Distribution and physiological function of atrial and brain natriuretic peptides in lower vertebrates.

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1. Atrial and brain natriuretic peptides (ANP, BNP).(1) The distribution of binding sites for ANP was studied by whole-body autoradiography in medaka, which was injected [ ^<125>I]-rat ANP intraperitoneally. High levels of radioactivity were found in the gill. ANP may act on the gill and play some roles, such as regulation of water and salt balance in fish. (2) By the double immunogold labeling, it was found that ANP and BNP coexist in the same secretory granules of atrial and ventricular cardiocytes in the representative animals of all vertebrate classes (two species each of teleosts and amphibians, and one species each of reptiles, birds and mammals). In most of animals examined, the immunoreactivity for ANP was stronger than that for BNP, but the opposite was true in the medaka and the soft-shelled turtle. Both peptides may be secreted simultaneously into circulation to play some coordinated functions. (3) Porcine BNP elicited a significant increase in water intake, when administered intraperitoneally (5 and 10 mug/bird) or intracerebroventricularly (0.3 mug/birds) in the water-replete Japanese quail. BNP may act on the brain to stimulatd drinking behavior.2. Endothelin (ET)(1) Immunoreactive ET was estimated in plasma of nonmammalian vertebrates (one species each of cyclostomes, elasmobranchs, amphibians, reptiles and birds and two species of teleosts, employing radioimmunoassay for ET-1. Levels of immunoreactive ET-1 were similar to those of human in all animals (mean : 3.3-9.6 pg/ml), except that it was rather low in the hagfish (0.7 pg/ml). These results suggest an endocrine function of the peptide in these species. (2) Immunoreactive ET-1 was found in the hypothalamo-hypophysial system of the representative animals of all vertebrate classes. Further, the double immunogold labeling revealed in rats that immunoreactive ET-1 coexisted with neurohypophysial hormones in the secretory granules of the vasopressinergic and oxytocinergic axons, respectively.