Noradrenaline stimulation of a membrane pump in frog skeletal muscle

Resting membrane potentials were recorded from the frog sartorius before and during exposure to noradrenaline (NA; 10(-2) M) in the presence or absence of ouabain (10(-3) M) and during manipulation of the extracellular potassium concentration. In potassium-free Ringer, NA produced no significant change in the resting membrane potential. With the extracellular potassium concentration at 2.5 or 5mM, NA caused a hyperpolarization. This reversed to a depolarization when extracellular K+ was greater than 7 mM. In all cases NA-induced changes were blocked by ouabain. These effects suggest that NA stimulates a membrane cation pump, the Na : K coupling ratio of which is affected by the available ion concentrations.