DOES INSULIN ANTAGONIZE THE ACTION OF ATROPINE ON THE CARDIAC VAGUS ENDINGS?

Insulin administered by vein in a dosage of five units has been reported to antagonize promptly the paralytic action of atropine on the cardiac vagus mechanism of the rabbit (Frey, 192). This was explained on the basis of a displacement of the atropine. Ruickholdt (1929), likewise reported that insulin counteracted the action of atropine on the rabbit vagal endings; but he was unable satisfactorily to explain the reaction since the simultaneous administration of insulin and piocarpine did not result in a sum-mative reaction. Since we were unable to obtain any evidence of an insulin action of any kind on the parasympathetic endings of the perfused frog heart or the isolated rabbit intestine, it seemed quite possible that the apparent displacement or antagonism of atropine by insulin should be explained independently of any action of insulin on the vagal endings. In this connection, it may be recalled that Rossbach (1879), and Fleischmann (1910), showed that the paralytic action of atropine alone on the vagal mechanism of rabbits is quite transient, due to the ability of the serum rapidly to destroy the alkaloid, and the duration of action is related to the dosage administered. Although the dosages of atropine tested by Fleischmann were somewhat smaller than those used by Frey, the recovery of the vagal excitability with atropine alone was quite comparable to that noted by Frey on the subsequent injec-tion of insulin. The following experiments were made to test the adequacy of this explanation