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We measured changes in membrane potential (MP), extra-and intracellular potassium and intracellular sodium ion activities (4, 4, and aNa) in papillary muscle of the guinea-pig heart induced by repetitive stimulation at various frequencies and periods in vitro, using doublebarrelled ion-selective microelectrodes. Stimulation (2-4 Hz) depolarized the cells, and termination of stimulation induced hyperpolarization. The a K and aNa (stimulated at 0.2 Hz) were 92.3 + 4.6 and 7.8 ± 2.0 mM (mean + S.D.), respectively. Prolongation of the stimulation period (0.5-2 min) increased aNa time-dependently, but there was no further increase by stimulation for longer than 3 min. The increase in aNa was dependent on stimulus frequency. After termination of the stimulation, aNa declined exponentially. a K slightly decreased by the stimulation (3-4 Hz). a K increased during the stimulation period, and decreased below the initial level after termination of the stimulation. Results suggest that in guineapig papillary muscle, the. contribution of an electrogenic component of Na-K pump to the hyperpolarization after stimulation would be small, since the hyperpolarization could be explained mostly by depletion of 4 induced by Na-K pump activity.

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.

We examined the relationship among intracellular sodium activity (aiNa), intracellular pH (pHi), and delayed afterdepolarization (DAD) in guinea pig ventricular papillary muscle fibers exposed to K-free solution in vitro, using Na+- and H+-selective microelectrodes. In unstimulated papillary muscles, exposure to K-free solution caused a rapid increase in aiNa of 16.2-22 mM at the end of a 20-min superfusion period with rates of 0.51-0.73 mM/min. This was paralleled by a decrease in intracellular pH of 0.10-0.15 units. DADs were induced after a train of driven action potentials 5-15 min after superfusion with K-free solution at the elevated aiNa from the steady-state level. Prevention of intracellular acidosis associated with the increased aiNa in K-free solution by external alkalosis at pH 8.0 enhanced the amplitude of the DAD, whereas restoration of external pH to 7.4 and a further acidification of external pH to 6.4 suppressed DADs. We concluded that intracellular acidosis associated with the increased aiNa per se increased the likelihood for inhibition of sarcoplasmic reticulum Ca2+ release in spite of increased cytosolic Ca2+.

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