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

Background: The trabecular meshwork is a tissue actively involved in the re gulation of intraocular pressure via contractile mechanisms. The present st udy was performed to investigate the effects of muscarinic m2-receptor anta gonists on trabecular meshwork contractility and to identify the m2 muscari nic receptor in human and bovine trabecular meshwork cells. Methods: Isomet ric tension measurements of bovine trabecular meshwork strips were performe d using a custom-made force length transducer. Western blot and immunopreci pitation analysis was used to detect the m2-receptor proteins in membrane p reparations of human and bovine trabecular meshwork cells. Results: Immunob lotting results showed the expression of an m2-receptor protein band at 56 kDa in both human and bovine trabecular meshwork cells. Two different m2-re ceptor antagonists were tested on trabecular meshwork contractility. After carbachol-induced contraction (10(-6)M set to 100% contractile force), spec ific m2-receptor antagonists: were applied. 3 alpha -Chloroimperaline (10(- 6)M) had no effect on the maximal carbachol-induced contraction in trabecul ar meshwork strips. Methoctramine induced a significant relaxation at conce ntrations of 10(-7), 10(-6) and 5x10(-6) M even in the presence of m1- and m3-receptor antagonists. Conclusion: These data indicate that in addition t o the m3-receptor subtype present in the trabecular meshwork this tissue al so features the m2 receptor. This receptor is partly involved ill the regul ation of trabecular meshwork contractility, suggesting that outflow facilit y might be influenced through this receptor.