Species differences in inhibitory effect of Li+ on high K+-induced contraction in the ileal longitudinal smooth muscle

The inhibitory effect of Li+ on contractile elements of intestinal smooth muscle

The mechanism of the inhibitory effect of Li+ on contraction was examined in guinea pig ileal longitudinal smooth muscle. Li(+)-substitution (68.4 mM) reversed contractions induced by high K+ (45.4 mM), carbachol (1 microM) and histamine (1 microM) without changing the cytosolic Ca2+ level. Li+ also had no effect on the increase in 45Ca2+ uptake stimulated by high K+. High K+ transiently increased myosin light chain (MLC) phosphorylation, reaching a peak at 6-9 sec. Li(+)-substitution inhibited the high K(+)-induced MLC phosphorylation. In permeabilized ileal strips, contraction induced by 1 microM Ca2+ was inhibited by 10 mM Li+. The inhibitory effect was antagonized by increasing the concentration of Ca2+ or calmodulin. In the permeabilized muscle in which MLC was previously thiophosphorylated with 1 mM ATP gamma S and 3 microM Ca2+, ATP induced contraction in Ca2+ free buffer. Li+ added during this contraction did not show an inhibitory effect. In contrast, when 30 mM Li+ was added during the thiophosphorylation, the contraction induced by the subsequent addition of ATP was inhibited. Li+ (30 mM) changed neither the rate of relaxation induced by removing external Ca2+ in permeabilized muscle nor the rate of dephosphorylation of MLC induced by crude phosphatase extracted from the ileum. Li+ (15 mM), on the other hand, inhibited the rate of phosphorylation of MLC caused by crude MLC kinase extracted from the ileum. Li+ did not inhibit the calmodulin activity as measured with the (Ca2+ +Mg2+)-ATPase activity of the erythrocyte membrane. These results suggest that the inhibitory effect of Li+ on contractions is attributable to the inhibition of MLC kinase in guinea pig ileum.

Inhibitory effect of lithium on muscle contractions caused by various stimulants in guinea-pig ileum

The tonic contractions induced in the guinea-pig ileal longitudinal muscle by 60 mM KCl, carbachol, histamine or A23187 were inhibited similarly by Li+-substituted solution. There was a good correlation between the Li+-induced inhibition of contractions induced by these stimulants and the intracellularly accumulated Li+ content. In the experiment with fura-2, Li+-substituted solution inhibited the high-K+-induced contraction without decreasing the cytosolic Ca2+ level elevated by high-K+ solution. The concentration-response curve for Ca2+ in the skinned fiber was shifted to the right by pretreatment with Li+-substituted solution. These results suggest that the inhibitory mechanism of Li+ on the contractions by various stimulants partly involves the direct inhibition of the contractile proteins in the guinea-pig ileal longitudinal muscle.