THE MIOTIC EFFECT OF SUBSTANCE-P ON THE ISOLATED RABBIT IRIS

Abstract

The effect of substance P on the rabbit iris was investigated by continuously monitoring the pupil size in vitro. Continuous perfusion with substance P caused a concentration-dependent contraction of the iridial sphincter muscle. The threshold was 2 .times. 10-10 M. A concentration-response curve was made for 60 s exposures of the iris to substance P. Blockade of muscarinic, nicotinic, .alpha.- and .beta.-adrenergic receptors and blockade with the putative presynaptic substance P-blocking agent baclofen did not affect the constrictor response to substance P. Pretreatment of the rabbits with indomethacin had no effect. No effect of substance P was seen on the isolated iridial dilator muscle. Although the miotic response to substance P was observed in a Ca-free perfusion fluid, [ethylene glycol bis(.beta.-amino ethyl ether) N,N,N'',N''-tetraacetic acid] added to Ca-free perfusate diminished the effect of substance P. The response of the iris to substance P and the time course of the response suggest that substance P is involved in the miosis of the nerve-mediated irritative response of the eye.