Ultrastructural and immunocytochemical study of the myoendocrine cells of the fish Hypostomus cordovae.

The myoendocrine cells of the heart of Hypostomus cordovae (Günther 1880), a teleost fish from South America, were investigated by electron microscopy and immunohistochemistry. By applying antibodies raised against synthetic cardiodilatin 99-126 (CDD/ANP 99-126), a specific labeling of this hormone was found in the heart of this fish, mainly in myoendocrine cells of atrial trabeculae, where specific secretory granules are stored. The distribution of secretory granules exhibited striking seasonal variations. In winter there were fewer differentiated myoendocrine cells, which were easily recognized by the presence of specific secretory granules, most of which occur clustered in perinuclear areas of the cells. By contrast, in summer the majority of the myocardic cells of the atrium are active endocrine cells. They contain abundant secretory granules widely scattered in the cytoplasm, many of them polarized toward the subendocardial aspect of the cell. The secretory granules can be easily differentiated from the Weibel-Palade granules of endothelial cells, the shape, size and content of which were typical at electron-microscopic level. In addition, these endothelial granules did not display CDD immunoreactivity. The presence of cardiodilatin in a fish such as Hypostomus cordovae further supports the view that cardiac hormones are present in many Vertebrates and may preserve analogous roles such as those reported in other species throughout the group.

Relaxation of non-contracted smooth muscle by atrial natriuretic peptide.

Atrial natriuretic peptide (alpha-hANP-99-126), 1 x 10(-8) to 5 x 10(-10) M was able to further relax the non-contracted aortic smooth muscle of rabbit after complete recovery from a previous challenge with human angiotensin II (AII), 1 x 10(-6) M. The relaxation was directly proportional to the response of the ring or strip to the previous challenge with AII. ANP does not have effect on basal tension in isolated strips or rings of the rabbit aorta not previously exposed to AII. Norepinephrine (NE), 10(-7) M was less potent in inducing reactivity of the vessel to ANP, ie, only a small relaxant effect on basal tension could be observed. A similar vasorelaxant effect of ANP on basal tension could be obtained in the absence of extracellular calcium: Ca(2+)-free Ringers' solution containing 2 mM ethylene-glycol-bis (beta-aminoethyl ether)N',N' tetraacetic acid (EGTA-Ringer). In contrast, sodium nitroprusside 10(-8) M does not affect basal tension. Present results demonstrate the role of the physiological state of the vessel in the reactivity to ANP.