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(Circulation. 1997;95:1568-1576.)
© 1997 American Heart Association, Inc.

Articles

ß₂-Adrenergic Dilation of Resistance Coronary Vessels Involves K_ATP Channels and Nitric Oxide in Conscious Dogs

Zhi Ming, MD, PhD; Robert Parent, PhD; Michel Lavallée, PhD

From the Department of Physiology, Faculty of Medicine, Université de Montréal and Institut de Cardiologie de Montréal (Québec), Canada.

Correspondence to Michel Lavallée, Institut de Cardiologie de Montréal, 5000, Bélanger St E, Montréal, Québec, Canada H1T 1C8.

	Abstract

Top Abstract Introduction Methods Results Discussion References

 
Background We considered that ß₂-adrenergic stimulation may dilate resistance coronary vessels by opening ATP-sensitive potassium (K_ATP) channels, thereby triggering NO formation.

Methods and Results In conscious instrumented dogs after ß₁-adrenergic blockade, intracoronary (IC) injections of acetylcholine (ACh), nitroglycerin (NTG), and pirbuterol (PIR), a selective ß₂-adrenergic agonist, were performed before and after blockade of NO formation with IC N-nitro-L-arginine methyl ester (L-NAME, 50 µg·kg^-1·min^-1x12 minutes) or blockade of K_ATP channels with IC glibenclamide (25 µg·kg^-1·min^-1x12 minutes followed by 2 µg·kg^-1·min^-1). PIR (50.0 ng/kg) increased coronary blood flow (CBF) by 32±6 from 43±7 mL/min and by only 11±2 (P<.01) from 40±7 mL/min after L-NAME. Increases in CBF to ACh were also reduced by L-NAME, but NTG responses were not. Before glibenclamide, PIR increased CBF by 33±5 from 45±7 mL/min and by only 14±3 (P<.01) from 36±5 mL/min thereafter. CBF responses to ACh and NTG were maintained after glibenclamide. Lemakalim, a selective opener of K_ATP channels, caused dose-dependent increases in CBF that were partially inhibited by L-NAME. In experiments in which CBF was controlled, the fall in distal coronary pressure caused by PIR was less after L-NAME or glibenclamide than before.

Conclusions ß₂-Adrenergic dilation of resistance coronary vessels involves both the opening of K_ATP channels and NO formation. L-NAME antagonized lemakalim responses consistent with a link between the opening of K_ATP channels and NO formation in canine resistance coronary vessels.

Key Words: endothelium • endothelium-derived factors • microcirculation • receptors, adrenergic, beta

	Introduction

Top Abstract Introduction Methods Results Discussion References

 
There is growing evidence that vasodilators acting through cAMP-dependent mechanisms involve hyperpolarization of vascular smooth muscle triggered by the opening of K_ATP channels. In this connection, blockade of K_ATP channels has been reported to blunt CBF increases to ß₁-adrenergic stimulation in canine resistance vessels,¹ to block ß₂-adrenergic dilation of isolated canine saphenous veins,² and to antagonize ß₁- and ß₂-adrenergic dilation of the rat isolated superior mesenteric arterial bed.³ K_ATP channels also intervene in coronary vasodilator responses to other cAMP-dependent agents, such as prostacyclin⁴ and adenosine.^{5 6} Endothelial cell hyperpolarization may also contribute to vascular relaxation. Because endothelial cells lack voltage-gated Ca²⁺ channels, the membrane potential is primarily involved in controlling Ca²⁺ movements across the cellular membrane by adjusting the electrochemical gradient driving Ca²⁺ entry.^{7 8 9 10} By increasing intracellular Ca²⁺ levels, hyperpolarization could therefore influence the activity of the constitutive NO synthase,^{11 12} a calcium/calmodulin–dependent enzyme.¹³ Several studies have reported the existence of K_ATP channels on endothelial cells.^{7 8 9 10} In fact, openers of the K_ATP channels, such as cromakalim and pinacidil, hyperpolarize endothelial cells and elevate intracellular calcium levels, which promotes NO formation in vitro.^{7 8} This may explain why in the pulmonary circulation in vivo, blockade of NO formation reduced pinacidil-induced vasodilation.¹⁴ The possibility that hyperpolarization of endothelial cells secondary to the opening of K_ATP channels triggers endothelial NO formation in resistance coronary vessels has not been studied in vivo.

