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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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Effects of membrane depolarization on Ca2+ transients elicited by repetitive stimulation. A–D Ca2+ transients elicited by 100 Hz stimulation in a fiber held at −100, −80, −60 and −55 mV, respectively. E–H First and last Ca2+ transients of A–D displayed in expanded time scales, respectively. The dashed lines in A–H indicate the resting [Ca2+]. I–L Electrical records corresponding to the Ca2+ transients shown in A–D, respectively. M–P Expanded time presentation of AP’s recorded simultaneously with the Ca2+ transients shown in E–H, respectively. Different voltage scales were used for M–P. The pulse amplitude was not changed. The dashed lines in I–P indicated the resting and zero potentials. The numbers in panels A–P indicate the position of responses along the train. Records were taken ~3 min after changing membrane potential
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Fig6: Effects of membrane depolarization on Ca2+ transients elicited by repetitive stimulation. A–D Ca2+ transients elicited by 100 Hz stimulation in a fiber held at −100, −80, −60 and −55 mV, respectively. E–H First and last Ca2+ transients of A–D displayed in expanded time scales, respectively. The dashed lines in A–H indicate the resting [Ca2+]. I–L Electrical records corresponding to the Ca2+ transients shown in A–D, respectively. M–P Expanded time presentation of AP’s recorded simultaneously with the Ca2+ transients shown in E–H, respectively. Different voltage scales were used for M–P. The pulse amplitude was not changed. The dashed lines in I–P indicated the resting and zero potentials. The numbers in panels A–P indicate the position of responses along the train. Records were taken ~3 min after changing membrane potential

Mentions: To assess the effect of depolarization on Ca2+ release during repetitive activation in the absence of ionic concentration changes, fibers were depolarized at various levels by current injection and stimulated with 10 pulses applied at 100 Hz (Fig. 6). Except for the potentiation of the first transient of the train, depolarization from −100 to −80 mV had little effect on Ca2+ release along the train (Fig. 6A, B). In contrast, depolarization to −60 mV had differential effects on Ca2+ transients along the train. Maximal potentiation was seen in response to the first pulse of the train whereas depression of Ca2+ transients was observed from the second to the last stimulus (Fig. 6E, F). It can also be seen that the release is irregular along the train, i.e. transients of alternating amplitude can be detected along the train. Nevertheless, opposite to the case for the first transient, the amplitudes of the 2nd to 10th Ca2+ transients at −60 mV were always smaller than those detected from −100 to −80 mV. At −55 mV, only a highly depressed Ca2+ transient elicited by the first stimulus is seen. Potentiation along the train (2nd to 10th pulses) was observed in a narrower potential range as compared with the responses to single pulses. An example of potentiation of the Ca2+ release from a fiber depolarized to −70 mV is shown in Fig. 8B. As can be seen, all Ca2+ transients along the train (in response to the 2nd to 10th pulses) were larger than those recorded at −100 to −80 mV. As expected, the effects of depolarization on the first pulse of the trains are identical to those observed in response to single stimulation. Figure 6E–H compare, in an expanded time scale, the first and last Ca2+ transient (if present) of every train. Figure 6G highlights the potentiation and prolongation of the first Ca2+ transient at −60 mV, as compared to those recorded at more negative membrane potentials. The electrical records corresponding to the Ca2+ transients shown in Fig. 6A–H are presented in Fig. 6I–P. Other than the imposed depolarization, little differences are detected among the AP trains elicited between −100 and −80 mV (Fig. 6I, J and M, N). Although active responses are elicited by all pulses in the train, only the first AP display a significant overshoot at a holding potential of −60 mV (~20 mV, Fig. 6K, O). As seen with single stimulation, this smaller AP is associated with a potentiated Ca2+ transient, while the rest of APs along the train elicits depressed releases. It can also be seen that variability in the Ca2+ transient amplitude is associated with corresponding, but smaller, variations in AP amplitude. It is remarkable that a difference of ~3 mV between the overshoots of the 9th and 10th AP is associated with relatively larger changes in Ca2+ transient amplitude (Fig. 6G, O). At −55 mV only the first pulse of the train elicits a regenerative active response; the rest of responses along the trains are abortive (Fig. 6L, P). The fact that at this potential the fiber cannot sustain active responses to 100 Hz stimulation explains why there is no Ca2+ release along the train but the depressed Ca2+ release associated with the first AP (Fig. 6D). Thus, although single small APs can be generated at −55 mV, at this potential the excitability is so compromised that active responses to trains of stimuli cannot be generated. It is important to note that the lack of electrical response at highly depolarized potentials occurs even when the applied pulses depolarize the surface membrane to about −5 mV (Fig. 6O, P). The pattern of responses described in Fig. 6 and Fig. 8B was confirmed in 5 fibers.Fig. 6


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)

