On the nature of the antagonism atropine-acetylcholine on the guinea pig heart.

The antagonism atropine-acetylcholine was studied in the perfused isolated heart (chronotropism) and atria (inotropism) of the guinea pig by means of the double reciprocal plot method. It was not possible to study the antagonism of atropine towards the negative chronotropic effect of acetylcholine in the spontaneously beating right atria, because this response was always preceded by the negative inotropism and required higher concentration of the agonist even in the presence of neostigmine. Quite similar results were obtained with the use of carbachol and choline. The dose-response curves in the presence of atropine and consequently the calculation of the pA2 values were determined after the equilibrium of the antagonist with the preparation has been attained. The results are consistent with a competitive type of inhibition. The pA2 values as well as the time of recovery from inhibition were higher than those described for the antagonism atropine-acetylcholine upon the guinea pig ileum which is suggestive of more potent binding forces between the antagonist and cardiac muscarinic receptors.