The fact that cAMP-dependent vasodilation involves K_ATP channels led us to consider that opening of these channels may intervene in the pathway triggering NO formation during ß₂-adrenergic stimulation of resistance coronary vessels.¹⁵ Therefore, the effects of selective blockade of NO formation or K_ATP channels on CBF responses to ß₂-adrenergic stimulation were studied in conscious dogs. The consequences of blockade of NO formation for coronary responses elicited by opening of K_ATP channels with lemakalim were also investigated.

	Methods

Top Abstract Introduction Methods Results Discussion References

 
Instrumentation
Under general anesthesia with sodium pentobarbital (30 mg/kg IV) and under sterile conditions, seven mongrel dogs (weight, 32±1 kg) underwent a left thoracotomy at the fifth intercostal space under artificial ventilation. A Tygon catheter (Norton Plastics and Synthetic Division) was implanted in the thoracic aorta. This catheter was connected to an external transducer (model 800, Bentley Trantec) to measure arterial pressure. A solid-state pressure transducer (model P6.5, Konigsberg Instruments) was inserted into the LV cavity to record LVP and to obtain LV dP/dt. A catheter was also implanted in the LV cavity to cross-calibrate the miniature pressure gauge. A cardiotachometer (model 9857, SensorMedics) triggered by the LV pressure pulse was used to monitor HR. An ultrasonic Doppler blood flow transducer was placed around the left circumflex coronary artery 20 to 30 mm from the bifurcation of the left main coronary artery. CBF was monitored with a 10-MHz pulsed Doppler flowmeter.¹⁶ At necropsy, the internal circumference of the vessel under the probe was measured to obtain the vessel cross-sectional area and to calculate a calibration factor (in mL·min^-1·kHz^-1). A Silastic (Dow Corning Co) IC catheter implanted proximal to the flow probe by the approach described by Gwirtz¹⁷ was used for IC drug delivery. The portion of the catheter within the coronary vessel had an external diameter of 0.6 mm. Analgesia was provided after surgery with buprenorphine (0.3 mg IM; Temgesic, Reckitt and Colman Pharmaceuticals). Prophylactic procaine penicillin G (300 000 U IM) and benzathine penicillin G (300 000 U IM) were administered for 10 days after the surgery. Enteric coated aspirin (650 mg/d PO) was given on a daily basis.

Hemodynamic variables were recorded on a VHS tape with a PCM recording adaptor (model 4000A, A.R. Vetter Co) and monitored on a direct ink-writing strip-chart recorder (model 2800s, Gould).

Protocols
ß₂-Adrenergic Dilation and Blockade of NO Formation
Experiments were performed 2 to 6 weeks after surgery in conscious healthy dogs lying quietly on their right sides in a dimly illuminated laboratory. While HR, LVP, LV dP/dt, phasic arterial pressure and MAP, and phasic and mean CBF were continuously monitored in animals pretreated with 1.0 mg/kg atenolol IV (Sigma Chemical Co) to eliminate ß₁-adrenergic influences, IC bolus injections of pirbuterol hydrochloride (Pfizer Inc), a selective ß₂-adrenergic agonist¹⁸ ; of acetylcholine (Sigma); and of nitroglycerin (Parke-Davis) were administered. Drugs injected IC were freshly dissolved in 0.2 mL warm saline (38.9°C), slowly injected, and then flushed with an additional 1.0 mL saline delivered with a pump-driven syringe over a 15-second period. The sequence of drug administration was randomly selected. At least 3 to 5 minutes was allowed between injections for the return to a steady-state hemodynamic baseline. The same procedure was repeated 10 minutes after the completion of IC administration of 50 µg·kg^-1·min^-1 for 12 minutes of L-NAME (Sigma) delivered in 0.5 mL/min saline.

