Endothelin–1–Induced Calcium–Free Contraction in Trabecular Meshwork: Interaction With RhoA

Abstract

Abstract: : Purpose:Treatment of glaucoma focuses on one of the major risk factors, the intraocular pressure (IOP). Trabecular meshwork (TM) and ciliary muscle (CM) actively participate in the regulation of IOP through contractile mechanisms. Endothelin–1 (ET–1) is probably involved in the pathogenesis of glaucoma. Its concentrations were found to be increased in plasma and aqueous humor of POAG–patients. ET–1 is known to induce contraction of TM and CM. Anti–endothelin effects on TM appear to be ideal mechanisms for IOP lowering drugs. Methods:The effects of intra– and extracellular Ca²⁺ deprivation on carbachol and ET–1 induced contraction of isolated bovine TM and CM strips were investigated using a force length transducer system. Western blot analysis was used to detect RhoA and Rho kinase in whole cell lysates of bovine TM and CM on protein level. Results:Under intra– and extracellular Ca²⁺ free conditions the carbachol (10^–6 M) –induced contraction was 23.1 ± 2.6 % (n=8) in TM and 16.3 ± 2.1 % in CM (n=7) versus control value (carbachol (10^–6 M) with Ca²⁺). ET–1 (10^–8 M) induced contraction in TM (69.2 ± 10.5 %, n=7) and CM (67.7 ±5.6 %, n= 6) vs. carbachol (10^–6 M). In Ca²⁺ free solution ET–1 (10^–8 M) induced contraction in TM was 13.9 ± 2.5 % (n=8, vs. carbachol (10^–6 M) with Ca²⁺). Under these conditions ET–1 failed to induce contractility in CM. The Ca²⁺–free ET–1–induced contraction of TM was completely blocked by the Rho kinase inhibitor Y–27632 (10^–6 M). In Western blot experiments the Rho A protein and Rho kinase could be detected in bovine TM and CM. Conclusions:In TM Ca²⁺ independend contraction is elicited by carbachol as well as by ET–1, whereas in CM only carbachol is able to induce contraction under Ca²⁺ free conditions. Since the Rho A pathway seems to be expressed in both tissues we conclude different intracellular signaling pathways of endothelin receptors in TM and CM. Inhibition of this ET–1–dependent pathway in TM may allow specific modulation of TM contractility to enhance outflow facility and lower IOP without affecting the CM, circumventing side effects such as accommodative changes and miosis. CR: none Support: Deutsche Forschungsgemeinschaft (DFG: Th 751/4–1)

Keywords: trabecular meshwork • signal transduction: pharmacology/physiology • outflow: trabecular meshwork