Endocardial endothelium in the avascular frog heart: role for diffusion of NO in control of cardiac O2 consumption

Abstract

We investigated the role of nitric oxide (NO) in the control of myocardial O2 consumption in the hearts of female Xenopus frogs, which lack a coronary vascular endothelium and in which the endocardial endothelium is the only source of NO to regulate cardiac myocyte function. Hence, frogs are an ideal model in which to explore the role of diffusion of NO from the endocardial endothelium (EE) without vascular endothelial or cardiac cell NO production. In Xenopus hearts we examined the regulation of cardiac O2 consumption in vitro at 25°C and 37°C. The NO-mediated control of O2 consumption by bradykinin or carbachol was significantly ( P < 0.05) lower at 25°C (79 ± 13 or 73 ± 11 nmol/min) than at 37°C (159 ± 26 or 201 ± 13 nmol/min). The response to the NO donor S-nitroso- N-acetyl penicillamine was also markedly lower at 25°C (90 ± 8 nmol/min) compared with 37°C (218 ± 15 nmol/min). When Triton X-100 was perfused into hearts, the inhibition of myocardial O2 consumption by bradykinin (18 ± 2 nmol/min) or carbachol (29 ± 4 nmol/min) was abolished. Hematoxylin and eosin slides of Triton X-100-perfused heart tissue confirmed the absence of the EE. Although endothelial NO synthase protein levels were decreased to a variable degree in the Triton X-100-perfused heart, NO2 production (indicating eNOS activity) decreased by >80%. It appears that the EE of the frog heart is the sole source of NO to regulate myocyte O2 consumption. When these cells are removed, the ability of NO to regulate O2 consumption is severely limited. Thus our results suggest that the EE produces enough NO, which diffuses from the EE to cardiac myocytes, to regulate myocardial O2 consumption. Because of the close proximity of the EE to underlying myocytes, NO can diffuse over a distance and act as a messenger between the EE and the rest of the heart to control mitochondrial function and O2 consumption.