NA+ CURRENTS NEAR AND AWAY FROM END-PLATES ON HUMAN FAST AND SLOW-TWITCH MUSCLE-FIBERS

Fast and slow twitch muscle fibers have distinct contractile properties. Here we determined that membrane excitability also varies with fiber type. Na+ currents (I(Na)) were studied with the loose-patch voltage clamp technique on 29 histochemically classified human intercostal skeletal muscle fibers at the endplate border and >200 mum from the endplate (extrajunctional). Fast and slow twitch fibers showed slow inactivation of endplate border and extrajunctional I(Na) and had increased I(Na) at the endplate border compared to extrajunctional membrane. The voltage dependencies of I(Na) were similar on the endplate border and extrajunctional membrane, which suggests that both regions have physiologically similar channels. Fast twitch fibers had larger I(Na) on the endplate border and extrajunctional membrane and manifest fast and slow inactivation of I(Na) at more negative potentials than slow twitch fibers. For normal muscle, the differences between I(Na) on 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. (C) 1993 John Wiley & Sons, Inc.