Acceleration of H+ Extrusion via Na+-H+ Exchange in Guinea-Pig Ventricular Papillary Muscle under Intracellular Acidic Condition.

We investigated mechanisms by which intracellular pH was regulated under intracellular acidic condition in resting guinea-pig ventricular papillary muscles in vitro. Intracellular sodium ion activity (aiNa), intracellular and surface pH (pHi and pHs) were measured with Na(+)- and H(+)-selective microelectrodes and resting tension was measured. By exposure to 0 mM K solution aiNa and resting tension increased progressively while pHi decreased but reached the steady level of pH 6.95. pHs which was lower than external bulk pH (pHo) decreased progressively by exposure to 0 mM K solution. In 4 mM K solution, amiloride (1 mM), an inhibitor of Na(+)-H+ exchange, induced a reversible decrease in both aiNa and pHi, and an increase in pHs. Changes in pHi and pHs induced by application of amiloride in 0 mM K solution were larger than those in 4 mM K solution. The rate of decrease in pHi induced by amiloride became larger at longer exposure to 0 mM K solution. Lowering pHo from 7.4 to 6.4 induced a larger decrease in pHi in 0 mM K solution than that in 4 mM K solution. Lowering pHo from 7.4 to 5.4 reversed the difference between pHs and pHo. These results suggest that in guinea-pig papillary muscle, Na(+)-H+ exchange is active to regulate intracellular H+ under resting condition and under intracellular acidic condition, H+ extrusion via the Na(+)-H+ exchange would be accelerated not only by the net thermodynamic driving force for Na+ and H+ but also by other factors.