Potassium accumulation and depletion in frog atrial muscle.

Abstract

SUMMARY1. In atrial wall trabeculae of Rana catesbeiana and R. ridibunda very slowly decaying membrane currents have been consistently observed in decay tails following voltage clamp depolarizing and hyperpolarizing pulses. It is not thought that these currents are carried by time-dependent conductance channels but rather result from potassium ion accumulation or depletion.2. Since voltage clamp techniques generally impose a non-physiological situation on the membranes of excitable cells, evidence that potassium ion accumulation occurs in unclamped atrial tissue is presented.3. When potassium ions accumulate, the reversal potentials for both atrial delayed conductance mechanisms, iXfat and ix,, should be shifted in a positive direction, the magnitude of the shifts being a function of the charge transferred during depolarization. Experiments have been performed to test this prediction quantitatively, and as a result, a simple accumulation model is developed. 4. A second important effect of accumulation should be upon the timeindependent potassium conductance, iK.. It was found that this effect produces current tails whose decay becomes exponential when the amount of accumulation is small. The time constant of this exponential is shown to be equal to the time constant of decay of accumulation, Tacc. Experimental verification of this assumption is presented. This 'crossover' effect allows current changes due to accumulation to show an apparent 'reversal potential' and so to appear like a conductance mechanism. 6. Potassium depletion is shown to occur during hyperpolarizing pulses.This depletion process must be allowed for in a direct kinetic analysis of the pace-maker current, ih51, at potentials negative to the resting potential (ER).

Potassium currents in frog ventricular muscle: evidence from voltage clamp currents and extracellular K accumulation.

“…10, 13) is qualitatively similar to those described in other muscle preparations (e.g. Almers, 1972;Noble & Tsien, 1968;Noble, 1976 (Fig. 13).…”

“…Inward rectifying currents have been found in 136 K+ CURRENTS IN FROG VENTRICLE skeletal muscle (Adrian, 1969;Almers, 1972), frog atrial muscle (Noble, 1976) and sheep Purkinje fibres (Dudel, Peper, Rudel & Trautwein, 1967;Noble & Tsien, 1968;Hauswirth, Noble & Tsien, 1972). The present study shows that inward rectification occurs in frog ventricular muscular in much the same way as in other muscle preparations.…”

“…By introducing non-linear current-voltage relations and limited ionic selectivity, the individual current components could be described using modified Hodgkin-Huxley formalism. Similarly, analysis of nonpacing frog atrial muscle indicated the presence of two time dependent current components (Ixfast, Ixslow) and a third component attributed to K accumulation (Brown, Clark & Noble, 1976a, b;Noble, 1976). K accumulations were also observed in sheep and calf ventricular muscle during voltage clamp pulses (McGuigan, 1974).…”

“…Weidmann (1956) observed the shortening of action potential in response to brief injection of K+ during the plateau of the action potential and suggested that elevation of [K]0 increases the outward movement of K. Such an increase in outward membrane current has been observed in several cardiac muscle preparations (Hall, Hutter & Noble, 1963;Noble & Tsien, 1969b;McGuigan, 1974;Noble, 1976). To compare the effect of [K]0 with that caused by local K+ accumulation one must have a reliable method to measure K+ accumulation.…”

Potential dependency of the electrogenic Na+-pump current in bullfrog atrial muscles.

“…1) Depletion may occur during the hyperpolarizing pulses, which could decrease the concentration of extracellular cation close to the cell membrane necessary for carrying inward current (NOBLE, 1976).…”