Antimuscarinic action of quinidine on the heart? A study in myocardial preparations from cat hearts.

Quinidine exerts anticholinergic effects which have been ascribed to atropine-like properties of the drug. We have examined the effects of acetylcholine on the force of contraction in isolated heart muscle preparations from cats and compared the inhibitory effects of atropine with those of quinidine. The effects of acetylcholine were antagonized competitively in the presence of atropine. The Schild-plot yielded a straight line; the slope was not significantly different from unity. In the presence of quinidine, the concentration-response curve of acetylcholine was shifted to the right as with atropine, however, the Schild-plot yielded a regression line which was not linear; the slope was statistically different from unity. The negative inotropic response to acetylcholine in cat ventricular heart muscle (revealed in the presence of the phosphodiesterase inhibitor, papaverine) was antagonized by atropine but not influenced by quinidine. We conclude that the inhibitory action of quinidine on the effects of acetylcholine in atrial heart muscle is not merely antimuscarinic. The antagonistic effects of acetylcholine and quinidine on atrial heart muscle may also be due to the opposite effects of the drugs on potassium conductance of the myocardial cell membrane.