Localization and quantification of angiotensin II (A II) binding sites in the kidney of Xenopus laevis--lack of A II receptors in the adrenal tissue.

The distribution and properties of angiotensin binding sites in the kidney of the clawed toad Xenopus laevis were studied using quantitative in vitro autoradiography. Specific binding sites for [125I]-[Val5]-angiotensin II (A II) were located in the glomeruli of the kidney but not in the adrenal tissue. [125I]-[Val5]-A II binding was equilibrated after 45 min. Scatchard and Hill analyses of saturation experiments showed that [125I]-[Val5]-A II binds to a single class of binding sites with a dissociation constant (Kd) of 1.884 +/- 0.535 nM and a maximum binding capacity (Bmax) of 0.484 +/- 0.144 fmol/mm2 (n = 8). Various angiotensin analogues displaced [125I]-[Val5]-A II in the rank order [Sar1, Ile5]-A II greater than human A II greater than [125I]-[Val5]-A II = [Val5]-A II = human A III much greater than human A I. Unrelated peptides did not alter the binding of [125I]-[Val5]-A II. Acclimation to 1.5% seawater increased [125I]-[Val5]-A II binding in glomeruli after 12 hr but returned to control levels after 7 days. Steroidogenic and catecholaminergic actions of [Val5]-A II on the adrenal tissue were examined in vitro and in vivo. Compared with known interrenal stimulators [human ACTH(1-39) and AVT] minimal effects were obtained only in vitro with high doses of [Val5]-A II while catecholamine release was unaffected. In vivo a single injection of 3 nmol [Val5]-A II per 100 g body wt did not change serum levels of corticosterone, aldosterone, epinephrine, norepinephrine, or dopamine.