Regulation of the transfected Na⁺/H⁺-exchanger NHE3 in MDCK cells by vasotocin

NHE3 is most likely the isoform involved in renal reabsorption of HCO₃^- and Na⁺. The functional properties of the 'cloned' NHE3 isoform, including its transport regulation by extra- and intracellular stimuli, have so far been studied using non-epithelial expression systems. In the present report we stably transfected NHE3 cDNA (rabbit isoform) into Madin-Darby canine kidney cells (MDCK) cells and compared the sensitivity to inhibitors and the regulation of the Na⁺/H⁺-exchanger by vasotocin in NHE3 transfectants to that of the intrinsic basolateral Na⁺/H⁺-exchanger in untransfected and control transfected MDCK cells. By Southern blot analysis we documented that the NHE3 transcript is expressed in NHE3 transfectants. Na⁺/H⁺-exchange activity, measured as sodium-dependent recovery of intracellular pH from an acid load using 2',7'-bis(carboxymethyl)-5(6)-carboxy-fluorescein (BCECF), was equally present at the basolateral cell surface of all cell lines; however, NHE3 transfectants demonstrated transport activity in the apical membrane that was significantly higher than that in untransfected or control transfected MDCK cells. Studies with ethylisopropylamiloride (EIPA) have shown that there is a similar sensitivity to inhibitors of the apical and/or basolateral Na⁺/H⁺-exchanger in transfected and untransfected MDCK cell lines. In contrast, the apical Na⁺/H⁺-exchanger (as compared to the basolateral Na⁺/H⁺-exchanger) of NHE3 transfectants was found to be relatively insensitive to the inhibitor HOE 694. Vasotocin decreased the activity of the apical Na⁺/H⁺-exchanger in NHE3 transfectants and stimulated the activity of the basolateral Na⁺/H⁺-exchanger in transfected (with NHE3 or pMAMneo) and untransfected MDCK cells. Phorbol ester, as expected, increased the activity of the Na⁺/H⁺-exchanger in the basolateral membrane of all cell lines; also, it stimulated transport activity at the apical cell surface of NHE3 transfectants. No change of Na⁺/H⁺-exchange activities was seen in studies with 8-bromo-cAMP. The PKC inhibitor calphostin C completely suppressed regulation of the apical and/or basolateral Na⁺/H⁺-exchanger by vasotocin, it partially blocked activation of the apical Na⁺/H⁺-exchanger in NHE3 transfectants by phorbol 12-myristate 13-acetate (PMA), and completely blocked stimulation of basolateral Na⁺/H⁺-exchanger by PMA. Consistent with a V₁ receptor action, the effects of vasotocin in NHE3 transfectants and in MDCK cells were blocked by the V₁ receptor antagonist, d(CH₂)₅Tyr(Me)-AVP, but were not reproduced by the V₂ receptor agonist desmopressin. It is concluded that NHE3 in the apical membrane of NHE3-transfected MDCK cells contributes to the differential regulation of the apical and basolateral Na⁺/H⁺-exchanger by vasotocin; NHE3 is inhibited and endogenous Na⁺/H⁺-exchange activity is stimulated by vasotocin via V₁ receptor activation of the protein kinase C pathway.