DIFFERENT CALCIUM-MOBILIZING RECEPTORS SHARE THE SAME GUANINE NUCLEOTIDE-BINDING PROTEIN POOL IN HEPATOCYTES

High affinity binding of epinephine to the alpha-1-adrenoceptor reflects the association of the ligand-receptor complex with a guanine nucleotide-binding protein (G protein) and thereby allows the receptor-G protein interaction to be assessed by radioligand binding methods. We have used [H-3]prazosin/epinephrine competition binding to rat liver plasma membranes to examine the effects of other Ca2+-mobilizing hormones on the interaction between the alpha-1-adrenoceptor and its G protein. The aim of our experiments was to test whether the different Ca2+-mobilizing receptors in liver share the same limited pool of G proteins. [Arg8] Vasopressin (AVP) caused a concentration-dependent (EC50 = 0.49 +/- 0.03 nM) inhibition of the extent to which epinephrine formed a high affinity complex with the alpha-1-adrenoceptor; antagonist binding was unaffected by AVP. The effect of AVP was competitively antagonized (K(d) = 0.27 +/- 0.10 nM) by a selective peptide antagonist of the V1 vasopressin receptor. We conclude that, in rat hepatocytes, alpha-1-adrenoceptors and V1 vasopressin receptors converge to interact with the same pool of G proteins.