Vasopressin stimulates sodium transport in A6 cells via a phosphatidylinositide 3-kinase-dependent pathway

The enzyme phosphatidylinositide 3-kinase (PI3K) phosphorylates the D-3 position of the inositol ring of inositol phospholipids and produces 3-phosphorylated inositides. These novel second messengers are thought to mediate diverse cellular signaling functions. The fungal metabolite wortmannin covalently binds to PI3K and selectively inhibits its activity. The role of PI3K in basal and hormone-stimulated transepithelial sodium transport was examined using this specific inhibitor. Wortmannin, 50 nM, did not affect basal, aldosterone-stimulated, or insulin-stimulated transport in A6 cells. Wortmannin completely inhibits vasopressin stimulation of transport in these cells. Vasopressin stimulates PI3K activity in A6 cells. Vasopressin stimulation of transport is also blocked by 5 mu M LY-294002, a second inhibitor of PI3K. One-hour preincubation with wortmannin blocked vasopressin stimulation of protein kinase A activity in the cells. Sodium transport responses to exogenous cAMP and forskolin, which directly activates adenylate cyclase, were not affected by wortmannin. These results indicate that wortmannin inhibits vasopressin stimulation of Na(+) transport at a site proximal to activation of adenylate cyclase. The results suggest that PI3K may be involved in receptor activation by vasopressin.