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 mechanism of vasopressin-dependent Ca2+ inflow in hepatocytes was investigated using both antibodies against the carboxyl termini of trimeric G-protein alpha subunits, and carboxyl-terminal alpha-subunit synthetic peptides. An anti-G(i1-2 alpha) antibody and a G(i2 alpha) peptide (G(i2 alpha) ILe(345)-Phe(355)), but not a Gi(3 alpha) peptide (G(i3 alpha) Ile(344)-Phe(354)), inhibited vasopressin- and thapsigargin-stimulated Ca2+ inflow, had no effect on vasopressin-stimulated release of Ca2+ from intracellular stores, and caused partial inhibition of thapsigargin-stimulated release of Ca2+. An anti-G(q alpha) antibody also inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. Immunofluorescence measurements showed that G(i2 alpha) is distributed throughout much 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 periphery. It is concluded that the trimeric G-protein, G(i2) is required 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.