ß₂-Adrenergic Dilation and Blockade of K_ATP Channels
At least 48 hours later, and after atenolol (1.0 mg/kg IV), the effects of IC injections of pirbuterol, acetylcholine, and nitroglycerin were studied before and after blockade of K_ATP channels with glibenclamide (Sigma) delivered as an IC infusion of 25 µg·kg^-1·min^-1x12 minutes followed by 2 µg·kg^-1·min^-1 for the duration of the experiment. The adequacy of K_ATP channel blockade with glibenclamide was confirmed by blunted CBF responses to IC adenosine (Sigma) and to lemakalim (SmithKline Beecham Pharma), a selective K_ATP channel opener.^{19 20} Lemakalim was prepared from a stock solution containing 1% DMSO in saline, and the final concentration of DMSO in the injectate was 0.03%. The vehicle alone had no significant effect on CBF. Glibenclamide was freshly dissolved in saline containing 2.0 mmol/L sodium hydroxide and was injected at a rate of 1.0 mL/min IC during 12 minutes. Administration of the vehicle containing sodium hydroxide used to dissolve glibenclamide did not interfere with pirbuterol-induced CBF responses. Blood glucose levels were maintained within the normal range during glibenclamide infusion by intravenous 10% dextrose infusion.

Opening of K_ATP Channels and Blockade of NO Formation
On a separate day, the effects of IC lemakalim, acetylcholine, and nitroglycerin were studied before and after IC L-NAME.

Controlled CBF and Blockade of NO Formation or K_ATP Channels
Coronary responses were also studied while CBF increases were prevented, and DCP was used as an index of coronary vasomotion. In six additional dogs, a Doppler flow probe was placed close to the origin of the left circumflex coronary artery, and a hydraulic constrictor placed next to this probe was used for preventing drug-induced increases in CBF. An IC catheter was implanted distal to the hydraulic constrictor to inject the drugs and to measure DCP with an external transducer (model 800, Bentley Trantec). The tip of this catheter was located before the first marginal coronary artery. After atenolol (1.0 mg/kg IV), baseline hemodynamics and DCP were recorded, and bolus doses of drugs (0.2 mL) were injected into the IC catheter and flushed with 1.0 mL saline over a period of 8 seconds. DCP recordings were obtained immediately thereafter. When CBF was controlled, the hydraulic constrictor was inflated at the time CBF started to increase, and care was taken to limit CBF increases to <10% above baseline levels and to avoid even slight decreases below baseline levels. Lemakalim and pirbuterol were injected during normal and controlled CBF conditions before and after IC L-NAME. On a separate day, the effects of pirbuterol before and after glibenclamide were studied with and without controlled CBF. Dextrose was administered as previously described.

Data Analysis
Hemodynamic data were read directly from the strip charts during baseline conditions and at peak increases of CBF after boluses injections of the various drugs. With a computer-assisted digitizing tablet, the area under mean CBF recordings in excess of baseline CBF, called volume response, was planimetered from the rise in CBF to the return of CBF to baseline and reported in milliliters. This approach was used to analyze responses to acetylcholine, pirbuterol, nitroglycerin, and adenosine. Because of the long-lasting effects of lemakalim on CBF, volume responses were measured from the rise in CBF to the point of 50% decline from peak CBF. The duration of CBF responses was established by direct measurement on the strip charts of the time corresponding to the measured volume responses. In experiments in which CBF was controlled, mean DCP was reported before and at peak decreases after drug injection. Data are reported as mean±SEM throughout. Overall simultaneous comparisons of baseline levels (before the various drugs) before and after L-NAME or before and after glibenclamide were made with two-way ANOVA for repeated measurements.^{21 22} A similar approach was used for comparing responses to a specific drug before and after L-NAME or glibenclamide. For protocols involving the injection of a single dose of acetylcholine, nitroglycerin, or lemakalim, paired t tests were used for comparing responses before and after L-NAME or glibenclamide. Comparisons of LVP, LV dP/dt, MAP, and HR at peak CBF with baselines were made with paired t tests.

