RAPID DESENSITIZATION OF VASOPRESSIN-STIMULATED PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE AND PHOSPHATIDYLCHOLINE HYDROLYSIS QUESTIONS THE ROLE OF THESE PATHWAYS IN SUSTAINED DIACYLGLYCEROL FORMATION IN A10 VASCULAR-SMOOTH-MUSCLE CELLS

The kinetics of vasopressin-stimulated PtdIns(4,5)P2 and phosphatidylcholine (PtdCho) hydrolysis in relation to sustained diacylglycerol (DAG) formation was investigated in A10 vascular-smooth-muscle cells in culture. Vasopressin stimulated a transient increase in Ins(1,4,5)P3 mass formation, which was mirrored by a decrease in PtdIns(4,5)P2 mass levels. Vasopressin stimulated sustained accumulation of total [H-3]inositol phosphates ([H-3]IP) in the presence of Li+; however, this was significantly decreased by adding a vasopressin-receptor antagonist at different times after initial stimulation. Vasopressin-stimulated phospholipase D (PLD) activity was found to be a transient phenomenon lasting approx. 2 min. Experiments involving agonist preincubation with subsequent addition of butanol confirmed that vasopressin-stimulated PLD activity was desensitized. Vasopressin stimulated an increase in formation of choline, but not of phosphocholine, suggesting that PLD was the major catalytic route of PtdCho hydrolysis in this cell line. The roles of choline and inositol phospholipid hydrolysis in the prolonged phase of DAG formation was examined by comparing vasopressin-stimulated changes in DAG levels in the presence of butanol, the protein kinase C inhibitor Ro-31-8220 or a V1a-receptor antagonist. Vasopressin-stimulated DAG formation was decreased by 40-50 % in the presence of butanol between 1 and 10 min; however, during more prolonged stimulation butanol was without significant effect. In cells pretreated with Ro-31-8220, vasopressin-stimulated DAG formation was decreased by approx. 30 % at 2 min, but was significantly potentiated at later times. This coincided with an enhancement of vasopressin-stimulated [H-3]IP accumulation. In cells exposed to the V1a-receptor antagonist 5 min after addition of vasopressin, subsequent DAG formation was significantly decreased, indicating that sustained formation of DAG, like [H-3]IP accumulation, was dependent on continual agonist receptor activation. The results are discussed in terms of different phospholipid-hydrolytic pathways providing DAG generation.