ENDOCARDIAL ENDOTHELIUM IN THE AVASCULAR HEART OF THE FROG - MORPHOLOGY AND ROLE OF NITRIC-OXIDE

Endocardial endothelial morphology and the physiological modulatory ro le 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, fi t face confocal scanning laser microscopy of samples of perfused fixed hearts demonstrated the presence of NO synthase as a cytoplasmic cons tituent of the endocardial endothelial cells, Stroke volume (as a meas ure of performance in paced frog hearts) and stroke work (as an index of systolic function) increased by approximately 5% after inhibition o f the NO-cGMP pathway with 10(-4)moll(-1) N-G-nitro-L-arginine methyl ester and by approximately 8% after inhibition with 10(-6)moll(-1) Met hylene Blue, In contrast, stroke volume and stroke work decreased by a pproximately 22 % after activation of the NO-cGMP pathway with sodium nitroprusside (10(-4)moll(-1)), while 3-morpholinosydnonimine (5x10(-8 ) to 10(-5)moll(-1)) caused a decrease of between 15 and 30 % and 8-br omo-cGMP (10(-6)moll(-1)) a decrease of approximately 8%, These respon ses were significantly attenuated after exposure of the ventricular lu minal to Triton X-100 (0.05 %, 0.1 ml), which itself increased perform ance (by over 10%) without detectable morphological changes. These res ults show that the endocardial endothelium of Rana esculenta produces amounts of NO sufficient to modulate ventricular performance.