Statistical significance was reached when P<.05 in all cases. All experimental procedures were approved by an ethical committee on animal care and performed in accordance with the Guide to the Care and Use of Experimental Animals (Canadian Council on Animal Care publication No. [ISBN] 0-919087-18-3, Ottawa, 1993).

	Results

Top Abstract Introduction Methods Results Discussion References

 
ß₂-Adrenergic Dilation and Blockade of NO Formation
Overall simultaneous comparisons made between baseline values (values before all drugs) revealed slight increases in LVP and MAP and decreases in HR and LV dP/dt after L-NAME. Baseline CBF was not altered after L-NAME, as reported in Table 1. Except for increases in CBF, other hemodynamic effects of pirbuterol, acetylcholine, and nitroglycerin were trivial and did not differ statistically before and after L-NAME.

View this table: [in this window] [in a new window]   Table 1. Overall Baseline Hemodynamics Before and After Blockade of NO Formation With Intracoronary L-NAME After ß1-Adrenergic Blockade in Conscious Dogs

Pirbuterol. A recording of responses elicited by pirbuterol (100.0 ng/kg) before and after L-NAME is displayed in Fig 1. L-NAME resulted in a smaller CBF response to pirbuterol. Before L-NAME, pirbuterol 50.0 ng/kg increased CBF by 32±6 from 43±7 mL/min. Pirbuterol resulted in smaller (P<.01) CBF responses (11±2 from 40±7 mL/min) after L-NAME. Overall, peak increases in CBF and the volume and duration of CBF responses to pirbuterol were smaller (P<.01) after L-NAME, as reported in Fig 2.

View larger version (83K): [in this window] [in a new window]   Figure 1. Responses to an IC bolus injection of pirbuterol 100.0 ng/kg before and after blockade of NO formation with L-NAME 50 µg·kg-1·min-1x12 minutes. Recordings of LVP and LV dP/dt, phasic and mean arterial pressure (AP), phasic and mean CBF, and HR are included. L-NAME reduced CBF increases caused by pirbuterol.

View larger version (14K): [in this window] [in a new window]   Figure 2. Mean±SEM peak increases in CBF (top) and corresponding values of flow volume (middle) and duration (bottom) of CBF responses to graded IC doses of pirbuterol before and after L-NAME 50 µg·kg-1·min-1x12 minutes. L-NAME reduced all aspects of CBF responses induced by pirbuterol.

Acetylcholine. Acetylcholine 3.0 ng/kg increased CBF by 49±6 from 42±7 mL/min before L-NAME. After blockade of NO formation, acetylcholine-induced increases in CBF were limited (P<.01) to 28±4 from 44±8 mL/min. Overall, peak increases in CBF and the volume and duration of CBF responses to acetylcholine were smaller (P<.01) after L-NAME.

Nitroglycerin. Nitroglycerin 50.0 ng/kg increased CBF by 18±2 from 42±7 mL/min before L-NAME. Similar responses (22±4 from 46±8 mL/min) were elicited by nitroglycerin after L-NAME. The volume and the duration of CBF responses to nitroglycerin did not differ statistically before and after L-NAME.

ß₂-Adrenergic Dilation and Blockade of K_ATP Channels
Overall, simultaneous comparisons made between baseline values (values before all drugs) revealed slight increases in MAP, decreases in LV dP/dt, and no changes in LVP and HR after glibenclamide. Baseline CBF fell (P<.05) after glibenclamide, as reported in Table 2. Except for increases in CBF, other hemodynamic effects of pirbuterol, acetylcholine, adenosine, lemakalim, and nitroglycerin were trivial and did not differ statistically before and after glibenclamide. Lemakalim 100.0 ng/kg increased CBF by 45±3 mL/min before glibenclamide and by only 6±1 mL/min (P<.01) thereafter. Thus, adequate blockade of K_ATP channels was achieved.

