Extracellular potassium accumulation in voltage‐clamped frog ventricular muscle.

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Abstract
1. Application of voltage clamp pulses (1‐‐10 sec) to frog ventricular strips causes temporary changes in the extracellular K concentration. 2. The changes in the extracellular K concentration can be estimated from (a) slowly decaying post‐clamp after‐potentials, (b) changes in the action potential duration, and (c) measurements with a K‐selective micro‐electrode. 3. The depolarization of the resting potential and the shortening of the action potential are present in approximately the same proportions during voltage‐clamp induced extracellular K accumulation and during perfusion with a K‐ricn Ringer solution but small consistent differences are noticed. 4. The measurements of the after‐potential, the action potential shortening, and the K‐electrode response were analysed as indicators of extracellular K+ activity and it was concluded that the after‐potential provides the most convenient and reliable estimate of the absolute magnitude of the voltage‐clamp induced extracellular K accumulation. 5. The depolarizing after‐potentials decay more slowly than the hyperpolarizing after‐potentials but it is found that this reflects the selectivity of the membrane to K+ concentrations as predicted by the Nernst or the Goldman equations. 6. 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 sec). A much longer time constant is required for a smaller component of the 'tail' in order to bring [K]o to the normal resting state. 7. 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 versus the clamped membrane potential. The maxima of these curves are located around ‐40 mV and the minima around ‐20 mV. 8. In spite of a significant outward membrane current (1‐‐1.5 microamperemeter) in the minimum region (‐20 mV), the post‐clamp after‐potential is often hyperpolarizing in nature suggesting extracellular K depletion. 9. These findings indicate that the K efflux is lower at ‐20 mV than at both higher and lower potentials and suggest that the N‐shape 'steady state' current‐voltage relation mainly reflects the voltage dependency of the K current. 10. A theory for K accumulation in a single compartment is presented which predicts that a simple linear RC‐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. It is tentatively suggested that the accumulation space is equivalent to the subendothelial fraction of the extracellular space.