The role of the Na+/H+ exchange system in the regulation of the internal pH in cultured cardiac cells

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Abstract
The Na+/H+ exchange system is not the major mechanism that regulates the internal pH value (pHi) of chick cardiac cells in culture under normal physiological conditions in the absence of carbonate. In cardiac cells in which the internal pH was lowered to 6.6-6.7, the Na+/H+ exchanger became the major mechanism to bring back pHi to normal values (pHi = 7.3). The blockade of the Na+/H+ exchange activity with an active amiloride derivative, ethylisopropylamiloride, prevented internal pH recovery. The internal pH dependence of the Na+/H+ exchanger activity was carefully studied. The [H+]i-dependence was very cooperative. For an external pH of 7.4, the system was nearly completely inactive at pHi 7.8 and nearly completely active at pHi 6.9-7.0 with half-maximum activation at pHi = 7.35. The increased activity of the Na+/H+ exchange system which follows the acidification of the internal medium produces an activation of the (Na+, K+)-ATPase.