View this table: [in this window] [in a new window]   Table 2. Overall Baseline Hemodynamics Before and After Blockade of KATP Channels With Intracoronary Glibenclamide After ß1-Adrenergic Blockade in Conscious Dogs

Pirbuterol. A recording of responses elicited by pirbuterol 100.0 ng/kg before and after glibenclamide is displayed in Fig 3. Glibenclamide resulted in a smaller CBF response to pirbuterol.

View larger version (81K): [in this window] [in a new window]   Figure 3. Responses to an IC bolus injection of pirbuterol 100.0 ng/kg before and after blockade of KATP channels with glibenclamide 25 µg·kg-1·min-1x12 minutes followed by 2 µg·kg-1·min-1. Recordings of LVP and LV dP/dt, phasic and mean arterial pressure (AP), phasic and mean CBF, and HR are included. Glibenclamide reduced CBF increases caused by pirbuterol.

Before glibenclamide, pirbuterol 50.0 ng/kg increased CBF by 33±5 from 45±7 mL/min. After blockade of K_ATP channels, pirbuterol resulted in smaller (P<.01) increases in CBF (14±3 from 36±5 mL/min). Overall, peak increases in CBF and the volume and duration of CBF responses to pirbuterol were smaller (P<.01) after glibenclamide, as reported in Fig 4.

View larger version (15K): [in this window] [in a new window]   Figure 4. Mean±SEM peak increases in CBF (top) and corresponding values of flow volume (middle) and duration (bottom) of CBF responses to graded IC doses of pirbuterol before and after glibenclamide 25 µg·kg-1·min-1x12 minutes followed by 2 µg·kg-1·min-1. Glibenclamide reduced all aspects of CBF responses induced by pirbuterol.

Acetylcholine. Acetylcholine 3.0 ng/kg increased CBF by 49±3 from 45±7 mL/min before glibenclamide. After blockade of K_ATP channels, acetylcholine-induced increases in CBF did not differ (46±6 from 37±6 mL/min). Overall, peak increases in CBF and the volume and duration of CBF responses to acetylcholine did not differ statistically before and after glibenclamide, as reported in Fig 5.

View larger version (10K): [in this window] [in a new window]   Figure 5. Mean±SEM peak increases in CBF (top) and corresponding values of flow volume (middle) and duration (bottom) of CBF responses to IC doses of acetylcholine before and after glibenclamide 25 µg·kg-1·min-1x12 minutes followed by 2 µg·kg-1·min-1. Glibenclamide failed to alter coronary responses to acetylcholine.

Nitroglycerin. Nitroglycerin 50.0 ng/kg increased CBF by 15±1 from 42±7 mL/min before glibenclamide. Similar responses (15±2 from 39±6 mL/min) were elicited by nitroglycerin after glibenclamide. The volume and duration of CBF responses to nitroglycerin did not differ before and after glibenclamide.

Opening of K_ATP Channels and Blockade of NO Formation
The effects of lemakalim 50.0 ng/kg before and after L-NAME are illustrated in Fig 6. Lemakalim caused a dose-dependent increase in CBF before L-NAME. Blockade of NO formation resulted in reductions (P<.01) of CBF responses (Fig 7). The percent inhibition of CBF responses created by L-NAME was inversely related to the dose of lemakalim. Both the volume and the duration of CBF responses to lemakalim were smaller (P<.01) after L-NAME.

View larger version (141K): [in this window] [in a new window]   Figure 6. Responses to an IC bolus injection of lemakalim 50.0 ng/kg before and after blockade of NO formation with L-NAME 50 µg·kg-1·min-1x12 minutes. Recordings of LVP and LV dP/dt, phasic and mean arterial pressure (AP), phasic and mean CBF and HR are included. L-NAME reduced CBF increases caused by lemakalim.

