little systemic effect (see Table 1) and was chosen because a larger dose (3 mg/kg iv) results in coronary blood flow oscillations (22, 31).

                              
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  		Table 1.   Hemodynamic and metabolic variables at rest and during graded treadmill exercise

Coronary blood flow, aortic pressure, and heart rate were continuously measured while the dogs were resting in a sling and during three levels of treadmill exercise: 1) 3 mile/h (mph), 5% grade; 2) 4 mph, 10% grade; 3) 5 mph, 15% grade. Exercise levels were chosen to obtain a graded increase in myocardial oxygen consumption. On the basis of preliminary experiments, the highest level of exercise increased myocardial oxygen consumption approximately three to four times of resting levels. The highest level was probably submaximal exercise for these animals, but it is difficult to determine maximal exercise for a dog. Arterial and coronary venous blood samples were collected when hemodynamic variables stabilized at each level. Each exercise period was ~2 min in duration, and the animals were allowed to rest between each level until hemodynamic variables returned to baseline. Control vehicle and glibenclamide experiments were conducted on separate days, and the animals were allowed at least 2 days of recovery between experiments. Eight of the ten chronic animals in the present study were also used in another investigation (33).

Drugs. Glibenclamide (1 mg/kg; Sigma Chemical, St. Louis, MO) was placed in 1.5 ml equal parts of 1 N NaOH, ethanol, and propylene glycol and was gently warmed until dissolved. Final volume was adjusted to 30 ml with warm 5.0% glucose. Glibenclamide was infused intravenously over a 10-min period. The adenosine stop solution was made in isotonic saline and included 1 µM iodotubercin (RBI, Natick, MA), 10 µM EHNA (RBI), 32 µM dipyridamole (Sigma Chemical), and 20 µM theophylline (Sigma Chemical).

Statistical analyses. Hemodynamic variables were recorded with Windaq data analysis software (Dataq Instruments, Akron, OH). Analog signals from the recording instruments were digitized and stored on disk at a rate of 200 samples/s. The values for mean coronary blood flow, mean aortic pressure, and heart rate at rest and during exercise were averaged over a 30-s period.

For the four key postulated response variables (coronary venous oxygen tension, coronary blood flow, coronary venous and interstitial adenosine concentrations), statistical testing was directed to overall treatment effects (control vehicle vs. glibenclamide). These tests were chosen for their specificity to the hypotheses and to avoid inappropriate multiple comparisons. All analyses accounted for the effects of drug and dog. Multiple linear regression was used to compare slopes of the two treatments for the response variables vs. myocardial oxygen consumption relationship [SAS; generalized linear model procedure (proc glm)]. Analysis of covariance was employed to adjust the response variables for linear dependence on myocardial oxygen consumption after testing for parallel regression lines (SAS, proc glm). Table 1 presents means by exercise level and does not include P values because the P values pertain to the overall treatment effects as shown in the figures. Data are presented as means ± SE unless otherwise noted.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Hemodynamic and metabolic data for the 10 dogs are given in Table 1. Point-by-point statistical comparisons are not given in the table to avoid excessive inappropriate multiple comparisons. The overall effects are given in the figures (see Statistical analyses). The hemodynamic response to treadmill exercise for one dog during control vehicle and with glibenclamide is shown in Fig. 1. Heart rate (A) and mean aortic pressure (B) are plotted vs. myocardial oxygen consumption in Fig. 2. Heart rate increased ~2.5-fold both during control and after glibenclamide, whereas mean aortic pressure was unchanged with exercise both during control and with glibenclamide. Coronary venous oxygen tension is plotted vs. myocardial oxygen consumption in Fig. 3. Coronary venous oxygen tension during control vehicle at rest was 19 ± 1 Torr, fell to 14 ± 1 Torr at exercise level 1, and was little changed at the second and third level of exercise. Coronary venous oxygen tension after glibenclamide was 13 ± 1 Torr at rest, and it decreased to 11 ± 1 Torr for all three exercise levels. Mean coronary venous oxygen tension after glibenclamide was significantly lower than that for control vehicle after linear adjustment for myocardial oxygen consumption (P < 0.00l; Fig. 3). However, the slope of the coronary venous oxygen tension vs. myocardial oxygen consumption relationship was not significantly different from control vehicle after glibenclamide (P = 0.56), demonstrating KATP+ channels are not required for exercise-induced coronary vasodilation.


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  		Fig. 1.   Recording of coronary blood flow, mean coronary blood flow, aortic pressure, and heart rate from 1 dog at rest and during treadmill exercise, before and during ATP-sensitive potassium channel blockade with glibenclamide.



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  		Fig. 2.   Relationship between myocardial oxygen consumption and heart rate (A) and mean aortic pressure (B) during control vehicle and after ATP-sensitive potassium channel blockade with glibenclamide. n, No. of dogs. Heart rate increased ~2.5-fold both during control and after glibenclamide. Mean aortic pressure was unchanged with exercise both during control and with glibenclamide.



