MEMBRANE CURRENTS IN HUMAN INTERCOSTAL MUSCLE AT VARIED EXTRACELLULAR POTASSIUM

Hyperpolarizing and depolarizing square steps were imposed on the membrane potential of excised human intercostal muscle fibers by means of a 3-microelectrode voltage clamp. The steady-state am-plitudes of the membrane currents inducing such steps were inves-tigated as a function of the membrane potential, while the muscle was bathed in solutions varying in potassium content (K, = 1,3.5,7,20, and 60 mM). At all potassium concentrations, the membrane acted as a rectifier, both in the inward- and outward-going directions. Inward cur-rents were much reduced when K, was lowered from 3.5 to l mM, and were increased when K, was raised beyond 3.5 mM. The delayed out-ward current was reduced when K, was increased from 3.5 mM to 7 mM and higher potassium concentration. The results were qualitatively similar to those reported for rat skeletal muscle