Alterations in calcium homeostasis reduce membrane excitability in amphibian skeletal muscle |
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Authors: | Juliet A. Usher-Smith Wei Xu James A. Fraser Christopher L.-H. Huang |
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Affiliation: | (1) Physiological Laboratory, University of Cambridge, Downing Street, Cambridge, CB2 3EG, UK |
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Abstract: | The effects of alterations in intracellular calcium homeostasis on surface membrane excitability were investigated in resting Rana temporaria sartorius muscle. This was prompted by initial results from a fatiguing stimulation protocol study that demonstrated a fibre subpopulation in which action potential generation in response to a standard 1.5 V electrical stimulus failed despite mean membrane potentials [E m, −69±2.3 mV (n=14)] compatible with spike firing in a control set of quiescent muscle fibres. Intracellular micro-electrode recordings showed a similar reversible loss of excitability, attributable to an increased threshold, despite only small (7.1±1.8 mV) positive changes in E m after approximately 60-min exposures to nominally 0 Ca2+ Ringer solutions in which Ca2+ was replaced by Mg2+. This effect was not reproduced by addition of Mg2+ to the Ringer solution and persisted under conditions of Cl− deprivation. The effects of three pharmacological agents, cyclopiazonic acid (CPA), caffeine and 4-chloro-m-cresol (4-CmC), each known to deplete store Ca2+ and increase cytosolic Ca2+ through contrasting mechanisms without influencing E m, were then investigated. All three agents produced a more rapid, but nevertheless still reversible, loss of membrane excitability than in 0 Ca2+ Ringer solution alone. This reduction in membrane excitability persisted in fibres studied in solutions containing a normal [Ca2+] following prior depletion of store Ca2+ using CPA- and 4-CmC-containing solutions. These novel findings suggest that sarcoplasmic reticulum Ca2+ content profoundly influences surface membrane excitability, thereby providing a potential mechanism by which spike firing fails in well-polarised fibres during fatigue.The authors Usher-Smith and Xu were equal contributors to this paper. |
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Keywords: | Membrane excitability Calcium store Fatigue Skeletal muscle |
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