Interactions between Ca(2+)-mobilizing receptors and their G proteins in hepatocytes

In hepatocytes, different receptors that share an ability to stimulate inositol 1,4,5-trisphosphate formation by activating guanine nucleotide-binding proteins (G proteins) evoke Ca2+ signals that are characteristic of the receptor that evoked them. High affinity, guanine nucleotide-sensitive binding of agonists to their receptors provides a convenient means of assessing receptor-G protein interactions. We used these methods to examine the extent to which different Ca(2+)-mobilizing receptors share G protein pools. Although our earlier results (Dasso, L. L. T., and Taylor, C.W. (1992) Mol. Pharmacol. 42, 453-457) showed that activated V1-vasopressin receptors prevented alpha 1-adrenoreceptors from associating with G proteins, our present results show that neither alpha 1-adrenergic agonists nor angiotensin II influences the interaction between V1 receptors and their G proteins. This asymmetric scavenging of G proteins by alpha 1-adrenoreceptors and V1 receptors is not a consequence of there being more of the latter and may result either from V1 receptors binding more tightly to G proteins or from their ability to interact with a second G protein pool. We speculate that the different interactions among V1 receptors, alpha 1-adrenoreceptors, and their G proteins contribute to the differently shaped Ca2+ spikes evoked by these receptors.