Cs⁺ causes a voltage-dependent block of inward K currents in resting skeletal muscle fibres

Abstract

WHEN frog skeletal muscle fibres are bathed in solutions containing Cs⁺ and K⁺ in the ratio 1 : 4,000, a reduction is observed in the size of inward K currents through the resting membrane. This effect is enhanced by an increase in either hyperpolarisation or external Cs⁺ concentration. It can be predicted from these findings that regenerative changes in membrane potential should be obtainable in fibres, in the presence of Cs⁺, that are hyperpolarised by means of a current electrode. Such responses are described in the last part of this report. In squid axon and frog node^1,2, internal Cs⁺ produces a voltage-dependent block of the delayed, outward K currents, though the ratio of Cs⁺ to K⁺ required for this effect is far greater than that used in the experiments reported here. A closer parallel can be drawn between our findings and those recently reported on the inward K currents in the starfish egg cell³.