Fluprostenol–Induced Endothelin–Antagonism on Trabecular Meshwork Contractility: The Fast IOP Lowering Mechanism of Prostaglandin Analogues?

Abstract

Purpose: : Prostaglandin F receptor (FP–R) agonists have been shown to reduce intraocular pressure (IOP) by enhancement of aqueous humor flow via the uveoscleral pathway. The trabecular meshwork (TM) is actively involved in the regulation of IOP via contractile mechanisms. Contraction of TM is induced by endothelin–1 (ET–1). ET–1 levels in aqueous humor were found to be elevated in glaucoma. A possible intervention of the FP–R agonist fluprostenol in the ET–1 effects on TM function was investigated to analyze additional mechanisms behind the ocular hypotensive effect of this drug.

Methods: : The effects of fluprostenol (fluprostenol–isopropylester = travoprost) on carbachol–and ET–1–induced contraction of isolated bovine TM strips were investigated using a force length transducer. Intracellular Ca²⁺ [Ca²⁺]_i was measured using Fura–2AM. FP–R expression was examined in bovine and human TM cells using Western blot analysis.

Results: : ET–1 (10^–8 M) caused contractions of bovine TM from baseline level to 61.5 ± 8.4 % vs 100 % carbachol (10^–6 M) induced contraction (n=6). The ET–1–induced contractions were partially blocked by fluprostenol (10^–6 M) to 25.0 ± 6.5 % (=6). Fluprostenol had no effect on baseline tension or carbachol–induced contraction. In the presence of Al–8810 (10^–6 M), an FP–R antagonists, the ET–1–induced contraction of BTM was not significantly altered by fluprostenol (50.3 ± 6.1 %, n = 8). In cultured BTM cells ET–1 induced an increase of [Ca²⁺]_i to 203.8 ± 23.3 % (n = 4) of the baseline level (81.6 ± 9.6 nM, n = 10). In the presence of fluprostenol (5*10^–6 M), which has no effect on [Ca²⁺]_i, the ET–1–induced increase was strongly diminished to 141.5 ± 6.0 % ( = 5) of the baseline level. FP–R was detected at 64 kDa in bovine and human TM cells.

Conclusions: : This study suggests that the anti–endothelin effect on TM contractility which probably increases aqueous humor outflow via the conventional route is additionally involved in the IOP–lowering effect of fluprostenol. The inhibiting action of FP–R agonists on ET–1–induced TM contractility could explain the fast effect on IOP of these antiglaucomatous drugs. CR: Yes Support: Deutsche Forschungsgemeinschaft (DFG: Th 751/4–1)

Keywords: outflow: trabecular meshwork • intraocular pressure • trabecular meshwork