Veratridine-induced oscillations in membrane potential of cultured rat skeletal muscle: Role of the Na-K pump

1. The acute effects of veratridine on membrane potential (E_m) and Na-K pump activity in cultured skeletal muscle were examined. 2. At a concentration of 10^-4M, veratridine caused depolarization of E_m and a decrease in Na-K pump activity. At concentrations of 10^-5 and 10^-6M, veratridine caused oscillations of E_m and an increase in Na-K pump activity compared to untreated, control cells. The oscillations consisted of depolarization to about -40 mV followed by hyperpolarization to about -90 mV; the level of hyperpolarization was higher at 37 than at 23°C. 3. Veratridine-induced oscillations could be prevented by pretreatment with tetrodotoxin (10^-6M) and blocked or prevented by ouabain, which depolarizes E_m of cultured myotubes. In contrast, depolarization of E_m to -60 mV by excess K⁺ did not alter the amplitude or frequency of the oscillations. 4. The results demonstrate that veratridine-induced increase in Na influx both depolarizes cultured myotubes and increases the activity of the Na-K pump, which repolarizes E_m to levels higher than control. This sequence accounts for veratridine-induced oscillations in E_m. High concentrations of veratridine cause only depolarization of E_m and inhibition of Na-K pump activity.