DEPENDENCE OF HEMICHOLINIUM ACTION ON CALCIUM CONCENTRATION’

VINCENZI, FRANK F. AN1) THEODORE C. WEST: Dependence of h:micholiniuni action on calcium concentration. J. Pharmacol. 152: 1-7, 1966. The hypothesis that the action of hemicholinium (HC-3) is directly dependent on the extracellular Ca concentration Was tested in the spontaneously beating right atrial preparation of the rabbit. The negative (‘hronotroluc response following electrical stimulation of intrasinoatrial node vagal fibers (cholinergic electrorelease response, or CERR) was used to test the antagonistic action of HC-3. Hemicholinium was applied in nutrient solution containing Ca concentrations rang-ing from 0.1 to 10.0 niM. The amount of cholinergic blockade by HC-3 was a direct function of the Ca concentration. The aiiiount of blockade was also directly correlated with the niagnitude of the negative chronotropic response to vagal stimulation. On the assumption that the magnitude of the CERR was l)roPortional to the release of acetylchohne (ACh), the relationship was interpreted to illustrate that depletion of l)res’nal)tic stores of ACh underlies the cholinergit blockade produced by HC-3. It j ‘tll known that the excitatioii-iiiduce