In vitro cumulative dose-response studies were performed on cat iris strips with a variety of cholinergic agonists. Nearly a one thousandfold difference in potency was found between carbachol, the most potent, and acetylcholine, the least potent of these agents. On the other hand, differences in maximum efficacy did not exceed 50%. The results emphasize the need to distinguish between efficacy and potency of pharmacologic agents.
Pilocarpine differed substantially from all of the other agonists studied in having a latent onset of action, decreased maximum efficacy, and in proving difficult to wash from the tissues. These in vitro studies demonstrate ceiling effects similar to those seen in clinical studies of antiglaucoma therapeutic agents.
Harris LS, Shimmyo M, Hughes J. Dose-Response of Cholinergic Agonists on Cat Irides. Arch Ophthalmol. 1974;91(4):299-302. doi:10.1001/archopht.1974.03900060309013