Phorbol ester effect on the hydrosmotic response to vasopressin in frog skin

Serosal preincubation of frog skin with tetradecanoyl phorbol acetate, TPA, an activator of protein kinase C, inhibits the hydrosmotic response elicited by vasopressin (AVP) but not that induced by 8br-cAMP. This proves that serosal TPA primarily influences a pre-cAMP step. The TPA-induced inhibition of AVP response appears to be related to TPA-induced prostaglandin synthesis. The pretreatment with naproxen, in fact, prevents the inhibition induced by serosal TPA on the AVP response. On the contrary, mucosal TPA produces a more marked inhibition of the response to AVP and significantly diminishes the water flow induced by 8br-cAMP; this suggests that mucosal TPA interferes mainly with a post-cAMP step. Furthermore, naproxen is unable to completely prevent the inhibition induced by mucosal TPA on AVP response thus indicating that mucosal TPA may also activate a prostaglandin-independent mechanism able to inhibit one of the last steps of the hydrosmotic response to AVP.