Robert Parent

Nitric oxide formation contributes to beta-adrenergic dilation of resistance coronary vessels in conscious dogs.

The contribution of the L-arginine/nitric oxide pathway to beta-adrenergic dilation of resistance coronary vessels was examined in conscious dogs instrumented for measuring coronary blood flow (CBF), left ventricular (LV) wall thickening, and LV and aortic pressures and for intracoronary injections of acetylcholine (0.003 micrograms/kg), nitroglycerin (0.175 micrograms/kg), and graded doses of isoproterenol (0.0005 to 0.004 micrograms/kg). Peak increases in CBF with intracoronary isoproterenol (0.001 micrograms/kg) averaged 105 +/- 10% from baseline. With acetylcholine, CBF increased by 158 +/- 11%, and with nitroglycerin, CBF increased by 139 +/- 10%. After the administration of intracoronary N omega-nitro-L-arginine methyl ester (L-NAME, 10 micrograms/kg per minute for 12 minutes) to block nitric oxide synthesis from L-arginine, baseline CBF was not altered, and CBF increased by 49 +/- 7% with isoproterenol and by 94 +/- 6% with acetylcholine; both values were smaller (P < .01) than those before the arginine analogue. With nitroglycerin, CBF was increased by 145 +/- 11%, not significantly different from the value before L-NAME. Intracoronary L-arginine (1.0 mg/kg per minute for 12 minutes), the precursor of nitric oxide synthesis, partially reversed the inhibition of L-NAME on CBF responses to acetylcholine and isoproterenol. After beta 1-adrenergic blockade, CBF responses to isoproterenol and acetylcholine were also reduced (P < .05) by the arginine analogue. When increases in CBF were prevented, peak changes in coronary vascular conductance with intracoronary bolus doses of acetylcholine and isoproterenol were attenuated (P < .01) by L-NAME. Thus, nitric oxide formation is an important intermediate in beta-adrenergic dilation of resistance coronary vessels in conscious dogs.

β2-Adrenergic Dilation of Resistance Coronary Vessels Involves KATP Channels and Nitric Oxide in Conscious Dogs

BACKGROUND We considered that beta 2-adrenergic stimulation may dilate resistance coronary vessels by opening ATP-sensitive potassium (KATP) channels, thereby triggering NO formation. METHODS AND RESULTS In conscious instrumented dogs after beta 1-adrenergic blockade, intracoronary (IC) injections of acetylcholine (ACh), nitroglycerin (NTG), and pirbuterol (PIR), a selective beta 2-adrenergic agonist, were performed before and after blockade of NO formation with IC N omega-nitro-L-arginine methyl ester (L-NAME, 50 micrograms.kg-1.min-1 x 12 minutes) or blockade of KATP channels with IC glibenclamide (25 micrograms.kg-1.min-1 x 12 minutes followed by 2 micrograms.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 KATP 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 beta 2-Adrenergic dilation of resistance coronary vessels involves both the opening of KATP channels and NO formation. L-NAME antagonized lemakalim responses consistent with a link between the opening of KATP channels and NO formation in canine resistance coronary vessels.

Contribution of endogenous endothelin to large epicardial coronary artery tone in dogs and humans

Nitric oxide (NO) may normally impair endothelin (ET) activity in epicardial coronary arteries. Lifting this inhibitory feedback could reveal ET-dependent effects involving ETA- and/or ETB-receptor activation. In conscious dogs, the blockade of ETA receptors (intracoronary Ro-61-1790) increased external circumflex coronary artery diameter (CD) (sonomicrometry) by 0.10 ± 0.01 from 3.04 ± 0.12 mm ( P < 0.01) without altering coronary blood flow (Doppler). Similarly, CD increased (0.09 ± 0.01 from 2.91 ± 0.14 mm; P < 0.01) when Ro-61-1790 was given after blockade of NO formation with intracoronary N ω-nitro-l-arginine methyl ester (l-NAME). In contrast, ETB-receptor blockade (intracoronary Ro-46-8443) did not influence baseline CD with and without l-NAME. In vitro, increases in tension caused by N ω-nitro-l-arginine (l-NNA) or PGF2α in arterial rings were reduced by ETA- but not ETB-receptor blockade. ETA-receptor blockade also reduced the increase in tension caused byl-NNA in human coronary arterial rings. Thus ETA receptors, but not ETB receptors, account for ET-dependent constriction in canine epicardial coronary arteries in vivo. ET-dependent effects were independent of the level of NO formation in vitro and in vivo. In human epicardial coronary arterial rings, ETA-receptor blockade also caused significant relaxation.

