Molefhe, Tebogo Rosemary; (1992) Effects of extracellular magnesium on skeletal muscle function. Doctoral thesis (Ph.D.), University College London.

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Abstract

Calcium uptake by the sarcoplasmic reticulum is thought to be too slow to explain the rapid relaxation seen in frog skeletal muscle. It has been suggested that calcium binding by parvalbumin a myoplasmic calcium and magnesium binding protein, aids the calcium pump in achieving the rapid rate observed. This work, which investigates the role of magnesium in determining the rate of calcium removal by parvalbumin following activation shows that increasing extracellular magnesium ion concentration results in a decline in maximum force production during the latter stages of a prolonged contraction. High extracellular magnesium ion concentration also increases relaxation rate and labile heat production, processes which reflect calcium binding to parvalbumin during the substitution reaction: Pa-Mg + Ca = Pa-Ca + Mg. These observations suggest the removal of calcium from troponin and the myoplasm by parvalbumin when the concentration of calcium binding sites on parvalbumin is raised. The changes described above are thought to be mediated by an increase in intracellular magnesium levels when the extracellular magnesium ion concentration was high. The role of intracellular magnesium ion concentration was tested by the construction of a model in which the magnesium concentration in the intracellular compartment was raised directly. The model shows that increasing intracellular magnesium ion levels increases relaxation rate and diminishes force production in a manner similar to that seen when a muscle is bathed in medium with a high extracellular magnesium concentration. The changes are accompanied by an increase in parvalbumin-calcium at the expense of parvalbumin-magnesium. The findings of this study, therefore, suggest that seasonal changes in intracellular magnesium ion concentration could regulate changes in relaxation rate in frogs, which are poikilotherms, by increasing the concentration of parvalbumin-magnesium in the myoplasm and so the concentration of calcium buffer in the myoplasm.

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