Schlemm's Canal Cells and Trabecular Meshwork Cells as Potential Targets in IOP Modulation

Abstract

Abstract: : Purpose: Concentrations of aqueous humor adenosine are increased in patients with primary open–angle glaucoma. Topical administration of adenosine receptor agonists and antagonists in humans and other mammals has also been associated with alterations in intraocular pressure (IOP). Specific sites and mechanisms are unknown. A₁ and A_2a adenosine receptors (ARs) change outflow oppositely in vivo, we applied selective AR agonists to human Schlemm’s canal (SC) and trabecular meshwork (TM) cells to probe whether either cell type might mediate the opposing effects. Methods: We prepared inner–wall SC (IW–SC) cells by explant technique and enzyme–assisted rapid isolation, followed by cell selection with antibody to the SC–cell marker CD31/PECAM–1. Enzymatically–derived cells were studied at P1–3, and explant–derived cells at P3–5. Effects of AR agonists on intracellular Ca²⁺ of subconfluent cells were measured by fura–2 fluorescence, while the area of cells loaded with calcein–AM was used as an index of cell volume. Whole–cell currents were recorded. Results: A₁ increased and A_2aAR agonists reduced whole–cell currents of explant–derived SC cells; responses were non–uniform, possibly because of cell heterogeneity. The A₁AR agonist (S)–ENBA increased IW–SC whole–cell currents by 718±232% (100nM, 18/22 applications); while the A_2aAR agonist CGS–21680 reduced currents by –29±7% (30nM, 4/7 applications). CGS–21680 acted oppositely on currents of immortalized human TM (hTM5) cells, increasing currents by 66±15% (18/22 applications). (S)–ENBA also increased hTM5 currents (12/14, 68±18%). The A₃AR agonist Cl–IB–MECA increased hTM5 currents (30nM, 13/16, 42±14%) but acted inconsistently on SC cells. A₁, A_2a, and A₃ AR agonists increased intracellular Ca²⁺ of both explant and enzymatic derived SC cell preparations. Only the A₁AR agonist (S)–ENBA shrank the inner–wall SC cells (–2.4±0.1%, N=8) and only the A_2aAR agonist CGS–216380 shrank the hTM5 cells (–3.1±0.1%, N=7). Conclusion: Our new enzymatically–assisted technique can generate human inner–wall SC cells of low passage number to permit physiologic characterization. Opposing actions of A₁ and A_2a AR agonists on SC but, not hTM5, cells suggest that inner–wall SC cells may mediate AR modulation of outflow resistance.

Keywords: outflow: trabecular meshwork • electrophysiology: non–clinical • signal transduction: pharmacology/physiology