Substance P contracts the human iris sphincter: possible modulation by endogenous enkephalinase.

Substance P-immunoreactive neurons have been found in the irides of many species including humans. In several species, substance P has been shown to induce contraction of the sphincter muscle but this action of substance P has not been previously demonstrated in the human eye. Using an eye cup model in which the sensitivity of the iris muscle to substance P is increased compared to the isolated sphincter muscle, we have observed that nanomolar amounts of substance P induced contraction of the sphincter in the human iris. This contractile response was enhanced in eyes pretreated with thiorphan, an enkephalinase inhibitor, suggesting that endogenous enkephalinase (E.C. 3.4.24.11) may modulate the substance P contraction in the human iris. Further support for this hypothesis was the finding of enkephalinase-like immunoreactivity and enzyme activity in the human iris sphincter muscle.