Comparison of the action of La³⁺ and Ca²⁺ on contraction threshold and other membrane parameters of frog skeletal muscle

Summary

The influence of La³⁺ on contraction threshold, on membrane input resistance, and on action potential parameters was investigated in fibers of the sartorius muscle of the frog, and it was compared to that of Ca²⁺. The dependence of the contraction threshold on [La³⁺]₀ in the presence of 0.5mm Ca²⁺ gave a sigmoid relationship between 0.1 and 5mm La³⁺ with a shift of 23 to 34 mV to less negative potentials following a 10-fold increase of [La³⁺]₀. The membrane input resistance was increased to various degrees in La-containing solutions, the increase beingirreversible. The threshold of action potential generation was shifted to less negative potentials by 28 mV, and the duration at half-maximal amplitude was tripled by 0.5mm La³⁺. In comparison a 10-fold increase of [Ca²⁺]₀ in the range of 0.5 to 50mm shifted the contraction threshold by 15 mV to less negative potentials. 17mm Ca²⁺, a concentration having the same effect on contraction threshold as 0.5mm La³⁺, increased membrane input resistancereversibly, shifted the action potential threshold by 16 mV to less negative potentials, and had only minor effects on action potential duration. Conduction was never blocked by Ca²⁺ as it was with 1mm La³⁺. In a theoretical treatment, it is shown that the influence of Ca²⁺ on contraction threshold, butnot that of La³⁺, may be accounted for by its screening and binding to negative surface charges according to the Gouy-Chapman theory of the diffuse double layer. To describe the action of La³⁺ on the contraction threshold an additional interaction of La³⁺ with neutral but amphoteric sites was considered.