Abstract
Changes in [Na+]o and [K+]o occurring during high frequency stimulation has been proposed as a cause of muscle fatigue. We investigated this hypothesis by measuring the Ca2+ release elicited by short high frequency trains (100Hz, 10 pulses) in rested frog semitendinosus fibers exposed to various [Na+]o or [K+]o. Myoplasmic [Ca2+] changes (Ca2+ transients) elicited by action potentials (AP) were estimated from Ca2+-dependent OGB-5N fluorescence changes. Segments of fibers, stretched to 4.5-5μm, were mounted in an inverted double grease-gap chamber placed in an inverted microscope equipped for epifluorescence. Fibers were held at -100mV and stimulated with 0.5ms current pulses. Normal Ringer solution contained (mM): 115 NaCl, 2.5 KCl, 1.8 CaCl2, 10 MOPS, 10 dextrose, pH=7.2 with NaOH. [K+]o ([Na+]o) was increased (reduced) by equimolar replacement with Na+ (N-methyl-D-glucamine). Fibers were loaded (30min) with a solution containing (mM): 110 aspartate, 5 ATP-K2, 5 Na2-creatine-phosphate, 20 MOPS, 0.05-0.1 EGTA, 5 MgCl2, 0.2 OGB-5N, pH=7.2 with KOH. A complex interaction of the effects of changing [K+]o or [Na+]o on membrane potential, AP overshoot and duration, and Ca2+ release was found. Using normal Ringer, the amplitude of Ca2+ transients elicited by single pulses increased with depolarizations up to -65mV. Raising [K+]o had a dual effect on Ca2+ release. Ca2+ transient's amplitude increased between 2.5 to 10 mM, and decreased markedly for higher [K]o. Potentiation of Ca2+ release, but not depression, could be reverted by current injection. This suggests a depolarization independent effect of K+ ions on Ca2+ release. Halving [Na+]o did not affect Ca2+ release elicited by single stimulation, but impaired the release in response to repetitive stimulation. Further reduction of [Na+]o to 1/3 of normal highly reduced Ca2+ release.
Published Version
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