Endothelin-Dependent Tone Limits Acetylcholine-Induced Dilation of Resistance Coronary Vessels After Blockade of NO Formation in Conscious Dogs

Nitric oxide (NO) impairs endothelin (ET) formation and/or action in isolated vessels. We hypothesized that ET may magnify the consequences of NO formation blockade on receptor-operated dilation of resistance coronary vessels in conscious dogs. In conscious instrumented dogs, graded intracoronary (IC) doses of acetylcholine (ACh) were delivered before IC administration of Nomega-nitro-L-arginine methyl ester (L-NAME), after L-NAME, and after L-NAME plus IC bosentan, an ETA/ETB receptor blocker. Before L-NAME, ACh (100 ng. kg-1. min-1) increased coronary blood flow (CBF) by 43+/-4% from 47+/-6 mL. min-1. After L-NAME, ACh failed to increase CBF (-3+/-2% from 50+/-7 mL. min-1). CBF responses to ACh were partially restored (+10+/-2% from 50+/-7 mL. min-1, P<0.01) after the addition of bosentan. Bosentan alone (without L-NAME) did not alter CBF responses to ACh. Blockade of ETA (Ro 61-1790) but not ETB (Ro 46-8443) receptors partially restored CBF responses to ACh after L-NAME. Myocardial immunoreactive ET levels in the perfusion territories of the circumflex and left anterior descending coronary arteries did not differ. ETA-dependent tone magnified the inhibitory effects of blockade of NO formation on receptor-operated dilation to ACh in resistance coronary vessels. Presumably, stimulated NO release has an inhibitory action on endogenous ET production and/or action at the level of resistance coronary vessels.

Contrasting effects of blockade of nitric oxide formation on resistance and conductance coronary vessels in conscious dogs

OBJECTIVES To determine the differential effects of blockade of nitric oxide (NO) formation by an arginine analogue on basal and stimulated NO release in conductance and resistance coronary vessels. METHODS In conscious dogs, instrumented for measuring coronary blood flow (CBF) and external epicardial coronary artery diameter (CD), intracoronary (ic) acetylcholine (ACH, 3.0 ng/kg), adenosine (ADENO 100.0 ng/kg) and nitroglycerin (NTG, 10.0 ng/kg) were injected before and after ic N omega-nitro-L-arginine methyl ester (L-NAME, 50.0 micrograms.kg-1 min-1 for 12 min) to block NO synthesis. RESULTS Before L-NAME, ACH increased CBF by 65.3 +/- 9.0 from 42.4 +/- 2.9 ml/min and CD by 0.199 +/- 0.035 from 3.374 +/- 0.193 mm. L-NAME failed to alter baseline CBF but reduced (P < 0.01) CD to 3.220 +/- 0.199 mm. CBF responses to ACH were smaller (P < 0.01) (32.8 +/- 5.3 ml/min) after L-NAME. In contrast, ACH-induced increases in CD (0.184 +/- 0.053 mm) were not altered. L-NAME did not change CBF responses to NTG but increased CD responses (0.345 +/- 0.062 vs 0.217 +/- 0.043 mm, P < 0.01). ADENO-induced increases in CBF were smaller after L-NAME (46.5 +/- 5.6 vs 79.8 +/- 10.9 ml/min, P < 0.01). Increases in CD created by ADENO, a flow-dependent phenomenon, were nearly abolished after L-NAME (0.043 +/- 0.018 vs 0.195 +/- 0.026 mm, P < 0.01) and partially restored by ic L-arginine. The effects of L-NAME on CBF and CD responses to ACH and ADENO continuously delivered into the coronary artery were similar to those of boluses. CONCLUSIONS L-NAME selectively reduced ACH-induced dilation in resistance coronary vessels but failed to prevent responses of conductance coronary vessels in spite of reducing baseline CD and blocking flow-dependent effects of ADENO. Therefore, blockade of NO formation resulted in disparate effects on receptor-operated dilation of resistance and conductance coronary vessels.

