Antagonism of calcium-induced contractions by some non-specific spasmolytics in depolarized smooth muscles

The effects of papaverine MgCl2, cocaine, DNP, KCN and khellin on responses of some rabbit and rat tissues to CaCl2 were studied in vitro in a depolarizing medium. Guinea pig taenia coli preparation was used for comparison. In rabbit tracheal chain and vas deferens and guinea pig taenia coli preparations all spasmolytics shifted the concentration-response curves of CaCl2 to the right without affecting the maxima or slopes. In rat tracheal chain and vas deferens preparations all spasmolytics shifted the concentration-response curves of CaCl2 to the right. Furthermore all agents (except cocaine in tracheal chain preparations) depressed the maximum responses. The slopes were unaffected in either preparations. The initial competition and subsequent noncompetition observed in certain tissues is discussed in the light of the reported poor capacity of some tissues to retain Ca++ and the absence of releasable firmly bound Ca++ (11).