Endothelium-derived nitric oxide does not modulate metabolic coronary vasodilation induced by tachycardia in dogs

Prostacyclin-mediated compensatory mechanism in the coronary circulation during acute NO synthase blockade

Nitric oxide (NO) in contrast to most prostanoids, plays a major role in the maintenance of coronary arterial tone under physiological conditions. However, in case of endothelial damage or other NO-depleting situations the importance of other vasodilating mechanisms may be increased. The aim of the present study was to investigate the crosstalk between the L-arginine - NO and the prostanoid systems in isolated rat hearts. Coronary flow and cardiac dynamics were measured in a standard Langendorff perfusion system. Application of indomethacin in the perfusion media failed to change coronary flow. Administration of L-NA, however, significantly decreased coronary flow by 24.8 +/- 2.3% (p < 0.01 vs. untreated control). In the presence of indomethacin, L-NA decreased coronary flow to an even greater extent by 35.8 +/- 5.2% (p < 0.05 vs. L-NA alone). Treatment of the preparations with L-NA or indomethacin failed to change cardiac work, coapplication of both drugs together, however, decreased cardiac work by 45 +/- 11% (p < 0.05 vs. untreated control). Heart rate remained constant throughout the experimental period and did not differ significantly between the treatment groups. The prostacyclin content of the effluent from the L-NA treated hearts was significantly higher than that of controls. We conclude that in case of decreased NO levels in the coronary circulation, arterial tone is maintained by prostacyclin production.

Role of nitric oxide in regulation of coronary blood flow in response to increased metabolic demand in dogs with pacing-induced heart failure

The role of endothelium-derived nitric oxide (NO) in the metabolic control of coronary blood flow (CBF) in heart failure (HF) is poorly understood, so the present study investigated the effects of inhibitors of NO synthesis on the response of CBF to changes in myocardial oxygen consumption (MVO2) in dogs with HF produced by rapid ventricular pacing and in control dogs. The CBF, MVO2, and other hemodynamic parameters were measured in anesthetized animals. Before infusion of Nomega-nitro-L-arginine methyl ester (L-NAME), the increases in CBF and MVO2 during pacing tachycardia were not significantly different between the control and HF dogs. Intracoronary infusion of L-NAME did not alter the responses of CBF or MVO2 to pacing tachycardia in the control dogs, but in the HF dogs, it reduced the CBF response to pacing tachycardia without altering the tachycardia-induced changes in MVO2. Intracoronary infusion of L-arginine reversed the effect of L-NAME. These results suggest that in HF dogs NO contributes to the regulation of CBF in response to an increased metabolic demand.

Role of pertussis toxin-sensitive G protein in metabolic vasodilation of coronary microcirculation

We have previously demonstrated that pertussis toxin (PTX)-sensitive G protein (G(PTX)) plays a major role in coronary microvascular vasomotion during hypoperfusion. We aimed to elucidate the role of G(PTX) during increasing metabolic demand. In 18 mongrel dogs, coronary arteriolar diameters were measured by fluorescence microangiography using a floating objective. Myocardial oxygen consumption (MVO(2)) was increased by rapid left atrial pacing. In six dogs, PTX (300 ng/ml) was superfused onto the heart surface for 2 h to locally block G(PTX). In eight dogs, the vehicle (Krebs solution) was superfused in the same way. Before and after each treatment, the diameters were measured during control (130 beats/min) and rapid pacing (260 beats/min) in each group. Metabolic stimulation before and after the vehicle treatment caused 8.6 +/- 1. 8 and 16.1 +/- 3.6% dilation of coronary arterioles <100 microm in diameter (57 +/- 8 microm at control, n = 10), respectively. PTX treatment clearly abolished the dilation of arterioles (12.8 +/- 2. 5% before and 0.9 +/- 1.6% after the treatment, P < 0.001 vs. vehicle; 66 +/- 8 microm at control, n = 11) in response to metabolic stimulation. The increases in MVO(2) and coronary flow velocity were comparable between the vehicle and PTX groups. In four dogs, 8-phenyltheophylline (10 microM, superfusion for 30 min) did not affect the metabolic dilation of arterioles (15.3 +/- 2.0% before and 16.4 +/- 3.8% after treatment; 84.3 +/- 11.0 microm at control, n = 8). Thus we conclude that G(PTX) plays a major role in regulating the coronary microvascular tone during active hyperemia, and adenosine does not contribute to metabolic vasodilation via G(PTX) activation.

Functional characteristics of the coronary microcirculation

For over 50 years, it has been recognized that coronary blood flow is precisely matched to cardiac metabolism. The interactions which govern this matching remain unknown. In the current review, 3 specific aspects of coronary flow regulation will be discussed: Specialization of function in different microvascular domains, influence of cardiac region on microvascular function and the interactions of vasoactive agents in control of coronary blood flow. Each level of the coronary microcirculation is affected by different physical and chemical forces within the heart. These forces place special demands on these vessels and are in turn met by specialized vasodilator responses, including metabolic and flow-mediated vasodilation. Perfusion of the heart is also profoundly affected by the region perfused. The endocardium is affected by forces, notably cardiac contraction, in a different manner than the epicardium. Thus, the microcirculation has specialized to meet these demands. Finally, the factors determining microvascular tone appear to be coordinated such that the loss of any individual dilator, such as nitric oxide, can be compensated for by the increased contribution of another, such as adenosine. This interplay may serve to protect the heart from ischemia during the early phases of coronary vascular disease when individual dilators may be impaired.

