Cell-specific differential modulation of human trabecular meshwork cells by selective adenosine receptor agonists

Current understanding of adenosine receptor (AR) function in the trabecular meshwork (TM) remains incomplete. Prior studies have shown that A1 and A2A AR subtypes may influence aqueous humor outflow, but the mechanisms are unclear. Earlier research has suggested that TM and Schlemm's canal (SC) cells respond differently to AR activation. However, these findings were based on heterogeneous cell populations, making interpretation difficult. The variability in whole-cell current responses to selective AR agonists has raised questions about the reliability of earlier data. Additionally, measuring cell volume via projected area may not fully capture contractile changes. This uncertainty has limited the ability to pinpoint specific cell types responsible for AR-mediated effects. The need for more precise methods has driven recent efforts to use calcein fluorescence quenching for volume assessment. This approach allows for more accurate tracking of cell shrinkage and expansion. Understanding these responses is crucial for developing targeted therapies for intraocular pressure regulation.

The aim of this study was to clarify the differential effects of adenosine receptor (AR) agonists on human trabecular meshwork (TM) cells. The focus was on determining whether A1, A2A, or A3 AR agonists induce distinct cellular responses. The researchers sought to address inconsistencies in prior studies by using a uniform cell population. They used the hTM5 cell line, which provides a more homogeneous sample than explant-derived cells. The goal was to assess cell volume changes using calcein fluorescence quenching, a method less influenced by contractile state. Whole-cell current responses were also measured to evaluate electrophysiological effects. The study aimed to determine whether A1 and A2A AR agonists produce opposing effects on outflow resistance. By isolating a single cell type, the researchers hoped to eliminate variability caused by mixed cell populations.

The study utilized the hTM5 cell line, a uniform population of human trabecular meshwork cells. Cell volume was measured using calcein fluorescence quenching, which tracks changes in cell size independently of contraction. Whole-cell patch-clamp recordings were performed to assess electrophysiological responses. Selective agonists for A1, A2A, and A3 adenosine receptors were applied to the cells. The A1 agonist CPA was tested for its effect on cell volume and whole-cell currents. A2A agonist CGS 21680 and A3 agonist NECA were also applied for comparison. Cell shrinkage was quantified using fluorescence intensity changes. Whole-cell current responses were recorded before and after agonist application. The experimental setup allowed for precise measurement of both morphological and electrophysiological changes.

A1 AR agonists caused consistent cell shrinkage in hTM5 cells, while A2A and A3 agonists did not. The A1 agonist CPA induced a measurable decrease in cell volume using calcein fluorescence quenching. Both A1 and A2A agonists increased whole-cell currents by a similar magnitude. The A2A agonist CGS 21680 did not trigger cell shrinkage. The A3 agonist NECA had no significant effect on cell volume. Whole-cell current responses to A1 and A2A agonists were reproducible and consistent. These findings contrast with earlier studies that reported variable responses. The uniform hTM5 cell line reduced variability compared to explant-derived populations. The results suggest that A1 and A2A agonists act on different cell populations or mechanisms.

The study suggests that A1 and A2A adenosine receptor agonists have distinct effects on human trabecular meshwork cells. A1 agonists cause cell shrinkage, while A2A agonists do not. Both A1 and A2A agonists increase whole-cell currents similarly. The results indicate that previous variability in responses may have come from mixed cell populations. The opposing effects of A1 and A2A agonists on outflow resistance are likely due to multiple cell types. These effects may involve TM and Schlemm's canal cells or subpopulations within the TM. The findings do not support a single cell population mediating both responses. The use of a uniform cell line improved the reliability of measurements.