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

Quiñonez M, González F, Morgado-Valle C, DiFranco M - J. Muscle Res. Cell. Motil. (2010)

Bottom Line: Similar effects were found for the Ca(2+) transients elicited by the first pulse of 100 Hz trains.Changes in Ca(2+) transients along the trains were associated with impaired or abortive APs.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.

View Article: PubMed Central - PubMed

Affiliation: Laboratorio de Fisiología y Biofisíca del Músculo, IBE, UCV, Caracas, Venezuela. mquinonez@mednet.ucla.edu

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.

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Calcium transients elicited by single stimulation at different membrane potentials. A Traces 1–4 are Ca2+ transients calculated from OGB-5N fluorescence transients elicited at membrane potentials of −100, −80, −60 and −55 mV, respectively. B AP’s corresponding to Ca2+ transients in A. Note differences in time scales. C Ca2+ transients in A presented in a normalized scale. D Calcium transients calculated from OGB-5N fluorescence transients elicited at −100 before applying the depolarizing protocol (thin trace) and 3 min after repolarizing the fiber (thick trace). Sarcomere length: 4.5 μm
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Fig4: Calcium transients elicited by single stimulation at different membrane potentials. A Traces 1–4 are Ca2+ transients calculated from OGB-5N fluorescence transients elicited at membrane potentials of −100, −80, −60 and −55 mV, respectively. B AP’s corresponding to Ca2+ transients in A. Note differences in time scales. C Ca2+ transients in A presented in a normalized scale. D Calcium transients calculated from OGB-5N fluorescence transients elicited at −100 before applying the depolarizing protocol (thin trace) and 3 min after repolarizing the fiber (thick trace). Sarcomere length: 4.5 μm

Mentions: Opposite to the monotonic effect of depolarization on resting [Ca2+], fiber depolarization from −100 to −55 had a complex effect on Ca2+ release. Depolarization from −100 to −90 mV had no significant effects on Ca2+ transients, but further depolarization up to −55 mV produced profound changes on the features of the Ca2+ transients (Fig. 4A and C, traces 2–4). It can be seen that, in this range of potentials, depolarization has a dual effect on the amplitude of Ca2+ transients. The amplitude increases with depolarization from −90 to −60 mV. Trace 3 in Fig. 4A shows the maximum potentiation of Ca2+ transient by depolarization; reached in this fiber at −60 mV. It can also be seen that the increase in the amplitude of the Ca2+ transient is larger than the increase in the pre-stimulus (“resting”) [Ca2+]. Ca2+ transients are markedly depressed by further depolarization between −60 and −55 mV (Fig. 4A, trace 4), and totally absent thereafter (not shown). Figure 4C shows that biphasic changes in transient amplitude are accompanied with a monotonic increase in both the transient duration and the time to peak of the Ca2+ transient. The increase in transient duration is due to a slowing of both its rising and falling phases. All the depolarization-dependent changes in the Ca2+ transient could be completely reversed by repolarizing the fiber to −100 mV as shown by the similarity of Ca2+ transients calculated from fluorescence data obtained before depolarizing the fibers (thin trace, Fig. 4D) and after repolarizing it back to −100 mV (thick trace, Fig. 4D). The maximal potentiation of the amplitude of the Ca2+ transient was 17.2 ± 7.3% (n = 8, P < 0.05). This value is significantly smaller than the maximal twitch amplitude potentiation (~56%) seen in amphibian fibers in the presence of 9 mM (Renaud and Light 1992), but similar to the potentiation found in mammalian fibers exposed to 10 mM (Cairns et al. 1997). The difference in percentage potentiation of both variables in frog fibers probably reflects the non linear dependence of force on free [Ca2+]. The difference in force potentiation between amphibian and mammalian fibers is interesting and deserves further investigation.Fig. 4


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

Quiñonez M, González F, Morgado-Valle C, DiFranco M - J. Muscle Res. Cell. Motil. (2010)

Calcium transients elicited by single stimulation at different membrane potentials. A Traces 1–4 are Ca2+ transients calculated from OGB-5N fluorescence transients elicited at membrane potentials of −100, −80, −60 and −55 mV, respectively. B AP’s corresponding to Ca2+ transients in A. Note differences in time scales. C Ca2+ transients in A presented in a normalized scale. D Calcium transients calculated from OGB-5N fluorescence transients elicited at −100 before applying the depolarizing protocol (thin trace) and 3 min after repolarizing the fiber (thick trace). Sarcomere length: 4.5 μm
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Related In: Results  -  Collection

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Fig4: Calcium transients elicited by single stimulation at different membrane potentials. A Traces 1–4 are Ca2+ transients calculated from OGB-5N fluorescence transients elicited at membrane potentials of −100, −80, −60 and −55 mV, respectively. B AP’s corresponding to Ca2+ transients in A. Note differences in time scales. C Ca2+ transients in A presented in a normalized scale. D Calcium transients calculated from OGB-5N fluorescence transients elicited at −100 before applying the depolarizing protocol (thin trace) and 3 min after repolarizing the fiber (thick trace). Sarcomere length: 4.5 μm
Mentions: Opposite to the monotonic effect of depolarization on resting [Ca2+], fiber depolarization from −100 to −55 had a complex effect on Ca2+ release. Depolarization from −100 to −90 mV had no significant effects on Ca2+ transients, but further depolarization up to −55 mV produced profound changes on the features of the Ca2+ transients (Fig. 4A and C, traces 2–4). It can be seen that, in this range of potentials, depolarization has a dual effect on the amplitude of Ca2+ transients. The amplitude increases with depolarization from −90 to −60 mV. Trace 3 in Fig. 4A shows the maximum potentiation of Ca2+ transient by depolarization; reached in this fiber at −60 mV. It can also be seen that the increase in the amplitude of the Ca2+ transient is larger than the increase in the pre-stimulus (“resting”) [Ca2+]. Ca2+ transients are markedly depressed by further depolarization between −60 and −55 mV (Fig. 4A, trace 4), and totally absent thereafter (not shown). Figure 4C shows that biphasic changes in transient amplitude are accompanied with a monotonic increase in both the transient duration and the time to peak of the Ca2+ transient. The increase in transient duration is due to a slowing of both its rising and falling phases. All the depolarization-dependent changes in the Ca2+ transient could be completely reversed by repolarizing the fiber to −100 mV as shown by the similarity of Ca2+ transients calculated from fluorescence data obtained before depolarizing the fibers (thin trace, Fig. 4D) and after repolarizing it back to −100 mV (thick trace, Fig. 4D). The maximal potentiation of the amplitude of the Ca2+ transient was 17.2 ± 7.3% (n = 8, P < 0.05). This value is significantly smaller than the maximal twitch amplitude potentiation (~56%) seen in amphibian fibers in the presence of 9 mM (Renaud and Light 1992), but similar to the potentiation found in mammalian fibers exposed to 10 mM (Cairns et al. 1997). The difference in percentage potentiation of both variables in frog fibers probably reflects the non linear dependence of force on free [Ca2+]. The difference in force potentiation between amphibian and mammalian fibers is interesting and deserves further investigation.Fig. 4

Bottom Line: Similar effects were found for the Ca(2+) transients elicited by the first pulse of 100 Hz trains.Changes in Ca(2+) transients along the trains were associated with impaired or abortive APs.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.

View Article: PubMed Central - PubMed

Affiliation: Laboratorio de Fisiología y Biofisíca del Músculo, IBE, UCV, Caracas, Venezuela. mquinonez@mednet.ucla.edu

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.

Show MeSH
Related in: MedlinePlus