Na+ current densities and voltage dependence in human intercostal muscle fibres.
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.
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Department of Neurology, Cleveland Department of Veterans Affairs Medical Center, Case Western Reserve University Medical School, OH 44106.
Whittlesey
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D004558:Electric Stimulation; D006801:Humans; D066298:In Vitro Techniques; D007366:Intercostal Muscles / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D008875:Middle Aged; D015222:Sodium Channels / Q000502:physiology*; D013997:Time Factors
D015222:Sodium Channels