Extracellular potassium accumulation in voltage‐clamped frog ventricular muscle.

Abstract

Application of voltage clamp pulses (1-10 s) to frog ventricular strips causes temporary changes in the extracellular K concentration ([K]o). The changes in the extracellular K concentration can be estimated from slowly decaying post-clamp after-potentials, changes in the action potential duration and measurements with a K-selective microelectrode. The depolarization of the resting potential and the shortening of the action potential are proportional during voltage-clamp induced extracellular K accumulation and perfusion with a K-rich Ringer solution but consistent differences are noticed. Measurement of the after-potential, the action potential shortening, and the K-electrode response indicated extracellular K+ activity. The after-potential appears to provide the most convenient and reliable estimate of the absolute magnitude of the voltage-clamp induced extracellular K accumulation. Depolarizing after-potentials decay more slowly than the hyperpolarizing after-potentials; this reflects the selectivity of the membrane to K+ concentrations as predicted by the Nernst or the Goldman equations. Analysis of the redistribution of accumulated K+ from the decay of the after-potential suggests that the major part of the redistribution process can be described by a single time constant (2-4 s). A longer time constant is required for a smaller component of the tail to bring [K]o to the normal resting state. N-shaped relations similar to the steady state current-voltage relation are obtained when the post-clamp after-potential, the action potential shortening, and the K-electrode response are plotted vs. the clamped membrane potential. The maxima of these curves are located around -40 mV and the minima around -20 mV. In spite of an outward membrane current (1-1.5 .mu.A) in the minimum region (-20 mV), the post-clamp after-potential is hyperpolarizing suggesting extracellular K depletion. Indicating that the K efflux is lower at -20 mV than at higher and lower potentials and suggest that the N-shape steady state current-voltage relation reflects the voltage dependency of the K current. A theory for K accumulation in a single compartment is presented, predicting that a simple linear RC[Resistance-Capacitance]-circuit may describe the electrical response of the preparation in a limited potential range around the resting potential. The extracellular accumulation space was estimated to be 13-16% of the total volume of the preparation. The accumulation space is tentatively equivalent to the subendothelial fraction of the extracellular space.