The liver is a key organ for the clearing of many hormones from the circulatory system (e.g., epidermal growth factor, insulin, vasopressin). In the normal adult rat liver, these hormones do not act as activators of cell growth. During liver regeneration however, when the liver is capable of intense bursts of proliferation, or during the growth of primary hepatocytes in culture, these hormones become potent growth activators. We have focused on the V1 vasopressin receptor system in liver, which upon ligand binding, has been shown to activate the turnover of phosphoinositides. Although much of the work characterizing this turnover has been performed utilizing the V1 system, neither the V1 receptor nor its associated guanine- nucleotide binding proteins (G-proteins) have been isolated. We have recently purified the V1 vasopressin receptor from liver and have begun to characterize and purify the G-proteins associated with this receptor. In addition, we have reconstituted the V1 receptor and associated G-proteins, and have demonstrated that the V1 receptor from liver but not from smooth muscle must be exposed to ligand in order to cause an association of the V1 receptor with G-proteins. Additionally, this results in a change in the V1 receptor from a low affinity state (Kd = 30 nM) to a high affinity state (Kd = 6 nM). We have also observed that a number of other endogenous macromolecules, including gangliosides and phospholipids, have a significant effect on the affinity of the V1 receptor for ligand. We propose to purify the V1-associated G- proteins, study the interaction of the G-proteins with the V1 receptors, and investigate the role of phospholipids, gangliosides, and ions in this interaction. In addition, we propose to produce an antibody directed against the V1 receptor, allowing us to study the turnover and processing of the V1 receptor.