Nitric Oxide–Independent Dilation of Conductance Coronary Arteries to Acetylcholine in Conscious Dogs

NO and prostacyclin formation cannot entirely account for receptor-operated endothelium-dependent dilation of coronary vessels, since vasodilator responses are not completely suppressed by inhibitors of these agents. Therefore, we considered that another factor, such as an endothelium-derived hyperpolarizing factor described in vitro, may participate in NO- and prostacyclin-independent coronary dilator responses. In conscious instrumented dogs, intracoronary acetylcholine (ACh, 30.0 ng.kg-1.min-1) increased the external epicardial coronary diameter (CD) by 0.18 +/- 0.03 mm (from 3.44 +/- 0.11 mm) when increases in coronary blood flow (CBF) were prevented and increased the CD by 0.20 +/- 0.05 when CBF was allowed to increase. After the administration of intracoronary N omega-nitro-L-arginine methyl ester (L-NAME), CBF responses to ACh were abolished, but CD responses (0.23 +/- 0.05 from 3.22 +/- 0.09 mm) were maintained. Blockade of NO formation was confirmed by reduced CD baselines and blunted flow-dependent CD responses caused by adenosine and transient coronary artery occlusions after L-NAME administration. ACh-induced CD increases resistant to L-NAME and indomethacin were reduced after the administration of intracoronary quinacrine, an inhibitor of phospholipase A2, or proadifen, an inhibitor of cytochrome P-450. Quinacrine or proadifen alone (without L-NAME) did not alter CD responses to ACh, but L-NAME given after proadifen blunted ACh-induced increases in CD. The increases in CD caused by arachidonic acid given after L-NAME + indomethacin were antagonized by proadifen but not altered by quinacrine. Thus, a cytochrome P-450 metabolite of arachidonic acid accounts for L-NAME-resistant and indomethacin-resistant dilation of large epicardial coronary arteries to ACh. Conversely, NO formation is the dominant mechanism of ACh-induced dilation after blockade of the cytochrome P-450 pathway.

Endothelin-dependent effects limit flow-induced dilation of conductance coronary vessels after blockade of nitric oxide formation in conscious dogs.

OBJECTIVE To determine whether endothelin (ET)-dependent effects limit shear stress-induced dilation of large epicardial coronary arteries after blockade of nitric oxide (NO) formation. METHODS In conscious dogs instrumented for measuring coronary blood flow (CBF) and external diameter (CD) of the circumflex coronary artery, flow-dependent CD dilation was elicited by intracoronary (i.c.) adenosine (500 ng kg-1 min-1). RESULTS I.c. adenosine increased CBF by 28 +/- 4 from 38 +/- 5 ml min-1 and CD by 0.25 +/- 0.03 from 3.53 +/- 0.07 mm without other hemodynamic effects. After N omega-nitro-L-arginine methyl ester (L-NAME), baseline CD fell (P < 0.01) to 3.35 +/- 0.08 mm but CBF was not significantly altered (36 +/- 5 ml min-1). CBF increases caused by adenosine were smaller (17 +/- 2 ml min-1, P < 0.05) and CD responses were nearly abolished (0.02 +/- 0.01 mm, P < 0.01). I.c. Ro 61-1790, an ETA receptor blocker, given after L-NAME did not significantly influence baseline CBF (36 +/- 5 ml min-1) but increased (P < 0.01) CD to 3.45 +/- 0.09 mm. CBF responses to adenosine were not significantly altered by Ro 61-1790 but CD responses (0.10 +/- 0.01 mm) were partially restored (P < 0.01). In contrast, blockade of ETB receptors with Ro 46-8443 after L-NAME had no further effects on CD and CBF responses to adenosine. CONCLUSION ETA receptor-mediated effects limit flow-dependent dilation of large epicardial coronary arteries in conscious dogs. Suppression of the L-arginine/NO-dependent pathway with L-NAME reveals significant ET-dependent effects.

Disparate effects of substance P on systemic and coronary beds in conscious dogs.

