NA+ CURRENT DENSITIES AND VOLTAGE DEPENDENCE IN HUMAN INTERCOSTAL MUSCLE-FIBERS

1. Voltage-clamp Na+ currents (I(Na)) were studied in human intercostal muscle fibres using the loose-patch-clamp technique. 2. The fibres could be divided into two groups based upon the properties Of I(Na). The two groups of fibres were called type 1 and type 2. 3. Both type 1 and type 2 fibres demonstrated fast and slow inactivation Of I(Na). 4. Type 1 fibres had lower I(Na) on the endplate border and extrajunctional membrane than type 2 fibres and required larger membrane depolarizations to inactivate Na+ channels by fast or slow inactivation Of I(Na). 5. Type 2 fibres had a higher ratio of I(Na) at the endplate border compared to extrajunctional membrane than Type 1 fibres. 6. Measurement of membrane capacitance suggested that the increase in I(Na) at the endplate border was due to increased Na+ channel density. 7. Histochemical staining of some fibres suggested that type 1 fibres were slow twitch and type 2 fibres were fast twitch. 8. Differences in the properties of Na+ channels between fast- and slow-twitch fibres may contribute to the ability of fast-twitch fibres to operate at high firing frequencies and slow-twitch fibres to be tonically active.