title: Effects of membrane depolarization and changes in extracellular [K(+)] on the Ca (2+) transients of fast skeletal muscle fibers. Implications for muscle fatigue.
abstract: 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.
authors.affiliation: Laboratorio de Fisiología y Biofisíca del Músculo, IBE, UCV, Caracas, Venezuela. mquinonez@mednet.ucla.edu
authors.forename: Marbella
authors.initials: M
authors.lastname: Quiñonez
authors.forename: Fernando
authors.initials: F
authors.lastname: González
authors.forename: Consuelo
authors.initials: C
authors.lastname: Morgado-Valle
authors.forename: Marino
authors.initials: M
authors.lastname: DiFranco
mesh_terms: D000818:Animals; D001001:Anura; D002118:Calcium / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*
chemical_list: D011188:Potassium; D002118:Calcium