ROLE OF THE ATRIAL-NATRIURETIC-PEPTIDE FOR ADRENAL REGULATION IN THE TELEOST FISH CYPRINUS-CARPIO

The biological role of atrial natriuretic peptide (ANP) for the adrena l tissue of the teleost fish Cyprinus carpio, the carp, was investigat ed using in vitro autoradiography to study ANP binding sites, immunohi stochemistry to localize ANP-like peptides, and in vitro perifusion to measure adrenal cortisol and catecholamine release. I-125-labeled rat ANP-(99-126) [I-125-rANP-(99-126)] binding was present in adrenal tis sue exhibiting positive cooperativity with a half-maximal binding conc entration (EC(50)) of 87 +/- 15 pM and a maximal binding capacity (B-m ax) of 56 +/- 11 amol/mm(2) (n = 3). I-125-rANP-(99-126) binding was c ompetitively displaced by unlabeled ANP analogues with an intact disul fide bridge showing a lower affinity than the iodinated ligand. By the use of immunohistochemistry, ANP immunoreactivity was found in the ma jority of the phenylethanolamine N-methyl-transferase-immunoreactive, i.e., epinephrine-synthesizing cells. In vitro, ANP elevated both basa l and acetylcholine-stimulated cortisol release by carp adrenal tissue , whereas adrenal catecholamines were significantly affected only in a cetylcholine-stimulated epinephrine secretion. Thus ANP derived from t he ANP-immunoreactive chromaffin cells of the carp adrenal seems to ac t in an autocrine and/or paracrine manner on steroidogenic interrenal and catecholaminergic chromaffin cells. Furthermore, the study indicat es that the ANP effects on fish catecholamine and steroid release are reversed in mammals.