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 (a(Na)i), intracellular and surface pH (pH(i) and pH(s)) were measured with Na+- and H+-selective microelectrodes and resting tension was measured. By exposure to 0 mM K solution a(Na)i and resting tension increased progressively while pH(i) decreased but reached the steady level of pH 6.95. pH(s) which was lower than external bulk pH (pH(o)) 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 a(Na)i and pH(i), and an increase in pH(s). Changes in pH(i) and pH(s) induced by application of amiloride in 0 mM K solution were larger than those in 4 mM K solution. The rate of decrease in pH(i) induced by amiloride became larger at longer exposure to 0 mM K solution. Lowering pH(o) from 7.4 to 6.4 induced a larger decrease in pH(i) in 0 mM K solution than that in 4 mM K solution. Lowering pH(o) from 7.4 to 5.4 reversed the difference between pH(s) and pH(o). 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.