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 > human A II > [125I]-[Val5]-A II = [Val5]-A II = human A III ≫ 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. © 1992.