View larger version (19K): [in this window] [in a new window]   Figure 7. Mean±SEM peak increases in CBF (top left) and corresponding values of flow volume (lower left) and duration (lower right) of CBF responses to IC injections of lemakalim before and after L-NAME 50 µg·kg-1·min-1x12 minutes. Upper right, Percent inhibition of peak increases in CBF caused by L-NAME. L-NAME reduced all aspects of CBF responses elicited by lemakalim. On a percent basis, L-NAME inhibitory effects were greater at lower doses of lemakalim.

CBF responses to acetylcholine 3.0 ng/kg were reduced (P<.01) after L-NAME, but responses to nitroglycerin 50.0 ng/kg were maintained after blockade of NO formation, as reported above.

Controlled CBF and Blockade of NO Formation or K_ATP Channels
The effects of L-NAME on CBF and DCP (when CBF increases were prevented) with pirbuterol and lemakalim are reported in Fig 8. Consistent with responses elicited with normal CBF, decreases in DCP when CBF was controlled were smaller after L-NAME. With arterial constriction, CBF increases caused by pirbuterol and lemakalim were <5% over baseline and did not differ before and after L-NAME.

View larger version (19K): [in this window] [in a new window]   Figure 8. Mean±SEM peak increases in CBF with normal CBF (top) and peak decreases in DCP with controlled CBF (bottom) to IC injections of pirbuterol (PIR) and lemakalim (LEM) before and after L-NAME 50 µg·kg-1·min-1x12 minutes. After L-NAME, CBF and DCP responses to PIR and LEM were smaller. *P<.05 vs before L-NAME; P<.01 vs before L-NAME.

The effects of glibenclamide on coronary responses elicited by pirbuterol with normal and controlled CBF are reported in Fig 9. After glibenclamide, pirbuterol caused smaller decreases in DCP when CBF was controlled. The residual increases in CBF during arterial constriction did not differ before and after glibenclamide.

View larger version (16K): [in this window] [in a new window]   Figure 9. Mean±SEM peak increases in CBF with normal CBF (top) and peak decreases in DCP with controlled CBF (bottom) to IC injections of pirbuterol (PIR) before and after glibenclamide 25 µg·kg-1·min-1x12 minutes followed by 2 µg·kg-1·min-1. After glibenclamide, CBF and DCP responses to PIR were smaller. *P<.05 vs before glibenclamide; P<.01 vs before glibenclamide.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
The present study demonstrates that ß₂-adrenergic dilation of resistance coronary vessels involves both the opening of K_ATP channels and NO formation. Glibenclamide, a blocker of K_ATP channels, selectively antagonized ß₂-adrenergic dilation without altering acetylcholine- and nitroglycerin-induced dilation. NO formation was also important in ß₂-adrenergic dilation of resistance coronary vessels, as indicated by blunted CBF increases after L-NAME, a powerful inhibitor of NO formation. Lemakalim, a selective opener of K_ATP channels, also triggered NO formation, as demonstrated by blunted CBF responses after L-NAME. Thus, opening of K_ATP channels could be the primary mechanism involved in ß₂-adrenergic coronary dilation. The increase in NO formation may be a secondary phenomenon triggered by the hyperpolarization of endothelial cells. Presumably, this led to an augmented electrochemical gradient for Ca²⁺ entry into endothelial cells, thereby increasing NO formation.^{7 8} Whether these observations could be extended to other vasodilators acting through a cAMP-dependent mechanism, such as prostacyclin and adenosine, remains to be established. Nevertheless, the present observations could be helpful in understanding the action of blockers of K_ATP channels given as hypoglycemic agents in the clinical setting and of openers of these channels, such as nicorandil, used as vasodilators in humans.

