W e explored the role for protein kinase C (PKC) in modulating vasopressin (AVP)-stimulated hydraulic conductivity (Lp) in rabbit cortical collecting tubule (CCT) perfused in vitro at 37 degrees C. In control studies, 10 microU/ml AVP increased Lp (mean +/- SE, X 10(-7) centimeters/atmosphere per second) from 4.4 +/- 0.9 to 166.0 +/- 10.4. Pretreatment with dioctanoylglycerol (DiC8) suppressed AVP stimulated peak Lp (peak Lp, 21.9 +/- 3.1). Pretreatment with 10(-9) and 10(-7) M 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) also blocked the increase in Lp in a dose-dependent fashion (peak Lp, 59.3 +/- 7.5 and 18.6 +/- 4.8, respectively). Inactive phorbol ester, 4 alpha-phorbol 12 beta,13 alpha-didecanoate (10(-7) M), had no effect. PMA also suppressed the increase in Lp induced by 10(-4) M 8-p-chlorophenylthio-cyclic AMP (CcAMP): peak Lp was 169.4 +/- 14.9 in control, 79.2 +/- 5.5 with 10(-9) M PMA, and 25.7 +/- 2.9 with 10(-7) M PMA. Furthermore, when 10(-7) M PMA was added to the bath 10 min after exposure to AVP, the Lp response to AVP was blocked. Peak Lp was 52.4 +/- 9.6 with PMA vs. 165.1 +/- 10.0 in control. Phosphatidic acid (PA), which is thought to stimulate phosphatidylinositol (PI) turnover, produced similar inhibitory effects on AVP as well as CcAMP-stimulated Lp: PA suppressed 10-microU/ml AVP-induced peak Lp from a control value of 159.6 +/- 7.9 to 88.9 +/- 15.8, and 10(-4) M CcAMP induced peak Lp from 169.4 +/- 14.9 to 95.5 +/- 7.7. We conclude that PMA, at concentrations known to specifically activate PKC, suppresses the hydroosmotic effect of AVP on CCT; This suppression is primarily a post-cAMP event; Inhibition of AVP-stimulated Lp by DiC8 and PA also suggests an inhibitory role for the PKC system; The ability of pre- and post-AVP administration of PMA to blunt the AVP response suggests that agents that act through modulation of PI turnover in CCT may regulate the hydroosmotic effect of AVP.