Abstract
Abstract: :
Purpose: The main outflow resistance in the eye is located within the trabecular meshwork (TM). Originally, it has been postulated that this tissue is a passive filter whose pores are changed in size by the direct action of the ciliary muscle (CM). There has been recent evidence about smooth muscle-like activities in the TM while relaxation and contraction of TM ultimately leads to modulation of outflow facility (for review see: Wiederholt et al., Progr. Ret. Eye Res., 19:275-95, 2000). TM contractility is directly modulated by a large number of vasoactive substances including various antiglaucoma drugs. This study was performed to identify the contractile regulatory elements in the TM as well as CM and their responses to various contractile agents. Methods: Measurements of isometric tension were performed on isolated bovine TM and CM strips using ML-7 (5 x 10-6 M), a specific blocker for myosin light chain kinase (MLCK). The effects on carbachol (10-6 M) and endothelin (10-8 M) induced contraction was verified. Additionally, Western blot analysis was performed to indentify smooth muscle myosin and MLCK in bovine TM and CM tissue as well as in human trabecular meshwork cells. Furthermore MLCK phosphorylation status was tested by using anti-phosphoserine antibodies. Results: Carbachol induced contraction was only blocked by ML-7 in TM but not in CM (TM: 80.4+/-8.3% vs.100% Carbachol, p<0.05; CM 101.0+/-7.6 % vs. 100% Carbachol; n.s.). ML-7 was able to block endothelin induced contractile force in TM (TM: 35.7+/- 4.6% vs. 69.2+/-10.4%; p<0.05). Western blot analysis revealed a signal for myosin (smooth muscle) at 250 kDa in native bovine TM and CM as well as in human native TM, and TM cell cultures. MLCK was detected at 160 kDa in bovine TM and also in human native TM, and cell cultures of the same origin. Conclusion: These data further support our concept that the TM is a smooth muscle-like tissue responsible for the regulation of aqueous drainage through contractile mechanisms. The latter are differently regulated in TM and CM. MLCK and finally myosin appear to be the signal transduction pathway endpoints in the regulation of TM and CM contractility. Smooth muscle relaxing compounds appear to be promising antiglaucoma agents contributing to increased outflow facility and possibly increased ocular blood flow. CR: none Support: This work was supported by the Deutsche Forschungsgemeinschaft (#DFG Wi 328/19)
Keywords: 503 outflow: trabecular meshwork • 581 signal transduction: pharmacology/physiology • 601 trabecular meshwork