Am J Physiol Cell Physiol AJP: Renal Physiology
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Am J Physiol Cell Physiol (November 23, 2005). doi:10.1152/ajpcell.00469.2005
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Submitted on September 20, 2005
Accepted on November 17, 2005

Long-lasting muscle fatigue: partial disruption of excitation-contraction coupling by the elevated cytosolic [Ca2+] during contractions

Esther Verburg1*, Travis L Dutka1, and Graham D Lamb1

1 Zoology, Latrobe University, Melbourne, Vic, Australia

* To whom correspondence should be addressed. E-mail: e.verburg{at}latrobe.edu.au.

The repeated elevation of cytosolic [Ca2+] above resting levels during contractile activity has been associated with long-lasting muscle fatigue. The mechanism underlying this fatigue appears to involve elevated levels of cytosolic [Ca2+] inducing disruption of the excitation-contraction (EC) coupling process at the triad junction. It is however unclear what aspects of the activity-related [Ca2+] changes are responsible for the deleterious effects, in particular whether it depends primarily on the peak [Ca2+] reached locally at particular sites or on the temporal summation of the increased [Ca2+] in the cytoplasm as a whole. In this study we used mechanically-skinned fibers from rat extensor digitorum longus muscle, in which the normal EC-coupling process remains intact. The cytosolic [Ca2+] was raised, either by applying a set elevated [Ca2+] throughout the fiber or by using action-potential stimulation to induce release of sarcoplasmic reticulum Ca2+ by the normal EC-coupling system with or without augmentation by caffeine or buffering with BAPTA. We show that elevating the [Ca2+] in the physiological range of 2-20µM irreversibly disrupts EC-coupling in a concentration-dependent manner, but requires exposure for a relatively long time (1-3 min) to cause substantial uncoupling. The effectiveness of Ca2+ released via the endogenous system in disrupting EC-coupling indicates that the relatively high [Ca2+] attained close to the release site in the triad junction is a more important factor than the increase in the bulk cytoplasmic [Ca2+]. Our results suggest that during prolonged vigorous activity, the many repeated episodes of relatively high triadic [Ca2+] can disrupt EC-coupling and lead to long-lasting fatigue.




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