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Departments of Medicine and The Pharmacological and Physiological Sciences University of Chicago, Chicago, Illinois, U.S.A.

1. The membrane responses of rabbit papillary muscles to rapid changes in [K]_o and [Cl]_o were measured with open-tipped micropipettes and with closed micropipettes made from K-selective glass.

2. The muscle cells behaved primarily as a K electrode, and responses to changes in [K]_o with constant [Cl]_o or with constant [K]_o x [Cl]_o were substantially the same.

3. When [Cl]_o was changed at a constant [K]_o the membrane potentials changed rapidly and symmetrically by a small value and remained constant for 30 min.

4. Measurement of potential with K⁺-selective micro-electrodes in these experiments showed no change in intracellular K activity. In addition to permitting calculation of K permeability, these measurements reassured us that the K⁺-selective electrodes were well insulated and not influenced by electrical shunts at the impalement site.

5. Although the membrane response to changes in [Cl]_o was small, it was possible to calculate that the permeability ratio (P_Cl/P_K), 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 (g_Cl/g_K) of about 0·17.

6. The time course of depolarization by increase in [K]_o was rapid (half-time 5 sec), but repolarization on return to lower [K]_o was much slower (half-time 50 sec). 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]_o. These calculations did not fit the time course of repolarization, which was slowed in the fashion expected from an inward-rectifying membrane.

7. 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.

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