Membrane charge moved at contraction thresholds in skeletal muscle fibres

Abstract

1. The current I_Q due to membrane charge movement and the threshold pulse duration t_th required to produce microscopically just-detectable contraction were determined for pulses to a variety of membrane potentials in tendon-terminated short segments of cut frog skeletal muscle fibres voltage-clamped using a single gap technique.

2. The time course Q(t) of membrane charge movement was calculated as the running integral of I_Q. The threshold charge Q_th moved by pulses which produced just-detectable contraction was estimated as Q(t_th).

3. Q_th was constant for pulses to potentials ranging from about -45 mV, the rheobase potential for contraction, to about -15 mV, where t_th was about 9 msec. The mean Q_th from fourteen fibres was 11.5 nC/μF, when the holding potential was about -100 mV.

4. Prepulses of 50 msec which were themselves sub-rheobase for producing contraction decreased the t_th for an immediately following test pulse. The total threshold charge moved during the prepulse and during t_th of the test pulse was equal to Q_th for the test pulse without prepulse.

5. Items 3 and 4 above indicate that t_th is determined by the time required to move a set amount of intramembrane charge, independent of the kinetics of the charge movement.

6. Steady partial fibre depolarization to between -70 and -55 mV increased t_th at all membrane potentials and elevated the rheobase potential for contraction. Slight further steady depolarization totally eliminated contraction.

7. Steady partial depolarization decreased the total ON charge movement Q_ON by about the same factor for pulses to all potentials tested.

8. Q_th for partially depolarized but still-contracting fibres remained approximately independent of membrane potential from rheobase to about 0 mV but was slightly less than Q_th for the same fibres when fully polarized.

9. Steady partial depolarizations which reduced the mean (±s.d.) ON charge movement Q_ON to 60 ± 8% of its value under full polarization reduced Q_th to 86 ± 11% of its full polarization value (n = 10). These steady partial depolarizations produced no change in the linear capacitance measured with hyperpolarizing pulses.

10. Contraction was completely abolished by steady partial depolarizations which reduced Q_ON to 41% of its value under full polarization (mean of three runs). The maximum value of Q_ON was then 77% of the Q_th value for the same fibres under full polarization.

11. A prolonged tail, a shoulder, a second rising phase or an early relatively high flat segment were successively evident in the I_Q records as the depolarizing pulse was successively increased to and beyond the rheobase potential for contraction. It was found that t_th either coincided with or occurred slightly later than the start of such tails, shoulders or second rising phases.

12. When test pulse I_Q records with and without immediately preceding sub-rheobase prepulses were shifted in time so that their t_th times coincided, the record with prepulse coincided with the later part of I_Q without prepulse. This indicates that sub-rheobase prepulses moved the initial portion of the I_Q that occurs during the test pulse alone, whereas they did not alter the latter portion of the test pulse I_Q.

13. A model was developed which accounts for charge movement's voltage dependence and kinetics and for the relationship between charge movement and just-detectable contraction in both the fully polarized and partially depolarized states.

14. The model proposes that Q be composed of two components. Component A is due to the voltage and time-dependent movement of charges between two sites located within the membrane and separated by a single energy barrier. Component B is instantaneously proportional to an integer power n of the fraction of component A charges which have crossed the barrier.

15. The I_Q time courses were best approximated using n = 3, with which both the relatively early and late portions of the experimental I_Q time courses could be reproduced. The best theoretical records obtained with n = 3 still passed below the shoulders, second rising phases and later parts of the early constant phases in the various experimental I_Q records. Theoretical records did fit accurately the I_Q time courses observed under steady partial fibre depolarization. The relatively small current not reproduced by the model may be an electrical accompaniment of the activation of calcium release or the elevation of internal free calcium levels in the space between the transverse tubules (T-tubules) and the sarcoplasmic reticulum.

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