Muscarinic receptor subtypes in human iris-ciliary body measured by immunoprecipitation

Abstract

Purpose: To determine the relative levels of the five muscarinic receptor subtypes in the anterior segment of the human eye.

Methods: Antisera selective for each of the five muscarinic receptor proteins were incubated with [3H]-QNB bound receptors solubilized from human iris sphincter, ciliary muscle, and ciliary processes. Precipitation of the radiolabeled receptor-antibody complexes and scintillation counting enabled quantitation of the subtypes in the various tissues. Reverse transcription-polymerase chain reaction was performed on the tissues and cultured smooth muscle cells derived from them.

Results: Approximately 60% to 75% of the muscarinic receptors in the human iris sphincter and ciliary body are the m3 subtype. Lower levels (5% to 10%) of the m2 and m4 receptors are present in these tissues. The m1 receptor (7%) was detected in the ciliary processes and iris sphincter and the m5 receptor (5%), which is usually found only in the central nervous system, was present in the iris sphincter.

Conclusions: The m3 subtype is the predominant muscarinic receptor in the anterior segment of the human eye. The extensive heterogeneity of muscarinic receptors makes it difficult to predict whether subtype-selective drugs will have an improved efficacy and side-effect profile.