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

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.