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Relaxation, [Ca2+]i and [Mg2+]i during prolonged tetanic stimulation of intact, single fibres from mouse skeletal muscle


Relaxation, [Ca2+]i and [Mg2+]i during prolonged tetanic stimulation of intact, single fibres from mouse skeletal muscle



Journal of Physiology 480: 31-43



ISSN/ISBN: 0022-3751

PMID: 7853224

DOI: 10.1113/jphysiol.1994.sp020338

1. In skeletal muscle there is generally a slowing of relaxation with increasing tetanus duration and it has been suggested that this is due to Ca-2+ loading of parvalbumin (PA). To study this we have produced prolonged tetani in intact, single fibres from a mouse foot muscle which contain a high concentration of PA. We measured the rate of tension relaxation and also various aspects of Ca-2+ handling. 2. During 'interrupted' tetani (15 repeated cycles of 100 ms with stimulation and 50 ms without) we observed a marked slowing of the relaxation both under control conditions and in acidosis (obtained by increasing the bath CO-2 content). This slowing was not accompanied by any reduction of the initial rate of decline of the free myoplasmic Ca-2+ concentration ((Ca-2+)-i), which was measured with indo-1. 3. The functioning of the sarcoplasmic reticulum (SR) pump after tetani of various durations was analysed by plotting d(Ca-2+)-i/dt vs. (Ca-2+)-i during the final slow decline of (Ca-2+)-i after tetani. This analysis showed that the rate of SR Ca-2+ pumping after a 1 s tetanus is less than half of that after a 100 ms tetanus. 4. The amplitude of the tail of (Ca-2+)-i 250 ms into relaxation was measured after tetani of various durations. This amplitude increased with tetanus duration and could be fitted to the sum of one exponential and one linear function. The exponential component increased with a time constant of 0.17 s and probably reflects Ca-2+ loading of PA. 5. Ca-2+ binding to PA will displace Mg-2+ and hence the free myoplasmic concentration of Mg-2+ ((Mg-2+) will increase. To study this we used the fluorescent Mg-2+ indicator furaptra. The results showed an increase of (Mg-2+)-i during prolonged tetani which, after removing the Ca-2+ component of the fluorescent signal, amounted to about 0.5 mM. 6. A model of Ca-2+ movements and tension production in skeletal muscle was used. The model showed that the increase of the amplitude of (Ca-2+)-i tails after tetani of various durations can be explained by the combined effect of Ca-2+ loading of PA and slowed SR Ca-2+ pumping. In contrast to the experimental data, the model predicted a slight reduction of the initial rate of (Ca-2+)-i decline with increased tetanus duration. 7. In conclusion, we observed a marked slowing of relaxation during prolonged tetanic stimulation. Altered Ca-2+ handling, including Ca-2+ loading of PA, seems not to be important for this slowing. Thus, the slowing appears to be due to altered cross-bridge kinetics.

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