Effects of membrane depolarization and changes in extracellular [K(+)] on the Ca (2+) transients of fast skeletal muscle fibers. Implications for muscle fatigue.

Repetitive activation of skeletal muscle fibers leads to a reduced transmembrane K(+) gradient. The resulting membrane depolarization has been proposed to play a major role in the onset of muscle fatigue. Nevertheless, raising the extracellular K(+) K(+)(O) concentration ([K(+)](O)) to 10 mM potentiates twitch force of rested amphibian and mammalian fibers. We used a double Vaseline gap method to simultaneously record action potentials (AP) and Ca(2+) transients from rested frog fibers activated by single and tetanic stimulation (10 pulses, 100 Hz) at various [K(+)](O) and membrane potentials. Depolarization resulting from current injection or raised [K(+](O) produced an increase in the resting [Ca(2+)]. Ca(2+) transients elicited by single stimulation were potentiated by depolarization from -80 to -60 mV but markedly depressed by further depolarization. Potentiation was inversely correlated with a reduction in the amplitude, overshoot and duration of APs. Similar effects were found for the Ca(2+) transients elicited by the first pulse of 100 Hz trains. Depression or block of Ca(2+) transient in response to the 2nd to 10th pulses of 100 Hz trains was observed at smaller depolarizations as compared to that seen when using single stimulation. Changes in Ca(2+) transients along the trains were associated with impaired or abortive APs. Raising [K(+)](O) to 10 mM potentiated Ca(2+) transients elicited by single and tetanic stimulation, while raising [K(+)](O) to 15 mM markedly depressed both responses. The effects of 10 mM K(+)(O) on Ca(2+) transients, but not those of 15 mM K(+)(O), could be fully reversed by hyperpolarization. The results suggests that the force potentiating effects of 10 mM K(+)(O) might be mediated by depolarization dependent changes in resting [Ca(2+)] and Ca(2+) release, and that additional mechanisms might be involved in the effects of 15 mM K(+)(O) on force generation.

Modulation by caffeine of calcium-release microdomains in frog skeletal muscle fibers.

The effects of caffeine on the process of excitation-contraction coupling in amphibian skeletal muscle fibers were investigated using the confocal spot detection technique. This method permits to carefully discriminate between caffeine effects on the primary sources of Ca2+ release at the Z-lines where the triads are located and secondary actions on other potential Ca Release sources. Our results demonstrate that 0.5 mM caffeine potentiates and prolongs localized action-potential evoked Ca2+ transients recorded at the level of the Z-lines, but that 1mM only prolongs them. The effects at both doses are reversible. At the level of the M-line, localized Ca2+ transients displayed more variability in the presence of 1 mM caffeine than in control conditions. At this dose of caffeine, extra-junctional sources of Ca2+ release also were observed occasionally.

Modulation by caffeine of calcium-release microdomains in frog skeletal muscle fibers

The effects of caffeine on the process of excitation-contraction coupling in amphibian skeletal muscle fibers were investigated using the confocal spot detection technique. This method permits to carefully discriminate between caffeine effects on the primary sources of Ca2+ release at the Z-lines where the triads are located and secondary actions on other potential Ca Release sources. Our results demonstrate that 0.5 mM caffeine potentiates and prolongs localized action-potential evoked Ca2+ transients recorded at the level of the Z-lines, but that 1mM only prolongs them. The effects at both doses are reversible. At the level of the M-line, localized Ca2+ transients displayed more variability in the presence of 1 mM caffeine than in control conditions. At this dose of caffeine, extra-junctional sources of Ca2+ release also were observed occasionally.