Role of Endothelium-Derived Nitric Oxide in the Modulation of Canine Myocardial Mitochondrial Respiration In Vitro

Abstract

The mechanism responsible for the regulation of cardiac function by endogenous nitric oxide (NO) remains unclear. In this investigation, O 2 consumption by freshly isolated myocardial muscle segments from the left ventricular free wall of canine hearts was quantified by a Clark-type O 2 electrode at 37°C. S -nitroso- N -acetylpenicillamine (SNAP, 9±3% to 50±8%), bradykinin (BK, 14±3% to 30±5%), or carbachol (CCh, 15±4% to 29±4%) significantly attenuated tissue O 2 consumption at doses of 10 −7 to 10 −4 mol/L (mean±SE, P <.05). The effects of BK and CCh, but not SNAP, were blocked by 10 −4 mol/L N G -nitro- l -arginine, consistent with both BK and CCh stimulating NO biosynthesis and with SNAP decomposing to release NO, respectively. Similar doses of 8-Br-cGMP caused a respiratory inhibition, but to a lesser extent (9±2% to 14±6%). A mitochondrial uncoupler, 2,4-dinitrophenol (at 1 mmol/L), blocked the effects of 8-Br-cGMP, but not those of SNAP, BK, or CCh, suggesting that the major site of action of NO is on mitochondrial electron transport. Myocardial muscle from dogs with pacing-induced heart failure had a basal O 2 consumption rate of 251±21 nmol·min −1 ·g −1 , which was 54% higher than the rate seen in muscle from normal healthy canine hearts. The inhibitory effects of BK and CCh on O 2 consumption were not observed in failing cardiac tissue, but SNAP showed an unaltered inhibitory effect. Therefore, our results indicate that NO released from microvascular endothelium by BK, stimulation of muscarinic receptors, and perhaps flow velocity may play an important physiological role in the control of cardiac mitochondrial respiration, and the loss of this regulatory function may contribute to the development of heart failure.