Actions of manganese and lanthanum in smooth muscles

Abstract

Effects of Mn²⁺ and La³⁺ on the excitation, contraction, ion movement, and biochemistry of smooth muscles were reviewed. Both Mn²⁺ and La³⁺ block the action potential of smooth muscles without affecting membrane resting potential. However, depolarization or hyperpolarization by these ions and slow discharges which are not affected by these ions have also been reported in some smooth muscles. Mn²⁺ and La³⁺ inhibit the spontaneous contraction and high-K-induced contracture, although these ions can also initiate slow tension development in some preparations. The drug-induced contractions are relatively insensitive to these ions. Mn²⁺ blocks Ca influx, and La³⁺ blocks both Ca influx and efflux. However, La-resistant Ca movements such as Na-Ca exchange or active Ca extrusion have also been reported. La³⁺ also shows effects on the movement of other ions. In biochemical experiments, La³⁺ shows effects on Ca movement of the membranous and microsomal fractions of smooth muscles, with variations among the smooth muscles. Thus, we should be careful of using these ions as mere Ca blockers.