Desensitization of the isolated bovine iris sphincter muscles by pilocarpine

We investigated the effect of pilocarpine on the bovine iris sphincter. This muscle contracted in a dose-dependent manner from 2 x 10(-6) M, with a maximal response at 3 x 10(-3) M. The ED50 values was (1.6 +/- 0.3) x 10(-4) M. Pilocarpine generated a desensitization while desensitization was not significant in the case of other cholinergic agents such as acetylcholine (Ach) and carbachol. Desensitization was profoundly increased in the presence of Ach, neostigmine or eserine: the responses to the second and third trials of pilocarpine were reduced to approximately 8-10% or 30% of the corresponding first responses. Pilocarpine reportedly releases transmitters and alters choline-acetyltransferase activity. These results taken together suggest that either variable Ach synthesis, inhibitory transmitter release or possible toxic action in high concentrations may be involved in the pilocarpine-induced responses of the bovine iris sphincter muscle. The desensitization and partial agonist-antagonist action of pilocarpine could not explain the characteristics of the bovine iris sphincter muscle.