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  		Fig. 3.   Relationship between coronary venous oxygen tension and myocardial oxygen consumption during control vehicle and with glibenclamide. n, No. of dogs. Slope of coronary venous oxygen tension vs. myocardial oxygen consumption was unchanged by ATP-sensitive potassium channel blockade (multiple linear regression, P = 0.56), indicating that ATP-sensitive potassium channels are not involved in exercise coronary hyperemia. Mean coronary venous oxygen tension was significantly reduced with glibenclamide (analysis of covariance, P < 0.001).

Coronary blood flow vs. myocardial oxygen consumption is shown in Fig. 4. During administration of control vehicle, coronary blood flow increased ~3.5-fold from rest to exercise level 3. After glibenclamide administration, the increase in coronary blood flow from rest to exercise level 3 was also ~3.5-fold. Myocardial oxygen consumption during control vehicle increased 4.3-fold from rest to exercise level 3, and after glibenclamide it increased 3.3-fold. The slope of the coronary blood flow vs. myocardial oxygen consumption relationship was not significantly different from control vehicle after glibenclamide (P = 0.64).


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  		Fig. 4.   Relationship between coronary blood flow and myocardial oxygen consumption during control vehicle and with glibenclamide. n, No. of dogs. Slope of coronary blood flow vs. myocardial oxygen consumption was not significantly changed with glibenclamide (multiple linear regression, P = 0.64).

Arterial and coronary venous plasma adenosine concentrations are plotted vs. myocardial oxygen consumption in Fig. 5. Neither arterial (P = 0.75) nor coronary venous plasma (P = 0.10) adenosine concentrations were significantly different from control vehicle after glibenclamide.


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  		Fig. 5.   Relationship between arterial (A) and coronary venous (B) plasma adenosine concentration and myocardial oxygen consumption. n, No. of dogs. Slope of adenosine concentration vs. myocardial oxygen consumption was unchanged with glibenclamide for both arterial (multiple linear regression, P = 0.88) and coronary venous (multiple linear regression, P = 0.58) adenosine concentration. Mean adenosine concentration was not different after glibenclamide (analysis of covariance: arterial, P = 0.75; venous, P = 0.10), and slopes were not different from zero (multiple linear regression, arterial, P = 0.20; coronary venous, P = 0.06).

Estimated interstitial adenosine concentration (Fig. 6) remained well below the threshold necessary for coronary vasodilation (~117 nM during control and ~1,170 nM after glibenclamide) (32). The slopes of the relationship between estimated interstitial adenosine concentration and myocardial oxygen consumption during control vehicle and KATP+ channel blockade with glibenclamide did not differ significantly (P = 0.60), and there was no change in interstitial adenosine concentration from control vehicle with glibenclamide (P = 0.10). These results demonstrate that adenosine did not increase to compensate for the loss of KATP+ channel function after glibenclamide blockade.


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  		Fig. 6.   Relationship between estimated interstitial adenosine concentration and myocardial oxygen consumption during control vehicle and with glibenclamide. n, No. of dogs. Slope of relationship was not changed with glibenclamide (multiple linear regression, P = 0.60), and there was no change in the average interstitial adenosine concentration (analysis of covariance, P = 0.10). Estimated common slope was different from zero (multiple linear regression, P = 0.04). Estimated interstitial adenosine concentration remained below the threshold necessary for coronary vasodilation (~117 nM during control and ~1,170 nM after glibenclamide) (32).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

The present study is the first to combine KATP+ channel blockade with adenosine measurements during exercise. Blockade of KATP+ channels with glibenclamide reduced resting coronary blood flow in 7 of 10 experiments and significantly decreased coronary venous oxygen tension, demonstrating a decrease in the ratio of oxygen supply to myocardial oxygen consumption. However, during exercise, the increase in coronary blood flow and the slope of the coronary venous oxygen tension vs. myocardial oxygen consumption relationship were unchanged by KATP+ channel blockade. Therefore, KATP+ channels are not required for the control of coronary blood flow during exercise. In addition, neither coronary venous nor estimated myocardial interstitial adenosine concentration increased to compensate for the loss of KATP+ channel function or to overcome the blockade of adenosine coronary vasodilation by glibenclamide.

Role of KATP+ channels in control of resting coronary blood flow. The majority of studies that examined the relationship between coronary blood flow and KATP+ channel blockade with glibenclamide found that basal flow was reduced by 12-25% (5, 9, 15, 21, 28, 29, 31). However, Katsuda et al. (17) observed no change in baseline coronary blood flow with glibenclamide in conscious dogs. In the present study, there was a small nonsignificant increase in resting blood pressure and myocardial oxygen consumption and a 12% decrease in resting coronary blood flow after glibenclamide (Table 1). The small increase in myocardial oxygen consumption combined with the decrease in coronary blood flow produced a significant decrease in coronary venous oxygen tension (Fig. 3, Table 1), demonstrating a fall in the balance between oxygen delivery and consumption. Despite the fall in coronary venous oxygen tension with KATP+ channel blockade, coronary venous plasma adenosine concentration (Fig. 5B), estimated interstitial adenosine concentration (Fig. 6), and lactate extraction (Table 1) were unchanged during resting conditions.

