Ca-dependent slow action potentials in human skeletal muscle (original) (raw)

1988, Journal of Cellular Physiology

Slow Ca-action potentials (CaAP) were studied in normal human skeletal muscle fibers obtained during surgery (fibers with both ends cut). Control studies also were carried out with intact as well as cut rat skeletal muscle fibers. Experiments were performed in hypertonic CI-free saline with 10 or 84 m M Ca and K-channel blockers; muscles were preincubated in a saline containing Cs and tetraethylammonium. A current-clamp technique with two intracellular microelectrodes was used. In human muscle, 14.5% of the fibers showed fully developed CaAPs, 21 % displayed nonregenerative Ca responses, and 64.5% showed only passive responses; CaAPs were never observed in 10 m M Ca. In rat muscle, nearly 90% of the fibers showed CaAPs, which were not affected by the cut-end condition. Human and rat muscle fibers had similar membrane potential and conductance in the resting state. In human muscle (22-32" C, 84 m M Ca), the threshold and peak potential during a CaAP were + 26 k 6 mV and + 70 ? 3 mV, respectively, and the duration measured at threshold level was 1.7 2 0.5 sec. In rat muscle, the duration was four times longer. During a CaAP, membrane conductance was assumed to be a leak conductance in parallel with a Ca and a K conductance. In human muscle (22-32" C, 84 m M Ca, 40 p m fiber diameter), values were 0.4 2 0.1 pS, 1.1 f 0.7 p,S, and 0.9 * 0.4 p,S, respectively. Rat muscle (22-24" C, 84 m M Ca) showed leak and K conductances similar to those found in human fibers. Ca-conductance in rat muscle was double the values obtained in human muscle fibers. Ca-dependent action potentials have been observed in mammalian muscle, when C1 was replaced by a