Impact of coronary risk factors on contribution of nitric oxide and adenosine to metabolic coronary vasodilation in humans

OBJECTIVES

The contribution of nitric oxide (NO) and adenosine to the increase in coronary blood flow (CBF) induced by cardiac pacing was investigated in 28 subjects with angiographically normal coronary arteries with and without one or more risk factors for atherosclerosis.

BACKGROUND

NO and adenosine are important in the regulation of coronary circulation, and the inhibition of NO synthesis increases adenosine production during cardiac pacing in experimental models.

METHODS

Coronary artery diameters and CBF were assessed by quantitative coronary arteriography and Doppler flow velocity measurement. Plasma levels of nitrites and nitrates (NOx) (stable end products of NO), adenosine and lactate were measured, and blood gas analysis was performed.

RESULTS

The extent of CBF response to cardiac pacing did not differ between the 14 subjects with and the 8 subjects without risk factors for atherosclerosis. NOx (12.0+/-0.9 vs. 14.9+/-1.1 ,amol/liter [mean+/-SD], p < 0.05), but not adenosine (50.8+/-7.2 vs. 50.8+/-6.5 nmol/liter), levels in coronary sinus blood increased in the subjects without risk factors. In contrast, adenosine (58.9+/-7.5 vs. 77.4+/-9.8 nmol/liter, p < 0.05), but not NOx (11.1+/-1.1 vs. 12.2+/-1.1 micromol/liter), levels increased in subjects with risk factors. Aminophylline, an antagonist of adenosine receptors, blunted CBF response to cardiac pacing in six subjects with risk factors. The number of risk factors showed a negative correlation (p < 0.05) with NOx production and a positive correlation (p < 0.05) with adenosine production during cardiac pacing, respectively.

CONCLUSIONS

NO and adenosine are increased during metabolic coronary vasodilation induced by cardiac pacing. Adenosine production may be a compensatory mechanism when NO production is reduced.

Hypoxic dilatation of porcine small coronary arteries: role of endothelium and KATP-channels

1. The aim of the present study was to determine the cellular mechanims and potential mediators involved in hypoxic dilatation of porcine small coronary arteries. 2. Small coronary arteries were isolated from a branch of the left anterior descending artery of porcine hearts, cannulated with glass micropipettes and studied in a perfusion myograph system. At a transmural pressure of 40 mmHg, the arteries had an internal diameter of 167.8 +/- 6.6 microns (n = 37). 3. In arteries contracted with acetylcholine (ACh), hypoxia (0% O2, 30 min) caused dilatation (86.9 +/- 6.7% relaxation, n = 6) in vessels with endothelium but constriction in endothelium-denuded vessels. 4. Hypoxic vasodilatation occurring in arteries with endothelium was abolished by the KATP channel inhibitor, glibenclamide (0.44 microM), but was not affected by inhibition of nitric oxide synthase (L-NAME, 44 microM) or cyclo-oxygenase (indomethacin, 4.4 microM). 5. Bradykinin evoked endothelium-dependent relaxation that was inhibited by L-NAME (44 microM) but not glibenclamide 0.44 microM). Cromakalim (0.1-0.3 microM), a KATP channel opener, caused relaxation that was inhibited by glibenclamide, but was not affected by L-NAME (44 microM) and/or indomethacin (4.4 microM). 6. Endothelium-removal inhibited vasodilatation evoked by cromakalim, but increased vasodilator responses to the NO donor, SIN-1 (10(-8) to 10(-5) M). 7. These results indicate that hypoxia acted directly on vascular smooth muscle of small coronary arteries to cause contraction. However, this effect was overwhelmed by endothelium-dependent relaxation in response to hypoxia. This relaxation was most likely mediated by release of an endothelium-derived factor, distinct from nitric oxide or prostacyclin, that activated smooth muscle KATP-channels.

Role of endothelium-derived nitric oxide in coronary vasodilatation induced by pacing tachycardia in humans

Endothelium-derived NO contributes to the control of coronary perfusion. We investigated the roles of NO in the metabolic coronary vasodilatation induced by rapid pacing in humans. We evaluated the dilatation of large epicardial and resistance coronary arteries during rapid atrial pacing before and after intracoronary infusion of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthesis, in 19 patients without significant coronary artery disease. The diameter of the large epicardial coronary artery and coronary blood flow (CBF) were assessed by quantitative coronary arteriography and by a Doppler flow velocity measurement. An increase in the heart rate increased CBF (P < .01) and the coronary artery diameter (P < .05). L-NMMA at a total dose of 200 mumol reduced basal CBF but did not significantly affect basal coronary artery diameter, arterial pressure, or heart rate. L-NMMA inhibited the pacing-induced dilatation of the large coronary arteries (P < .05) but did not affect pacing-induced increases in CBF. L-NMMA inhibited the acetylcholine-induced increase in CBF (P < .01) and acetylcholine-induced dilatation of the large epicardial coronary artery (P < .05). These results show that the contribution of NO to the metabolic vasodilatation during rapid pacing may differ between large epicardial and resistance coronary arteries in patients without significant coronary artery disease.