BackgroundPrevious studies in anesthetized animals indicated that substance P is a coronary and peripheral vasodilator. However, coronary vasodilation was only transient perhaps because of tachyphylaxis. In the present study, the steady-state effects of intravenous substance P on systemic and coronary beds were investigated in conscious, instrumented dogs. Methods and ResultsWith intact autonomic reflexes, 5 ng/kg/min i.v. substance P resulted in increases (p < 0.01) in cardiac output by 22 ± 5%, in decreases (p < 0.01) in mean arterial pressure by 9 ± 2%, and in total peripheral resistance by 23 ± 4% 7–9 minutes after the beginning of substance P infusion. Heart rate increased (p < 0.01) by 35 ± 7% and left ventricular dP/dt (p < 0.05) by 13 ± 4%. In this situation, coronary blood flow decreased (p < 0.01) by 19 ± 4% and coronary vascular resistance increased (p < 0.05) by 13 ± 5%. Myocardial oxygen delivery was reduced (p < 0.05) by 13 ± 5% and the arteriovenous oxygen difference widened (p < 0.01). After ganglionic blockade, increases in cardiac output, heart rate, and left ventricular dP/dt with substance P administration were abolished, but total peripheral resistance and mean arterial pressure decreased (p < 0.01) by 12 ± 3% and 11 ± 3%, respectively. Under these conditions, coronary blood flow decreased (p < 0.01) by 37 ± 5% and coronary vascular resistance increased (p < 0.01) by 47 ± 8%, which were more (p < 0.01) than control responses. In this situation, myocardial oxygen delivery was reduced (p < 0.01) by 37 ± 4% and the arteriovenous oxygen difference widened (p < 0.01). Intracoronary infusion of substance P (0.4 ng/kg/min) resulted in significant and sustained decreases in coronary blood flow, which were similar before and after ganglionic blockade. ConclusionsThus, in conscious dogs, systemic vasodilation is the prevailing effect of substance P, but paradoxically, this peptide simultaneously elicits coronary vasoconstriction. (Circulation 1991;84:300–312)

Long-term daily study of blood volume in cardiac autotransplanted dogs.

Cardiac transplantation is followed by the interruption of afferent nerves to the heart. Knowing that some of the afferent nerves are responsible for the homeostasis of the blood volume, we undertook a serial and long-term study of the blood volume in 25 dogs with denervated hearts. The animals were autotransplanted in order to eliminate repercussions linked to rejection and to its treatment. To discern the effects of surgery and of extracorporeal circulation from those of denervation, a group of 11 control dogs was operated upon and subjected to a period of extracorporeal circulation. In both groups, serial and long-term studies of blood volume were carried out daily with 131I-labeled albumin. Analysis of the results demonstrated that in the hours following surgery, blood volume is significantly decreased by 11% in the control group and by 22% in group 2. By the 5th postoperative day, the blood volume had increased gradually to attain normal values in both groups. At the 2nd postoperative week, the total blood volume remained normal in the control group, whereas blood volume had increased by 5.7% in the autotransplanted dogs, this being due to a 38% increase of the plasmatic phase. This increment persisted from the 14th to the 42nd postoperative day and attained 7.4%. Our conclusion is that the variations in blood volume during the 1st postoperative week in the autotransplanted heart are inherent in surgery and in extracorporeal circulation. Afterwards, the hypervolemia shown in the transplanted dogs is secondary to cardiac denervation.

Time course of left ventricular function after cardiac denervation in conscious dogs

The time course of left ventricular (LV) function was compared in normal (N) and cardiac-denervated (CD) dogs over an 8-week period after instrumentation with solid-state LV pressure gauges and three pairs of ultrasonic crystals to measure LV long and short axes and wall thickness. Baseline LV systolic, end-systolic, and end-diastolic pressures did not differ in N and CD dogs. Heart rate was higher (p less than 0.01) and LV dP/dt was lower (p less than 0.05) in CD dogs. LV short-axis shortening, shortening fraction, velocity of circumferential fiber shortening, and ejection fraction were consistently lower (p less than 0.01) in CD dogs. With angiotensin II to increase LV afterload, relations of LV short-axis shortening, shortening fraction, velocity of circumferential fiber shortening, and ejection fraction to average LV systolic wall stress were shifted downward (p less than 0.01) in CD dogs at 2, 4, and 8 weeks. Relations of LV short-axis shortening to LV end-diastolic wall stress also differed (p less than 0.01) in N and CD dogs. Ganglionic blockade abolished differences in LV function between N and CD dogs during elevated LV systolic wall stress with angiotensin II. Thus, in conscious dogs, cardiac denervation results in a sustained reduction of LV function over a wide range of ventricular loading conditions.