Ca‐dependent slow action potentials in human skeletal muscle

Abstract

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 Cl-free saline with 10 or 84 mM 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 mM 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 degrees C, 84 mM Ca), the threshold and peak potential during a CaAP were +26 +/- 6 mV and +70 +/- 3 mV, respectively, and the duration measured at threshold level was 1.7 +/- 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 degrees C, 84 mM Ca, 40 micron fiber diameter), values were 0.4 +/- 0.1 microS, 1.1 +/- 0.7 microS, and 0.9 +/- 0.4 microS, respectively. Rat muscle (22-24 degrees C, 84 mM 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 slow action potentials in neuromuscular diseases

“…As previously reported (Nicola Siri et al, 1988) these responses can be elicited in spite of the cut end condition of the fibers. Muscle fibers can also produce abortive non-regenerative Ca responses, or no Ca responses at all.…”

“…It was observed that Ca responses in normal muscle are much more frequent in rat than in human muscles (Nicola Siri et al, 1988). The differences between our findings and those in dystrophic mouse are probably due to differences between species and between animal and human diseases.…”

“…Previous observation of CaAPs in normal human and rat skeletal muscle fibers (Nicola Siri et al, 1988) showed that neither the cut end condition of the fiber nor the experimental procedure (i.e., cut ends, incubation, temperature, etc.) could affect the frequency of observation of CaAPs.…”

“…The experiments were performed as described by Nicola Siri et al (1988). Briefly, Ca-dependent slow action potentials (CaAP) were recorded in C1-free tetraethyl ammonium-methanesulphonate (TEA-Mes) saline with 3,4-diamino pyridyne (3, in human skeletal muscle fibers (with cut ends) obtained from normal as well as pathological biopsies.…”

“…We have recently shown (Nicola Siri et al, 1988) that slow Ca-dependent action potentials (CaAP) can be observed in human skeletal muscle fibers (normal muscle), but in a much lower percentage of fibers than in other species. Human muscle fibers can also display abortive non-regenerative Ca-dependent responses, but most of the fibers show only passive capacitive responses.…”