Calcium and the Contractile Effect of Carbachol in the Depolarized Guinea-Pig Taenia Caecum

Carbachol (10−8–10−3 M) caused a phasic contraction in the guinea-pig taenia caecum suspended in the Ca-free, K-Tyrode solution. The action of carbachol was concentration-related and was completely antagonized by atropine (10−6 g/ml). The capacity to contract in the absence of external Ca was removed by shortterm exposure to a high concentration of carbachol (more than 5×10−4 M), but was restored after treatment with Ca (0.1–2.0 mM). Carbachol, 10−3 M, was used throughout the following experiments. The degree of restoration increased with the duration of treatment with Ca and with the concentration of Ca. The restored capacity was not maintained but wore off again after return to the Ca-free solution. The time courses of restoration and disappearance of the capacity following addition and withdrawal of Ca were much slower than those of the Ca-contracture and its relaxation. The rate constant for loss of Ca component responsible for the capacity was calculated as 0.063 min−1 by assuming that there is a hyperbolic function between steady isometric tension and [Ca]1. These results suggest that carbachol mobilizes Ca ions from stored Ca, which is in a dynamic equilibrium with [Ca]0, to activate the contractile proteins.