The Effect of Calcium on Potassium Contractures of Single Slow Muscle Fibres of Xenopus Laevis

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Abstract
Isolated slow muscle fibresof Xenopus laeviswere subjectcd to step increases in external potassium concentration at various external calcium concentrations. An increase in [Ca]oshifted the curve relating peak tension to log [K]oto higher K‐koncentrations. a decrease in [Ca]ocaused a shift in the opposite direction. Similar effects were seen on the relation between inactivation and log [K]o+. The peak tension was but little affected by [Ca]o; the time‐course of contractures was altered in such a way that the rate of relaxation was increased in low [Ca]oand decreased in high [Ca]o. Evidence was obtained that the effect of altered [Ca]ooccurs as rapidly as that of altered [K]o. Some observations were also made on contractures in Ca‐free solutions and in solutions with Na partially subatituted bv Ba.