Characterisation of Angiotensin II Binding to Glomeruli from Rainbow Trout Oncorhynchus mykiss Adapted to Fresh Water and Seawater

Specific binding of 1251-[Asn1, Val5]-angiotensin II to isolated glomeruli of freshwater-adapted and seawater-adapted rainbow trout, Oncorhynchus mykiss, reached a plateau after 20 min at 10° and was displaced by nonradiolabelled [Asn1, Val5]-angiotensin II. Seawater adaptation increased the rate of dissociation of 1251-[Asn1, Val5]-angiotensin II from putative receptors. Competitive binding experiments revealed a single population of high-affinity receptors with a KD 0.3-0.4 × 10-10M, which was unchanged by seawater adaptation. Seawater adaptation did, however, reduce the density (Bmax) of the binding sites, which may reflect down-regulation by the higher plasma concentration of angiotensin II in seawater-adapted trout.