In the present study, CBF responses were used as an index of changes of vascular tone in resistance vessels. Given that only 20% of the pressure loss across the coronary circulation occurs in vessels >200 µm²³ and that large epicardial coronary vessels contribute little to overall coronary resistance (2% to 5%),²⁴ CBF responses reported in the present study should primarily reflect changes of vascular tone in vessels <200 µm, where the highest resistance resides. Dilation of vessels with a higher caliber would be expected to have a minor influence on CBF responses. In the present study, the chronic treatment with aspirin, targeting the blockade of cyclooxygenase and the reduction of platelet aggregation for maintaining the patency of the IC catheters may have influenced our data by changing the level of NO synthase activity. This possibility should be kept in mind when the present data are considered.

The involvement of NO formation in ß-adrenergic dilation remains a debated issue. In isolated canine epicardial coronary vessels, ß-adrenergic relaxation has been reported to be endothelium-independent^{25 26} as well as partially endothelium-dependent.^{27 28} In contrast, in conscious dogs, L-NAME antagonized CBF responses triggered by ß₂-adrenergic stimulation, consistent with the involvement of an endothelium-dependent process.¹⁵ A receptor-operated process rather than a flow-dependent phenomenon was involved, because blockade of ß-adrenergic dilation by L-NAME could be demonstrated even when CBF increases were prevented.¹⁵ The apparent discrepancy between in vivo and in vitro data may be related to segmental differences in the vessels studied, ie, conductance and resistance coronary vessels.

Our data are in general agreement with several recent reports in which K_ATP channels intervened in ß-adrenergic effects.^{1 2 3 6} We can only speculate on the reason why our data differed from those reported by Narishige et al,¹ in which increases in CBF caused by ß₁-adrenergic stimulation were blocked by glibenclamide but ß₂-adrenergic responses were not. Aside from the fact that Narishige et al¹ studied anesthetized dogs, our study differed in the use of a higher dose of glibenclamide to block K_ATP channels. Perhaps a more complete blockade of K_ATP channels was achieved in our study. In our hands, glibenclamide reduced CBF responses to lemakalim 100.0 ng/kg by >85%, whereas in the study by Narishige et al,¹ glibenclamide effects were limited to a 45% attenuation of responses of pinacidil 30 µg/min.

Conceivably, the effects of glibenclamide on ß₂-adrenergic dilation could be secondary to the blockade of ß₂-adrenergic receptors by the K_ATP channel blocker. Our data do not directly allow us to rule out this possibility. However, in an earlier study, Randall and McCulloch³ showed that glibenclamide 1 nmol/L to 100 µmol/L did not displace the specific [³H]dihydroalprenolol binding from rat ß-adrenergic receptors. Consistent with this observation, glibenclamide did not prevent increases in contractility caused by isoproterenol in anesthetized dogs,¹ as would be expected if ß-adrenergic blockade were involved. In the present study, the highest rate of IC glibenclamide delivery created an estimated IC concentration in the range of 10^-5 mol/L, which should not have interfered with ß-adrenergic receptor activation.³

Lemakalim, the active enantiomer of cromakalim, acts as a hyperpolarizing agent and creates vasodilation by opening K_ATP channels.^{19 20} Our data are consistent with a selective action of lemakalim on K_ATP channels, because glibenclamide, a blocker of K_ATP channels,²⁹ was a powerful antagonist of lemakalim-induced dilation of resistance coronary vessels. Furthermore, the unaltered CBF responses to nitroglycerin and acetylcholine after glibenclamide suggest a maintained coronary reactivity after blockade of K_ATP channels. This also rules out the possibilities that a decrease in baseline CBF caused by glibenclamide led to smaller CBF responses or that blockade of K_ATP channels interfered with processes related to NO formation or its action on vascular smooth muscles.

