The activity of the Na+/H+ antiporter in cultured cardiac cells is dependent on the culture conditions used.

When chick cardiac cells are maintained in serum-free NCTC-135 medium, the pHi dependence of the Na+/H+ exchanger is much more acidic (pK = 7.0) than when a serum-free M199 medium is used (pK greater than 7.5). Phorbol esters activate the Na+/H+ exchanger and produce a cell alkalinization in cells maintained in NCTC-135 medium by shifting the pHi dependence of the system to more alkaline values. They have no action on cells maintained in M199 medium. The alkaline pHi dependence of the Na+/H+ exchanger and the loss of responsiveness of phorbol ester action in M199 medium correlate with increased rates of phosphatidylinositol turnover.