EVIDENCE THAT THE PERTUSSIS-TOXIN-SENSITIVE TRIMERIC GTP-BINDING PROTEIN G(I2) IS REQUIRED FOR AGONIST-ACTIVATED AND STORE-ACTIVATED CA2+ INFLOW IN HEPATOCYTES

The role of a trimeric GTP-binding protein (G-protein) in the mechanis m of vasopressin-dependent Ca2+ inflow in hepatocytes was investigated using both antibodies against the carboxyl termini of trimeric G-prot ein alpha subunits, and carboxyl-terminal alpha-subunit synthetic pept ides. An anti-G(i1-2 alpha) antibody and a G(i2 alpha) peptide (G(i2 a lpha) ILe(345)-Phe(355)), but not a Gi(3 alpha) peptide (G(i3 alpha) I le(344)-Phe(354)), inhibited vasopressin- and thapsigargin-stimulated Ca2+ inflow, had no effect on vasopressin-stimulated release of Ca2+ f rom intracellular stores, and caused partial inhibition of thapsigargi n-stimulated release of Ca2+. An anti-G(q alpha) antibody also inhibit ed vasopressin-stimulated Ca2+ inflow and partially inhibited vasopres sin-induced release of Ca2+ from intracellular stores. Immunofluoresce nce measurements showed that G(i2 alpha) is distributed throughout muc h of the interior of the hepatocyte as well as at the periphery of the cell. By contrast, G(q/11 alpha) was found principally at the cell pe riphery. It is concluded that the trimeric G-protein, G(i2) is require d for store-activated Ca2+ inflow in hepatocytes and acts between the release of Ca2+ from the endoplasmic reticulum (presumably adjacent to the plasma membrane) and the receptor-activated Ca2+ channel protein( s) in the plasma membrane.