Abstract

The antagonism by atropine of the response of the nictitating membrane to nerve stimulation is not due to the antagonism by atropine of acetylcholine. This is demonstrated by the following observations on the nictitating membrane: 1) more than a 100-fold larger dose of atropine was necessary to reduce a particular response to sympathetic nerve stimulation than to antagonize the same response to acetylcholine; 2) the minimal dose of atropine required to reduce a particular response to nerve stimulation was identical with the minimal dose required to antagonize the same response to epinephrine; 3) when responses of equal magnitude were compared, atropine was equieffective against nerve stimulation in normal and reserpine-pretreated preparations; and 4) scopolamine did not antagonize the response to nerve stimulation. The antagonism of atropine to sympathomimetic amines is of the surmountable type. Atropine exerted its antagonistic effects on the nictitating membrane of the cat and of the rabbit; on the blood pressure and the heart rate of the spinal cat, however, atropine failed to antagonize epinephrine. These observations rule out the possibility that significant amounts of acetyicholine act as transmitter between the terminals of sympathetic fibers and the smooth muscle of the nictitating membrane.