Abstract
1. In single barnacle skeletal muscle cells cell diameter has been measured as a function of external osmolality, and cell diameter and membrane potential have been measured during changes of external K and Cl concentrations ([K](o) and [Cl](o)) like those described in frog muscle by Hodgkin & Horowicz (1959). The diameter was monitored microscopically with a precision of 0.2-0.4% (S.D.). [K](o) was varied from 1 to 18 mM, a range of concentrations which does not cause contracture.2. At pH 8.0 the Cl permeability was so low that net KCl and water movements were absent. Such net movements were present at pH 4.5, corresponding to a change in the ratio (Cl conductance/K conductance) from approximately 1/12 at pH 8.0 to 1/2 at pH 4.5.3. Characteristically long time constants were observed for membrane potential responses to a change in [K](o) and/or [Cl](o), even at constant [K](o).[Cl](o). This phenomenon is attributed to a delayed equilibration by diffusion within the system of sarcolemmal invaginations and T-tubules. The delay in the response was increased by introducing polyvinylpyrrolidone (PVP) into this system, presumably because PVP raises intratubular viscosity.4. At pH 4.5 anomalous rectification for net movements of K was demonstrated by measurements of cell diameter and of membrane potential.