INFLUENCE OF CHANGES IN EXTERNAL POTASSIUM AND CHLORIDE-IONS ON MEMBRANE-POTENTIAL AND INTRACELLULAR POTASSIUM-ION ACTIVITY IN RABBIT VENTRICULAR MUSCLE

The membrane responses of rabbit papillary muscles to rapid changes in [K]0 and [Cl]0 were measured with open-tipped micropipettes and with closed micropipettes made from K-selective glass. The muscle cells behaved primarily as a K electrode, and responses to changes in [K]0 with constant [CL]0 or with constant [K]0 .times. [Cl]0 were substantially the same. When [Cl]0 was changed at a constant [K]0 the membrane potentials changed rapidly and symmetrically by a small value and remained constant for 30 min. Measurement of potential with K+-selective micro-electrodes showed no change in intracellular K activity. In addition to permitting calculation of K permeability, these measurements indicated that the K+-selective electrodes were well insulated and not influenced by electrical shunts at the impalement site. Although the membrane response to changes in [Cl]0 was small, it was possible to calculate that the permeability ratio (PCl/PK), was 0.11. The Cl and K conductances were about 0.015 mmho/cm2 and 0.09 mmho/cm2, respectively, resulting in a conductance ratio (gCl/gK) of about 0.17. The time course of depolarization by increase in [K]0 was rapid (half-time 5 s), but repolarization on return to lower [K]0 was much slower (half-time 50 s). The depolarization time course was easily fitted by the potential change calculated by assuming the need for K diffusion into the extracellular spaces and taking account of the logarithmic relation between membrane potential and [K]0. The influence of [K]i on membrane potential was investigated by changes in tonicity of the external solution. Hypotonic solution produced a change in intracellular K activity close to that produced by ideal water movement. However, in hypertonic solution, intracellular K activity did not rise as much as predicted, suggesting a change in intracellular activity coefficient.