Na+ current densities and voltage dependence in human intercostal muscle fibres.

Abstract

1. Voltage‐clamp Na+ currents (INa) 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 INa. 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 INa. 4. Type 1 fibres had lower INa 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 INa. 5. Type 2 fibres had a higher ratio of INa at the endplate border compared to extrajunctional membrane than Type 1 fibres. 6. Measurement of membrane capacitance suggested that the increase in INa 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. © 1992 The Physiological Society