Mechanisms of action of unoprostone on trabecular meshwork contractility

PURPOSE. This study was performed to clarify the possible mechanism behind the ocular hypotensive effect of unoprostone isopropyl (Rescula; Novartis O phthalmics AG, Basel, Switzerland), a new docosanoid that has been shown to reduce intraocular pressure (IOP) in patients with ocular hypertension or primary open-angle glaucoma. To gain insight into the possible mode of acti on, the effects of unoprostone on ciliary muscle (CM) and trabecular meshwo rk (TM) contractility, intracellular calcium levels, and membrane channels were investigated. METHODS. The effects of unoprostone (M1 metabolite = free acid, 10(-5) M) a nd endothelin (ET)-1 (10(-9) M) on bovine TM (BTM) and ciliary muscle (CM) strips were investigated, by using a custom-made force-length transducer sy stem. The effects of unoprostone and ET-1 (5 x 10(-8) M) on intracellular C a2+ mobilization in cultured human TM (HTM) were measured using fura-2AM as a fluorescent probe. Patch-clamp experiments were performed on HTM and BTM cells to investigate the unoprostone-dependent modulation of membrane curr ents. RESULTS. In isolated TM and CM strips, unoprostone almost completely inhibi ted ET-induced contractions (TM; 2.9% +/- 4.3% vs. 19.6% +/- 5.7%, P < 0.05 , n = 6; CM: 1.4% +/- 1.6% vs. 30.1% +/- 5.3%, P < 0.01, n = 6; 100% = maxi mal carbachol-induced (10(-6) M) contraction). However, neither carbachol-i nduced contraction nor baseline tension was affected by unoprostone. Furthe rmore, unoprostone had no effect on baseline intracellular calcium levels ( baseline: 126 +/- 45 nM versus unoprostone: 132 +/- 42 nM, n = 8) in HTM ce lls. The endothelin-induced increase (679 +/- 102 nM), however, was almost completely (P < 0.01) blocked by unoprostone (178 <plus/minus> 40 nM). In p atch-clamp recordings, unoprostone could be shown to double the amplitude o f outward current (HTM: 200% +/- 33%; n = 6 BTM: 179% +/- 20%; n = 8). This effect was blocked by the specific inhibitor of maxi-K channels, iberiotox in. CONCLUSIONS. This study presents evidence for direct interaction of unopros tone with the contractility of the TM and CM. This compound may lower IOP b y affecting aqueous outflow.