Verapamil-sensitive and less sensitive contractions in the intestinal smooth muscle of the guinea-pig taenia caeci.

Effect of verapamil, a Ca2+ channel blocker, on the contractions in the intestinal smooth muscle of taenia isolated from guinea-pig caecum was examined. In normal medium, 10 mM-80 mM KCl or 10(-7) M-10(-5) M carbachol induced a transient contraction followed by a sustained one. When the muscle strips were treated with a Ca2+-free solution for 2 min, these stimulants failed to induce contraction except an initial transient contraction induced by 10(-6) M-10(-5) M carbachol. Cumulative application of verapamil during the sustained contraction induced muscle relaxation. Higher concentration of verapamil was needed to inhibit the sustained contraction induced by 10 mM or 20 mM K+ than that induced by 40 mM or 80 mM K+. Similarly, the sensitivity to verapamil of the sustained contraction induced by 10(-7) M carbachol was lower than that induced by 10(-6) M or 10(-5) M carbachol. In the presence of verapamil, addition of either high K+ or carbachol induced an initial transient contraction, although the sustained contraction was strongly inhibited. These results suggest that high K+ and carbachol activate two types of Ca2+ channels; lower concentrations of the stimulants open the channel which is less sensitive to verapamil, and higher concentrations open the channel which is more sensitive to verapamil.