Muscarinic receptors of the M2 subtype in human and bovine trabecular meshwork

Background: The trabecular meshwork is a tissue actively involved in the regulation of intraocular pressure via contractile mechanisms. The present study was performed to investigate the effects of muscarinic m2-receptor antagonists on trabecular meshwork contractility and to identify the m2 muscarinic receptor in human and bovine trabecular meshwork cells. Methods: Isometric tension measurements of bovine trabecular meshwork strips were performed using a custom-made force length transducer. Western blot and immunoprecipitation analysis was used to detect the m2-receptor proteins in membrane preparations of human and bovine trabecular meshwork cells. Results: Immunoblotting results showed the expression of an m2-receptor protein band at 56 kDa in both human and bovine trabecular meshwork cells. Two different m2-receptor antagonists were tested on trabecular meshwork contractility. After carbachol-induced contraction (10–6M set to 100% contractile force), specific m2-receptor antagonists were applied. 3α-Chloroimperaline (10–6M) had no effect on the maximal carbachol-induced contraction in trabecular meshwork strips. Methoctramine induced a significant relaxation at concentrations of 10–7, 10–6 and 5×10–6 M even in the presence of m1- and m3-receptor antagonists. Conclusion: These data indicate that in addition to the m3-receptor subtype present in the trabecular meshwork this tissue also features the m2 receptor. This receptor is partly involved in the regulation of trabecular meshwork contractility, suggesting that outflow facility might be influenced through this receptor.