Abstract

Alterations in the pharmacologic properties of the iris of the cholinesterase inhibitor-treated eyes of the dog and rabbit have been investigated. One eye of each animal was treated with the cholinesterase inhibitor—in most cases, echothiophate iodide—three times daily for 10 to 18 days. After a few days of treatment the miotic (i.e., cholinergic) effectiveness of echothiophate gradually decreased until by the 5th day of treatment, it had virtually no miotic effect. These echothiophate-pretreated eyes also became refractory to the cholinergic effects of topically applied carbachol. Pilocarpine had a mydriatic rather than a miotic action on pretreated dog eyes. Cholinergic response could not be obtained in the echothiophate-pretreated rabbit eye even after intravitreal injection of 2000 times the normally effective dose of carbachol. In spite of these changes in its pharmacologic properties, the iris of the pretreated eye exhibited normal or, in the case of the dog, hyperactive physiologic responses to light. Atropine abolished these responses. These observations are discussed in the light of mechanisms known to control normal movements of the mammalian iris.