Characterization of the angiotensin II receptor in primary cultures of rat hepatocytes. Evidence that a single population is coupled to two different responses

The angiotensin II receptor of cultured rat hepatocytes was characterized using [3H]angiotensin II as radioligand. Binding at 23 degrees C was rapid (t1/2 = 0.65 min) with equilibrium being reached in 10-12 min. At this time, binding was completely reversible after 20 min (t1/2 = 3.5 min), indicating negligible internalization of the ligand. Analysis of the saturation binding curve showed one population of binding sites with an apparent KD of 8.6 nM and a Bmax of 35 fmol/mg of protein. The time courses of association and dissociation were also consistent with one class of binding sites with an apparent kinetically derived KD of 7.7 nM. The order of potency of different agonists and antagonists to increase cytosolic Ca2+ or phosphorylase a or inhibit the effects of angiotensin II on these parameters was the same as for their mimicry or reversal of angiotensin II inhibition of glucagon-induced cAMP accumulation, and was well correlated with their order of potency to inhibit angiotensin II specific binding. Treatment of cultured hepatocytes with dithiothreitol caused a time- and concentration-dependent inhibition of angiotensin II binding and corresponding alterations of angiotensin II effects on phosphorylase and cAMP. It also inhibited the actions of other hormones on phosphorylase. These results indicate that hepatocytes contain a homogeneous population of angiotensin II receptors that are coupled to two different biological effects apparently mediated by different G-proteins.