Changes in intra- and extracellular potassium and intracellular sodium activities induced by repetitive stimulation and their relation to membrane potential in guinea-pig papillary muscle.

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Abstract
We measured changes in membrane potential (MP), extra- and intracellular potassium and intracellular sodium ion activities (aKo,aKi, and aNai) in papillary muscle of the guinea-pig heart induced by repetitive stimulation at various frequencies and periods in vitro, using double-barrelled ion-selective microelectrodes. Stimulation (2-4 Hz) depolarized the cells, and termination of stimulation induced hyperpolarization. The aKi and aNai (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 aNai time-dependently, but there was no further increase by stimulation for longer than 3 min. The increase in aNai was dependent on stimulus frequency. After termination of the stimulation, aNai declined exponentially. aKi slightly decreased by the stimulation (3-4 Hz). aKo increased during the stimulation period, and decreased below the initial level after termination of the stimulation. Results suggest that in guinea-pig 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 aKo induced by Na-K pump activity.