Effects of the substance P antagonist [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP on miosis caused by echothiophate iodide or pilocarpine hydrochloride.

The anticholinesterase agent echothiophate iodide (EI) and the cholinergic agent pilocarpine hydrochloride (pilocarpine), drugs commonly used in glaucoma therapy, cause miosis in rabbits as well as in man. In rabbits the miotic effect decreases after a few days of treatment, a phenomenon possibly due to a drug-induced decrease in the number of muscarinic receptors. However, the muscarinic pupillary contraction caused by stimulation of the retina with light is intact. In this investigation the miosis caused by the doses of EI was found to be very resistant to muscarinic or nerve blockade but inhibited by the substance P (SP) analog [D-Arg1,D-Pro2,D-Trp7,9, Leu11]SP, which seems to be a SP/SPLI blocker in the rabbit pupillary sphincter. Miosis caused by pilocarpine was partly inhibited by muscarinic blockade and partly by the SP blocker. In eyes treated with EI topically twice daily for three weeks, SP or the red pepper extract capsaicin, a releaser of SP-like immunoreactivity (SPLI), had less miotic effect than in control eyes. Capsaicin caused more pronounced miosis in eyes treated with topical pilocarpine for three weeks than in controls. The radioimmunoassay technique did not reveal a significant change in the amount of SPLI in the retinas or iris-ciliary bodies from EI-treated eyes as compared with the controls. It is concluded that, besides cholinergic miosis, EI causes non-muscarinic miosis, probably by release of SP or a related substance and that pilocarpine may have similar effects.