Phorbol ester and A23187 have additive but mechanistically separate effects on vasopressin action in rabbit collecting tubule.

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Abstract
Activation of protein kinase C (PKC) and elevation of intracellular calcium ion concentration ([Ca++]i) result from phosphatidylinositol biphosphate (PIP2) breakdown. We previously demonstrated that PKC activation inhibits arginine vasopressin (AVP)-induced osmotic water flow in rabbit cortical collecting tubule (CCT) perfused in vitro at 37 degrees C. To estimate the potential significance of PIP2 turnover as a modulator of water transport in this nephron segment, we examined the effect of Ca on AVP action and explored the mechanisms of action of PKC and increased [Ca++]i. In rabbit CCTs perfused at 37 degrees C, pretreatment with bath A23187 (2 x 10(-8) M, 2 x 10(-6) M), a Ca ionophore, almost totally suppressed AVP (10 microU/ml)-induced peak hydraulic conductivity (Lp). The suppression by 2 x 10(-8) M A23187 was as potent as that by 2 x 10(-6) M A23187, and significant even when it was administered 10 min after AVP. When phorbol myristate acetate (PMA, 10(-9) M), a PKC activator, and A23187 (2 x 10(-8) M) were placed in the bath simultaneously, the combined suppressive effect on peak Lp was greater than that of either inhibitor alone. However, the mechanisms of inhibition by PMA and A23187 were different. While both 10(-7) and 10(-9) M PMA suppression are primarily post-cAMP, A23187 predominantly suppressed a pre-cAMP step: 10(-4) M chlorophenylthio-cAMP-induced peak Lp was not affected by 2 x 10(-8) M A23187, and only partially inhibited by 2 x 10(-6) M A23187. The PMA (10(-7) M) suppression of AVP-induced peak Lp was totally reversed by bath staurosporine (10(-7) M), a PKC inhibitor, but not attenuated by either bath indomethacin (5 x 10(-6) M) or low Ca (1-2 x 10(-6) M) bath medium. In contrast, the A23187 (2 x 10(-8) M) suppression of the peak Lp was not affected by staurosporine, but was significantly reversed by indomethacin or low Ca bath medium. We conclude: (a) Elevation of [Ca++]i, as well as activation of PKC, suppresses the hydroosmotic effect of AVP on CCT at 37 degrees C. (b) When stimulated simultaneously these two intracellular mediators are additive in their antagonism of AVP action. These results suggest that stimulated PIP2 breakdown may be an important modulator of water transport in CCT. (c) Different mechanisms underlie PKC and Ca-mediated suppression of the AVP-induced water transport. The inhibition of AVP action by increased [Ca++]i is primarily pre-cAMP, and involves a cyclooxygenase metabolite(s) of arachidonic acid, while the inhibition by PKC is post-cAMP, and independent of cyclooxygenase products of arachidonic acid.