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

We investigated the role of nitric oxide (NO) in the control of myocardial O(2) 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 O(2) consumption in vitro at 25 degrees C and 37 degrees C. The NO-mediated control of O(2) consumption by bradykinin or carbachol was significantly (P < 0.05) lower at 25 degrees C (79 +/- 13 or 73 +/- 11 nmol/min) than at 37 degrees 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 degrees C (90 +/- 8 nmol/min) compared with 37 degrees C (218 +/- 15 nmol/min). When Triton X-100 was perfused into hearts, the inhibition of myocardial O(2) 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, NO(2) 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 O(2) consumption. When these cells are removed, the ability of NO to regulate O(2) 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 O(2) 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 O(2) consumption.