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 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°C and 37°C. The NO-mediated control of O2 consumption by bradykinin or carbachol was significantly (P Ͻ 0.05) lower at 25°C (79 Ϯ 13 or 73 Ϯ 11 nmol/min) than at 37°C (159 Ϯ 26 or 201 Ϯ 13 nmol/min). The response to the NO donor S-nitroso-Nacetyl penicillamine was also markedly lower at 25°C (90 Ϯ 8 nmol/min) compared with 37°C (218 Ϯ 15 nmol/min). When Triton X-100 was perfused into hearts, the inhibition of myocardial O 2 consumption by bradykinin (18 Ϯ 2 nmol/min) or carbachol (29 Ϯ 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 Ͼ80%. 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 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 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. nitric oxide; Western blot analysis; Triton X-100; mitochondria; Griess reaction THE ENDOCARDIAL ENDOTHELIUM (EE) plays a vital role in the regulation of myocardial performance and electrical activity as recently reviewed by Brutsaert (3). Located between luminal blood and the underlying cardiac muscle, the EE synthesizes and releases nitric oxide (NO) in amounts sufficient to regulate myocardial contraction under basal conditions (4,24). NO is also known to regulate O 2 consumption under a variety of conditions (2,12,15,22,25). This regulation of myocardial metabolism may be one of NO's most significant roles. We have proposed that endothelial NO synthase (eNOS), which is the most highly expressed isoform of NOS in vascular tissue under physiological conditions, contributes to the control of tissue O 2 consumption by NO (16,25).As a model to determine the significance of the diffusion of NO in the control of cardiac cell O 2 consumption, the frog heart is of much interest because it has no coronary circulation and can therefore be used to study the interaction between the EE and the myocardium without interference from the vascular endothelium (...

Lysozyme, a mediator of sepsis that intrinsically generates hydrogen peroxide to cause cardiovascular dysfunction

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Role of Oxidative-Nitrosative Stress and Downstream Pathways in Various Forms of Cardiomyopathy and Heart Failure

“…NO produced by Ca 2+ -dependent NOS in endothelium, in cardiomyocytes and in cardiac nerves serves a number of important roles in the regulation of cardiac function including coronary vasodilation, inhibiting platelet and neutrophil actions, antioxidant effects, modulation of cardiac contractile function, and inhibiting cardiac oxygen consumption [ 3,7,[50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67]. In excess concentrations, NO could be potentially toxic.…”

TELOCYTES IN ENDOCARDIUM: Electron Microscope Evidence

“…The role of telocytes network is still speculative: heterocellular signalling, cardiac renewing, immune surveillance and/or mechanotransduction [11, 12, 19]. We cannot overlook the fact that endocardium is, at least conceptually, something like blood–brain barrier, mainly a ‘blood–heart barrier’ or a ‘blood–myocardium barrier’[29]. Apparently, telocytes, which seem to be the main population in the sub‐epithelial layer of endocardium, might have a role in this ‘blood–myocardium barrier’.…”