AUTONOMIC drugs may induce miosis by several mechanisms. For example, pilocarpine seems to act directly on the receptor system of the iris sphincter, whereas physostigmine and diisopropyl fluorophosphate (DFP) seem to stimulate contraction of the iris sphincter by an indirect mechanism. Their blocking action on tissue cholinesterases prevents destruction of the physiologically produced acetylcholine, thereby enhancing and prolonging the stimulatory effects of the latter on the sphincter.
It has generally been assumed that a direct stimulating drug, like pilocarpine, and an indirectly acting drug, like physostigmine, have additive effects if administered together because they act by different mechanisms. These drugs are, therefore, frequently used in combination in the treatment of acute glaucoma; however, in patients in whom it was possible to compare treatment responses of the two eyes we have observed that the combined use of these drugs in maximal doses seemed less effective both in degree of miosis and
SWAN KC, GEHRSITZ L. COMPETITIVE ACTION OF MIOTICS ON THE IRIS SPHINCTER. AMA Arch Ophthalmol. 1951;46(5):477-481. doi:10.1001/archopht.1951.01700020490001