Effects of membrane depolarization on Ca2+ transients elicited by repetitive stimulation. A–D Ca2+ transients elicited by 100 Hz stimulation in a fiber held at −100, −80, −60 and −55 mV, respectively. E–H First and last Ca2+ transients of A–D displayed in expanded time scales, respectively. The dashed lines in A–H indicate the resting [Ca2+]. I–L Electrical records corresponding to the Ca2+ transients shown in A–D, respectively. M–P Expanded time presentation of AP’s recorded simultaneously with the Ca2+ transients shown in E–H, respectively. Different voltage scales were used for M–P. The pulse amplitude was not changed. The dashed lines in I–P indicated the resting and zero potentials. The numbers in panels A–P indicate the position of responses along the train. Records were taken ~3 min after changing membrane potential
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Related In: Results  -  Collection

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Fig6: Effects of membrane depolarization on Ca2+ transients elicited by repetitive stimulation. A–D Ca2+ transients elicited by 100 Hz stimulation in a fiber held at −100, −80, −60 and −55 mV, respectively. E–H First and last Ca2+ transients of A–D displayed in expanded time scales, respectively. The dashed lines in A–H indicate the resting [Ca2+]. I–L Electrical records corresponding to the Ca2+ transients shown in A–D, respectively. M–P Expanded time presentation of AP’s recorded simultaneously with the Ca2+ transients shown in E–H, respectively. Different voltage scales were used for M–P. The pulse amplitude was not changed. The dashed lines in I–P indicated the resting and zero potentials. The numbers in panels A–P indicate the position of responses along the train. Records were taken ~3 min after changing membrane potential
Mentions: To assess the effect of depolarization on Ca2+ release during repetitive activation in the absence of ionic concentration changes, fibers were depolarized at various levels by current injection and stimulated with 10 pulses applied at 100 Hz (Fig. 6). Except for the potentiation of the first transient of the train, depolarization from −100 to −80 mV had little effect on Ca2+ release along the train (Fig. 6A, B). In contrast, depolarization to −60 mV had differential effects on Ca2+ transients along the train. Maximal potentiation was seen in response to the first pulse of the train whereas depression of Ca2+ transients was observed from the second to the last stimulus (Fig. 6E, F). It can also be seen that the release is irregular along the train, i.e. transients of alternating amplitude can be detected along the train. Nevertheless, opposite to the case for the first transient, the amplitudes of the 2nd to 10th Ca2+ transients at −60 mV were always smaller than those detected from −100 to −80 mV. At −55 mV, only a highly depressed Ca2+ transient elicited by the first stimulus is seen. Potentiation along the train (2nd to 10th pulses) was observed in a narrower potential range as compared with the responses to single pulses. An example of potentiation of the Ca2+ release from a fiber depolarized to −70 mV is shown in Fig. 8B. As can be seen, all Ca2+ transients along the train (in response to the 2nd to 10th pulses) were larger than those recorded at −100 to −80 mV. As expected, the effects of depolarization on the first pulse of the trains are identical to those observed in response to single stimulation. Figure 6E–H compare, in an expanded time scale, the first and last Ca2+ transient (if present) of every train. Figure 6G highlights the potentiation and prolongation of the first Ca2+ transient at −60 mV, as compared to those recorded at more negative membrane potentials. The electrical records corresponding to the Ca2+ transients shown in Fig. 6A–H are presented in Fig. 6I–P. Other than the imposed depolarization, little differences are detected among the AP trains elicited between −100 and −80 mV (Fig. 6I, J and M, N). Although active responses are elicited by all pulses in the train, only the first AP display a significant overshoot at a holding potential of −60 mV (~20 mV, Fig. 6K, O). As seen with single stimulation, this smaller AP is associated with a potentiated Ca2+ transient, while the rest of APs along the train elicits depressed releases. It can also be seen that variability in the Ca2+ transient amplitude is associated with corresponding, but smaller, variations in AP amplitude. It is remarkable that a difference of ~3 mV between the overshoots of the 9th and 10th AP is associated with relatively larger changes in Ca2+ transient amplitude (Fig. 6G, O). At −55 mV only the first pulse of the train elicits a regenerative active response; the rest of responses along the trains are abortive (Fig. 6L, P). The fact that at this potential the fiber cannot sustain active responses to 100 Hz stimulation explains why there is no Ca2+ release along the train but the depressed Ca2+ release associated with the first AP (Fig. 6D). Thus, although single small APs can be generated at −55 mV, at this potential the excitability is so compromised that active responses to trains of stimuli cannot be generated. It is important to note that the lack of electrical response at highly depolarized potentials occurs even when the applied pulses depolarize the surface membrane to about −5 mV (Fig. 6O, P). The pattern of responses described in Fig. 6 and Fig. 8B was confirmed in 5 fibers.Fig. 6

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