The contribution of NO formation to lemakalim-induced increases in CBF along with the blockade of ß₂-adrenergic dilation by either glibenclamide or L-NAME supports the possibility that opening of K_ATP channels triggers NO formation during ß₂-adrenergic stimulation. Consistent with this hypothesis, K_ATP channels have been demonstrated to intervene in the regulation of endothelial cell membrane potentials. Lückhoff and Busse^{7 8} have further established that the Ca²⁺ influx into endothelial cells and the formation of endothelium-derived relaxing factor triggered by openers of K_ATP channels are secondary to cellular hyperpolarization. Openers of K_ATP channels prolong K⁺ currents elicited by bradykinin and augment Ca²⁺ entry into those cells.^{7 8} Endothelial K⁺ channels activated by shear stress can also cause membrane hyperpolarization, which promotes Ca²⁺ influx and augments NO formation.^{30 31} K_ATP channels have been reported to account for the release of NO caused by adenosine.³² Our data suggest that a similar mechanism involving K_ATP channels as the triggering mechanism of NO formation intervenes in the cascade of reaction leading to ß₂-adrenergic dilation of resistance coronary vessels.

In addition to an action on the endothelium, a direct effect of pirbuterol on vascular smooth muscle could also be involved in glibenclamide-sensitive ß₂-adrenergic vasodilation. In this connection, Nakashima and Vanhoutte² reported that smooth muscle relaxation of the canine saphenous vein induced by ß₂-adrenergic agonists involved K_ATP channels. Although our data with L-NAME suggest that NO is a major intermediate in ß₂-adrenergic relaxation, the residual vasodilation after blockade of NO formation may reflect the involvement of a different mechanism, perhaps a direct smooth muscle relaxation caused by ß₂-adrenergic stimulation. An incomplete blockade of NO formation in resistance coronary vessels may also explain the residual acetylcholine and ß₂-adrenergic CBF responses after L-NAME, in which case the contribution of a direct smooth muscle effect of ß₂-adrenergic stimulation could only be overestimated by the present data.

The relative contribution of NO formation to overall lemakalim responses was inversely related to the dose of the agonist used. For example, NO formation accounted for 70% of peak increases in CBF caused by 20.0 ng/kg lemakalim but only for 30% of the response to 100.0 ng/kg lemakalim. This may explain why opposite conclusions regarding the role of NO formation were reached when a single dose of an opener of K_ATP channels was studied in vivo.^{14 33}

The possibility that L-NAME or glibenclamide interfered with pirbuterol and lemakalim effects on CBF by blocking a flow-dependent component was studied directly in separate experiments in which DCP was used as an index of vascular tone of resistance vessels when CBF increases were prevented. Consistent with our initial observations, in which L-NAME reduced CBF responses to pirbuterol and lemakalim when CBF was normal, L-NAME reduced the fall in DCP when CBF was controlled. Glibenclamide also antagonized the CBF responses to pirbuterol in normal conditions and the fall in DCP when CBF was controlled. Thus, L-NAME and glibenclamide did not interfere with pirbuterol and lemakalim responses solely by blocking a flow-dependent component, consistent with our contention that opening of K_ATP channels may be the mechanism triggering NO formation.

In conclusion, ß₂-adrenergic dilation of resistance coronary vessels involves both the opening of K_ATP channels and NO formation. A causal relationship between the opening of K_ATP channels and NO formation is suggested by the finding of smaller CBF responses to an opener of K_ATP channels after L-NAME.

	Selected Abbreviations and Acronyms

 

DCP = distal coronary pressure HR = heart rate IC = intracoronary KATP = ATP-sensitive potassium (channels) L-NAME = N-nitro-L-arginine methyl ester LV = left ventricular LVP = LV systolic pressure MAP = mean arterial pressure

	Acknowledgments

 
This work was supported through grants from the Medical Research Council of Canada, Canadian Heart and Stroke Foundation, Fonds de la Recherche en Santé du Québec, and Fonds de la Recherche de l'Institut de Cardiologie de Montréal. The authors thank Claude Mousseau, Jean-Pierre Turcotte, and Jhésabelle Voyer for skillful assistance in conducting these studies. Pirbuterol was generously supplied by Pfizer Inc, Groton, Conn, and lemakalim by SmithKline Beecham Pharma, Oakville, Ontario, Canada.

Received August 5, 1996; revision received October 12, 1996; accepted November 4, 1996.

	References

Top Abstract Introduction Methods Results Discussion References

 

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