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

Abstract

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 Ca3+ gave a sigmoid relationship between 0.1 and 5 mM 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 being irreversible. 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 0.5 mM La3+. In comparison a 10-fold increase of [Ca2+]0 in the range of 0.5 to 50 mM shifted the contraction threshold by 15 mV to less negative potentials. 17 mM Ca3+, a concentration having the same effect on contraction threshold as 0.5 mM La3+, increased membrane input resistance reversibly, 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 Ca3+ as it was with 1 mM La3+. In the theoretical treatment, it is shown that the influence of Ca3+ on contraction threshold, but not 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.

Effects of external cadmium ions on excitation-contraction coupling in rat soleus fibres

“…Since divalent cations do not easily cross the plasmalemma, and the effects of changing their concentrations are rapid, their immediate actions are thought to be at the fibre surface. The divalent cations reduce the electric field, "seen" by the voltage-sensitive processes, when they screen or bind to negatively charged groups on the membrane surface [4,6,9,16,21].…”

Potassium contractures and mechanical activation in rat skeletal muscle: effects of multivalent cations, temperature and tetracaine

“…Divalent cations in the extracellular bathing medium have been suggested to control these processes by binding or screening charged groups on the surface of the sarcolemma (D'Arrigo 1973;Dorrscheidt-Kafer, 1981). As the external cationic composition is reduced, an apparent depolarization is induced.…”

“…Some, but not all, divalent and trivalent cations can substitute for calcium in maintaining the amplitude and to a certain extent the time course of potassium contractures (Frank, 1962;Luttgau & Spiecker, 1979). The effects of di-and tri-valent cations on the excitation-contraction coupling of potassium contractures have been suggested to be exerted by changes in the surface membrane potential (D'Arrigo, 1973;Dorrscheidt-Kafer, 1981).…”

Pharmacological Investigations of Excitation‐Contraction Coupling

“…Large shifts in the contractile threshold are also produced by Ca 2+, Mg 2+, H+, and La 3+ (14, 61, 65, 78, 80a) and have been explained by binding and screening of negative fixed charges. Evidence favoring this view has been obtained with neuraminidase (80), which removes negative charges associated with sialic acids from the membrane, and ruthenium red (79), which is though to bind to and neutralize them. Both produced shifts of several millivolts in the contractile threshold.…”