SUMMARY1. 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.
Fast 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 microns from the endplate (extrajunctional). Fast and slow twitch fibers showed slow inactivation of endplate border and extrajunctional INa and had increased INa at the endplate border compared to extrajunctional membrane. The voltage dependencies of INa were similar on the endplate border and extrajunctional membrane, which suggests that both regions have physiologically similar channels. Fast twitch fibers had larger INa on the endplate border and extrajunctional membrane and manifest fast and slow inactivation of INa at more negative potentials than slow twitch fibers. For normal muscle, the differences between INa 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.
The voltage dependence and amplitude of Na+ currents (INa) were studied with the loose-patch voltage-clamp technique on 19 fast-twitch human intercostal skeletal muscle fibers at the endplate border and > 200 microns from the endplate (extrajunctional). The fibers were histochemically classified as fast-twitch oxidative-glycolytic (type IIa, n = 9) or fast-twitch glycolytic (type IIb, n = 10). The voltage dependence of activation and fast and slow inactivation of INa were similar for membrane patches recorded on the endplate border and on extrajunctional membrane for both fiber types. INa was about fivefold larger on the endplate border compared with extrajunctional membrane for both fiber types. Type IIb fibers had larger values of INa and manifest fast inactivation of INa at more negative potentials than type IIa fibers. The difference between type IIa and IIb fibers may enable IIb fibers to operate at higher firing frequencies for brief periods.
Muscle fibers from the lateral gastrocnemius or intercostal muscles of 7 normal adult males were chemically skinned (sarcolemma disrupted) and isolated fibers were divided into two parts for histochemical determination of fiber type and physiologic studies. The Ca- and Sr-induced tension relationships and maximum contraction velocities were measured. Slow twitch fibers developed tension at lower concentrations of Ca or Sr than fast twitch fibers. The difference between fast and slow twitch fibers was greatest when Sr was the activating cation. Fast and slow twitch fibers was greatest when Sr was the activating cation. Fast and slow twitch fibers generated similar maximum tensions. The contraction velocities of fast twitch fibers were more than two-fold greater than slow twitch fibers. Fast-oxidative-glycolytic (FOG, type IIA) and fast-glycolytic (FG, type IIB) fibers had similar Ca- and Sr-tension relationships and contraction velocities.
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