Na+currents near and away from endplates on human fast and slow twitch muscle fibers

摘要

AbstractFast and slow twitch muscle fibers have distinct contractile properties. Here we determined that membrane excitability also varies with fiber type. Na+currents (INA) were studied with the loose‐patch voltage clamp technique on 29 histochemically classified human intercostal skeletal muscle fibers at the endplate border and<200 μm from the endplate (extrajunctional). Fast and slow twitch fibers showed slow inactivation of endplate border and extrajunctionalINAand had increasedINAat the endplate border compared to extrajunctional membrane. The voltage dependencies ofINAwere similar on the endplate border and extrajunctional membrane, which suggests thatboth regions have physiclogically similar channels. Fast twitch fibers had largerINAon the endplate border and extrajunctional membrane and manifest fast and slow inactivation ofINAat more negative potentials than slow twitch fibers. For normal muscle, the differences betweenINAon fast and slow twitch fibers might: (1) enable fast twitch fibers to operate at high firing frequencies for brief periods; and (2) enable slow twitch fibers to operate at low firing frequencies for prolonged times. Disorders of skeletal membrane excitability, such as the periodic paralyses and myotonias, may impact fast and slow twitch fibers differently due to the distinctive Na+channel properties of each fiber type. © 1993 John Wiley&Sons,