Phosphatidylinositol metabolism in rat hepatocytes stimulated by vasopressin

In isolated rat hepatocytes, vasopressin evoked a large increase in the incorporation of [³²P]P(i) into phosphatidylinositol, accompanied by smaller increases in the incorporation of [1-¹⁴C]oleate and [U-¹⁴C]glycerol. Incorporation of these precursors into the other major phospholipids was unchanged during vasopressin treatment. Vasopressin also promoted phosphatidylinositol breakdown in hepatocytes. Half-maximum effects on phosphatidylinositol breakdown and on phosphatidylinositol labelling occurred at about 5 nM-vasopressin, a concentration at which approximately half of the hepatic vasopressin receptors are occupied but which is much greater than is needed to produce half-maximal activation of glycogen phosphorylase. Insulin did not change the incorporation of [³²P]P(i) into the phospholipids of hepatocytes and it had no effect on the response to vasopressin. Although the incorporation of [³²P]P(i) into hepatocyte lipids was decreased when cells were incubated in a Ca²⁺-free medium, vasopressin still provoked a substantial stimulation of phosphatidylinositol labelling under these conditions. Studies with the antagonist [1-(β-mercapto-β,β-cyclopentamethylenepropionic acid),8-arginine]vasopressin indicated that the hepatic vasopressin receptors that control phosphatidylinositol metabolism are similar to those that mediate the vasopressor response in vivo. When prelabelled hepatocytes were stimulated for 5 min and then subjected to subcellular fractionation, the decrease in [³H]phosphatidylinositol content in each cell fraction was approximately in proportion to its original phosphatidylinositol content. This may be a consequence of phosphatidylinositol breakdown at a single site, followed by rapid phosphatidylinositol exchange between membranes leading to re-establishment of an equilibrium distribution.