Effects of carbamylcholine on membrane potential and Na-K pump activity of cultured rat skeletal myotubes.

1. We measured changes in resting membrane potential (Em) and Na-K pump activity, assayed by ouabain-sensitive 86Rb uptake, in response to carbamylcholine (CCh) and its continued presence in single rat skeletal myotubes in culture. 2. CCh caused immediate depolarization from control Em (-80 to -85 mV) to near 0 followed by repolarization of varying degrees depending on the age of the culture and temperature of the recording medium; repolarization of Em was most apparent by culture age 8-9 days in vitro (DIV), Em reaching values as high as -60 mV by 5-10 min after peak depolarization at 37 degrees C. 3. Input resistance, which decreased during CCh depolarization, increased only slightly during the initial phase of repolarization and then remained essentially unchanged during the major component of membrane repolarization in the presence of CCh. 4. Ouabain, given before CCh, prevented repolarization of Em and, when given after repolarization had begun, reversed it and caused Em to return to about -7 mV. 5. Na-K pump activity was decreased in myotubes in which Em did not repolarize or did so only slightly, and was increased by over 40-50% in myotubes whose Em repolarized by 40-60 mV, even though CCh was still present in the medium. Inhibition of pump activity in non repolarizing myotubes was related to Na influx, inhibition being reversed to stimulation when CCh was administered to myotubes in Na-free medium. 6. Repeated (three or four times) or prolonged (up to 60-min) administration of CCh to myotubes in which repolarization was hardly expressed (age 6-7 DIV) caused increases both in the amount of repolarization and in 86Rb uptake, both being related to the number or duration of CCh exposures. 7. We conclude that repolarization of Em following CCh-induced depolarization of cultured rat skeletal myotubes depends to a large extent on an increase in activity of the electrogenic Na-K pump.