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

Area of Science:

• Ophthalmology
• Neuroscience
• Pharmacology

Background:

• Substance P is found in human irides and can contract sphincter muscles in various species.
• The contractile effect of substance P on the human iris sphincter muscle has not been previously demonstrated.
• Enkephalinase (E.C. 3.4.24.11) is an enzyme that may play a role in modulating neuropeptide actions.

Purpose of the Study:

• To demonstrate and characterize the contractile effect of substance P on the human iris sphincter muscle.
• To investigate the potential role of endogenous enkephalinase in modulating this contractile response.
• To confirm the presence of enkephalinase activity in the human iris.

Main Methods:

• Utilized an ex vivo human eye cup model to assess iris sphincter muscle response.
• Administered nanomolar concentrations of substance P to the iris tissue.
• Pretreated eyes with thiorphan, a specific enkephalinase inhibitor, to evaluate its effect on substance P-induced contraction.
• Assayed for enkephalinase-like immunoreactivity and enzyme activity in human iris sphincter muscle.

Main Results:

• Nanomolar amounts of substance P induced significant contraction of the human iris sphincter muscle.
• Pretreatment with thiorphan enhanced the contractile response to substance P.
• Enkephalinase-like immunoreactivity and enzyme activity were detected in the human iris sphincter muscle.

Conclusions:

• Substance P directly induces contraction of the human iris sphincter muscle.
• Endogenous enkephalinase activity in the human iris modulates the contractile response to substance P.
• These findings suggest a novel neuropharmacological pathway influencing human iris function.