Effects of Lanthanum on Potassium Contractures of Isolated Twitch Muscle Fibres of the Frog

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Abstract
The effects of lanthanum on the time course of potassium‐induced contractures of isolated muscle fibres from the frog were investigated. Two kinds of effects could be distinguished: 1. A reduction in the rate of tension development and a decrease in contracture amplitude; 2. A prolongation of the tension plateau and a decrease in the rate of relaxation during sustained depolarization. The effects under point 1 were obtained at lanthanum concentrations ≥ 0.5 mM, while those under point 2 were clearly visible at lanthanum concentrations as low as 0.01 mM. In a calcium‐free medium containing 0.1 mM lanthanum, a muscle fibre produced repeatable contractures to 117.5 mM potassium over many hours. The slowing of tension development and the diminished contracture amplitude is explainable on the basis of a reduction in the rate of release oi activator calcium in response to a depolarization step. The prolongation of the platmu and the slowing of relaxation caused by lanthanum is interpreted as reflecting a reduced rate of inactivation of thc calcium‐release mechanism following depolarization.