Muscarinic subsensitivity without receptor change in monkey ciliary muscle.

Intense muscarinic stimulation of the monkey ciliary muscle causes long-lasting muscarinic subsensitivity. This could be due to changes in number or affinity of muscarinic receptors which would cause a threshold elevation detectable in vivo. Since plasma levels of the agonists causing contraction in vivo were not available, the accommodation response to systemic muscarinic agents in subsensitized eyes was compared with that in the normal fellow eyes, usually seven days after a single subsensitizing dose of 100 micrograms carbachol to cornea. Subsensitivity was present whether the agonist tested was pilocarpine, carbachol or bethanechol but no evidence for threshold elevation was found. The conclusion is that changes in the muscarinic receptors are of minor importance for the kind of subsensitivity studied.