| Abstract: | Guanine nucleotides were observed to modify the binding of 125I-angiotensin II to rat hepatic plasma membrane receptors. GTP and its nonhydrolyzable analogues greatly increased the dissociation rate of bound 125I-angiotensin II and altered hormone binding to the receptor under equilibrium conditions. In the absence of GTP, 125I-angiotensin II labeled both high affinity sites (Kd1 = 0.46 nM, N1 = 650 fmol/mg) and low affinity sites (Kd2 = 4.1 nM, N2 = 1740 fmol/mg). In the presence of guanine nucleotides, the affinities of the two sites were unchanged, but the number of high affinity sites decreased markedly to 52 fmol/mg. In analogous experiments using the angiotensin II antagonist, 125I-sarcosine1,Ala8-angiotensin II (125I-saralasin), guanine nucleotides minimally affected the interaction of 125I-saralasin with its receptor, increasing the dissociation rate 1.9-fold and the Kd 1.4-fold. The guanine nucleotide inhibition of agonist binding required a cation such as Na+ or Mg2+, with a maximal effect occurring at about 1 mM Mg2+. In liver plasma membranes prepared in EDTA, angiotensin II inhibited basal and glucagon-stimulated adenylate cyclase activities by 30% and 10%, respectively. Angiotensin II also caused a 40% inhibition of glucagon-stimulated cyclic AMP accumulation in intact hepatocytes, with a half-maximal effect occurring at 1 nM. The inhibition by angiotensin II of adenylate cyclase in membranes and of cAMP levels in intact cells could be reversed by the antagonist sarcosine1,Ile8-angiotensin II. Vasopressin caused a smaller 26% inhibition of glucagon-stimulated cyclic AMP accumulation. The ability of angiotensin II to inhibit cyclic AMP synthesis may provide an explanation for the observed effects of guanine nucleotides on 125I-angiotensin II binding to plasma membranes. |
