Endocardial endothelial morphology and the physiological modulatory role of nitric oxide (NO) were studied in an in vitro preparation of the working intact heart of the frog Rana esculenta, which lacks coronary vasculature and is thus devoid of a coronary vascular endothelium. En face confocal scanning laser microscopy of samples of perfused fixed hearts demonstrated the presence of NO synthase as a cytoplasmic constituent of the endocardial endothelial cells. Stroke volume (as a measure of performance in paced frog hearts) and stroke work (as an index of systolic function) increased by approximately 5 % after inhibition of the NO-cGMP pathway with 10(-4 )mol l-1 NG-nitro-l-arginine methyl ester and by approximately 8 % after inhibition with 10(-6 )mol l-1 Methylene Blue. In contrast, stroke volume and stroke work decreased by approximately 22 % after activation of the NO-cGMP pathway with sodium nitroprusside (10(-4 )mol l-1), while 3-morpholinosydnonimine (5x10(-8) to 10(-5 )mol l-1) caused a decrease of between 15 and 30 % and 8-bromo-cGMP (10(-6 )mol l-1) a decrease of approximately 8 %. These responses were significantly attenuated after exposure of the ventricular luminal to Triton X-100 (0.05 %, 0.1 ml), which itself increased performance (by over 10 %) without detectable morphological changes. These results show that the endocardial endothelium of Rana esculenta produces amounts of NO sufficient to modulate ventricular performance.

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