IT IS GENERALLY accepted that acetylcholine and pilocarpine act directly upon effector cells innervated by cholinergic fibers, such as the sphincter muscles of the iris, to elicit their characteristic responses. Physostigmine and diisopropyl fluorophosphate act indirectly; they block tissue cholinesterase and thereby enhance and prolong the effect of acetylcholine, for instance, on the sphincter muscle of the iris.
It has been assumed that pilocarpine and physostigmine if administered together have additive miotic effects. The drugs are therefore frequently used in combination with the hope of obtaining a more intensive miotic effect, as, for example, in the treatment of glaucoma.
In a recent paper,1 Swan and Gehrsitz, working on albino rabbits, presented observations which indicated that, instead of an additive action, a weaker miotic (pilocarpine) may reduce the effectiveness of a stronger drug (physostigmine and diisopropyl flurophosphate). When pilocarpine was administered, the subsequent application of physostigmine and diisopropyl fluorophosphate was
LOWENSTEIN O, LOEWENFELD IE. EFFECT OF PHYSOSTIGMINE AND PILOCARPINE ON IRIS SPINCTER OF NORMAL MANIs It Competitive or Additive? Pupillographic Studies. AMA Arch Ophthalmol. 1953;50(3):311-318. doi:10.1001/archopht.1953.00920030318008