PGE₂ reverses AVP inhibition of HCO₃^- absorption in rat MTAL by activation of protein kinase C

In the medullary thick ascending limb (MTAL) of the rat, prostaglandin E₂ (PGE₂) reverses inhibition of HCO₃^- absorption (J_HCO3) by arginine vasopressin (AVP) by inhibiting AVP-stimulated adenosine 3′, 5′-cyclic monophosphate (cAMP) production. To determine whether this regulation by PGE2 involves protein kinase C (PKC), MTAL segments were perfused in vitro with physiological solutions containing 25 mM HCO₃^- (pH 7.4). With 10^-10 M AVP in the bath, addition of 10^-6 M PGE₂ to the bath increased J_HCO3 from 7.8 ± 0.4 to 13.0 ± 1.1 pmol·min^-1·mm^-1 (P < 0.01). This effect was blocked completely by pretreatment with the PKC inhibitors staurosporine or chelerythrine chloride (10^-7 M in the bath). With both AVP and PGE₂ in the bath, addition of staurosporine or chelerythrine to the bath decreased J_HCO3 from 12.2 ± 1.1 to 7.3 ± 0.6 pmol·min^-1·mm^-1 (P < 0.005). Neither staurosporine nor chelerythrine affected J_HCO3 under basal conditions or in the presence of AVP alone. With AVP in the bath, addition of phorbol 12-myristate 13-acetate (PMA, 10^-6 M) to the bath increased J_HCO3 from 5.0 ± 0.5 to 9.1 ± 1.0 pmol·min^-1·mm^-1 (P < 0.01). Similar to PGE₂, PMA had no effect on J_HCO3 in the absence of AVP or in the presence of 10^-6 M bath forskolin. The effect of PMA to stimulate J_HCO3 in the presence of AVP was abolished by pretreatment with pertussis toxin (2 × 10^-11 M). We conclude that 1) PGE₂ reverses AVP inhibition of HCO₃^- absorption by activation of PKC, 2) PKC likely increases J_HCO3 by inhibiting AVP-stimulated cAMP production via a G_i-dependent mechanism, and 3) PKC activity has no influence on basal HCO₃^- absorption rate. adenosine 3′, 5′-cyclic monophosphate; signal transduction; phorbol ester;.