Characterisation of angiotensin II binding to glomeruli from rainbow trout Oncorhynchus mykiss adapted to fresh water and seawater.

Specific binding of 125I-[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 degrees and was displaced by nonradiolabelled [Asn1, Val5]-angiotensin II. Seawater adaptation increased the rate of dissociation of 125I-[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 x 10(-10) M, 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.