Role of KATP+ channels in coronary blood flow regulation during exercise. Although KATP+ channels appear to play a role in the regulation of resting coronary blood flow, they are not required for the increase in flow that accompanies an increase in myocardial metabolism, on the basis of the results presented here. The increase in coronary blood flow during control exercise and exercise with glibenclamide was the same (3.5-fold; Table 1, Fig. 4). Similar findings have been reported in dogs by Duncker et al. (7-9), who found that glibenclamide decreased resting coronary blood flow but did not attenuate the increase in flow associated with exercise.

Role of adenosine during exercise with glibenclamide. It is well established that glibenclamide inhibits adenosine coronary vasodilation (2-4, 6, 9, 25, 27, 31). Glibenclamide decreased resting coronary venous oxygen tension from 19 to 13 Torr, indicating that the myocardium was close to underperfusion. Exercise further decreased coronary venous oxygen tension to 11 Torr at all levels of exercise (Fig. 3, Table 1), whereas coronary venous adenosine concentration was unchanged (Fig. 5B). Estimated interstitial adenosine concentration after glibenclamide was not different from control vehicle and remained well below the vasoactive level. This is in contrast to the hypothesis that adenosine increases to overcome the adenosine blockade, as has been previously suggested (8, 9, 16, 20).

The methods used in the present investigation are capable of detecting elevations in coronary venous adenosine concentration as occurs during hypoxia (13), coronary autoregulation (31), intracoronary norepinephrine infusion (34), and the release of endogenous adenosine by the inhibition of adenosine kinase and adenosine deaminase (32). Therefore, it is very likely that the present methods are adequate for detecting significant changes in coronary venous plasma adenosine concentration when oxygen consumption is increased during exercise. Furthermore, the studies cited above demonstrate that increases in myocardial interstitial adenosine concentration are reflected in coronary venous adenosine concentration despite avid uptake of adenosine by coronary vascular endothelium (19, 24).

Duncker et al. (9) observed that glibenclamide lowered coronary venous oxygen tension and that subsequent adenosine-receptor blockade with 8-phenyltheophylline further lowered coronary venous oxygen tension. In addition, coronary blood flow and myocardial oxygen consumption were both significantly reduced with combined glibenclamide and 8-phenyltheophylline treatment. They interpreted their results to indicate that an increase in adenosine concentration compensated for the loss of KATP+ channel function. In the present study, if adenosine were compensating for the loss of KATP+ channel function, the increase in coronary blood flow should be matched by an increase in interstitial adenosine. Although a small (nonsignificant) increase occurs at the first level of exercise, estimated interstitial adenosine is not further changed at the higher exercise levels even though coronary blood flow continues to rise. The results from the present study do not confirm the interpretation of Duncker et al. because adenosine concentration did not increase to vasoactive levels after glibenclamide. Because glibenclamide shifts the adenosine dose-response curve to the right 10-fold (31), the estimated interstitial adenosine concentration would have had to increase to ~1,170 nM to initiate coronary vasodilation.

Cardiac interstitial adenosine concentration was estimated by using a distributed model that accounts for flow heterogeneity in the myocardium. The model has been extensively tested by using enzyme and transport blockade (19). The most sensitive parameter determining the difference measured between venous plasma and estimated interstitial adenosine concentrations is the paracellular permeability of adenosine (PSg in the model). The model accounts for the changes in PSg with changes in measured blood flow, but what is the likelihood that PSg is sensitive to other variables that might change during exercise? Infusion of norepinephrine in buffer-perfused guinea pig hearts does not change the value of PSg (11). Adenosine infusion also does not change the value of PSg (J. B. Bassingthwaighte, personal communication). There is no a priori reason to postulate a non-flow-related change in PSg. However, an arbitrary and highly unlikely twofold decrease in PSg was modeled and the calculated interstitial concentration increased by ~20% (nonlinear function) in the present experiments. An ~20% increase in interstitial adenosine concentration does not change any of the conclusions drawn from the data in the present experiments (Fig. 6).

In summary, inhibition of KATP+ channels decreases the balance between resting coronary oxygen delivery and myocardial oxygen consumption, but KATP+ channels are not required for the increase in coronary blood flow when myocardial oxygen consumption increases during exercise. Coronary venous adenosine concentration did not increase with exercise, and the estimated interstitial adenosine concentration after glibenclamide was not different from control vehicle. Therefore, KATP+ channels are not necessary for regulation of coronary blood flow during exercise, and adenosine levels do not increase to compensate for the loss of KATP+ channel function.


    ACKNOWLEDGEMENTS

We thank Pamela Campbell for expert technical and editorial assistance in all phases of this research. We also thank Julie Kleeberger for expert surgical assistance.


    FOOTNOTES

This work was supported by National Institutes of Health Grants HL-49822, HL-49170, HL-07403, and RR-01243.

Address for reprint requests and other correspondence: E. O. Feigl, Dept. of Physiology and Biophysics, University of Washington School of Medicine, Box 357290, Seattle, WA 98195-7290 (E-mail: efeigl{at}u.washington.edu).

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

Received 8 February 2000; accepted in final form 23 March 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

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