The dynamic sodium current of human skeletal muscle.

The dynamic sodium current (INa) was investigated in intact external intercostal muscle bundles dissected from 5 volunteers. Only fibres with a resting membrane potential negative to -80 mV were used. Stepwise changes were imposed on the membrane potential using the "loose-patch" clamp technique. The following results concerning INa are means +/- SD obtained from 27 fibres: threshold at -42.7 +/- 5.2 mV; maximal amplitude of 9.7 +/- 7.8 mA/cm2 at -1 +/- 13 mV. Inactivation: starting from the resting potential, maximally 90% of INa max could be activated; 50% of INa max were inactivated at Eh = -66.2 +/- 5.8 mV; the slope of the inactivation (h infinity -) curve at this potential was kh = 7.1 +/- 1.6 mV.