In vitro contraction of the pupillary sphincter by substance P and its stable analogs.

The effects of substance P and substance P analogs have been studied quantitatively by use of an isolated preparation of bovine pupillary sphincter muscle. Substance P contracts the pupillary sphincter in a dose-dependent manner with a value for median effective dose (ED50) of 1.0 X 10(-6) M. The effects of substance P result from its interaction with a specific receptor in the pupillary sphincter. These results strengthen the view that substance P is involved in the oculopupillary reflex.