Species Differences in Inhibitory Effect of Li⁺ on High K⁺-Induced Contraction in the Ileal Longitudinal Smooth Muscle

Abstract

The effects of Li⁺ substitution for external Na⁺ in the medium on a 60 mM KCl (60K⁺)-induced contraction were compared in the ileal longitudinal smooth muscles isolated from monkey, rabbit, guinea pig and rat. Li⁺ substitution inhibited the 60K⁺-induced contraction in the ileum of all the animal species. Greater inhibitory effect was observed with the increase in external Li⁺ concentration and the increase in incubation period with Li⁺. Intracellular Li⁺ contents of these muscles also increased with the increase in external Li⁺ concentration and the incubation period. Thus, a good correlation was obtained between Li⁺-induced inhibition of 60K⁺-induced contraction and the increase in Li⁺ content. However, close examination revealed that the contraction in monkey and rat ileum was inhibited by much lower Li⁺ accumulation than that in guinea pig and rabbit ileum, suggesting the existence of species difference in the sensitivity of the ileal smooth muscle to the inhibitory effect of Li⁺ substitution, and a mechanism for the species differences in the inhibitory action of Li⁺-substitution was discussed.