Sodium and potassium permeability of membrane vesicles in a sarcolemma-enriched preparation from canine ventricle

Abstract

Vesicles in a highly enriched sarcolemma preparation from canine ventricle were found to develop membrane potentials in response to outwardly directed potassium and inwardly directed sodium concentration gradients. The magnitude of the potential measured by the fluorescent dye diS-C3-(5) suggested a sodium-to-potassium permeability ratio between 0.2 and 1.0 which is one to two orders of magnitude greater than values obtained for the myocardial cell. Radiotracer techniques were employed to evaluate the permeability coefficients of the isolated cardiac sarcolemma membrane for sodium and potassium under equilibrium conditions (i.e., equal salt concentrations in the intravesicular and extravesicular spaces). The uptake of sodium and potassium was best described by two exponential processes which followed an increment of uptake that occurred prior to the earliest assay time (i.e., 17 sec). The compartment sizes were linear, nonsaturable functions of the cation activity. Evaluation of the rate coefficients of cation uptake by the two exponential processes versus cation activity revealed that sodium influx via the slow process and potassium influx via the fast process varied linearly with cation activity, suggesting that the permeability coefficients were concentration independent for these compartments. Conversely, sodium influx via the fast process exhibited a nonlinear increase with increasing sodium activity, and potassium influx via the slow process appeared to saturate with increasing potassium activity. In general, the permeabilities of the sarcolemma-enriched preparation for sodium and potassium were of equal magnitude. The permeability coefficients were lower than that for the potassium coefficient reported for cardiac cells but are in the range of that reported for sodium.