Extrinsic Modulation of the Zonular Tension Induces Calcium Influx in Lens Fiber Cells That Is Mediated by Mechanosensitive TRPV1 and TRPV4 Channels

Purpose: The purpose of this study was to determine if changing the tension applied to the rat lens via the zonules increases intracellular Ca²⁺ levels via the activation of mechanosensitive Transient Receptor Potential Vanilloid (TRPV)1 and TRPV4 channels. Methods: Rat lenses were mounted in a recording chamber with zonular attachments intact and imaged using confocal laser-scanning microscopy. Epithelial cells were loaded with Cal520 AM, whereas fiber cells were microinjected with Fluo-8 pentapotassium salt. Changes in intracellular Ca²⁺ were monitored following pharmacological modulation of ciliary muscle contractility using Tropicamide to increase zonular tension and pilocarpine to decrease zonular tension. The contribution of TRPV1 and TRPV4 channels was assessed using selective inhibitors. Results: Increasing zonular tension with Tropicamide induced a transient Ca²⁺ increase in epithelial cells and a robust, sustained Ca²⁺ influx in differentiating fiber cells. Decreasing zonular tension with pilocarpine produced minimal epithelial responses but evoked distinct Ca²⁺ elevations in fiber cells. Isolation of the lens from the ciliary muscle abolished these responses, indicating that this influx depends on mechanical coupling through the zonular fibers. Furthermore, pre-incubation of lenses in the presence of TRPV1/4 inhibitors revealed that Tropicamide-induced Ca²⁺ responses in fiber cells are largely transduced by TRPV4, whereas activation of TRPV1 mediates the increase in Ca²⁺ induced by Pilocarpine. Conclusions: In this study, we have shown that extrinsic modulation of ciliary muscle contractility, through alterations in zonular tension, generates regionally specific influxes of Ca²⁺ mediated by mechanosensitive TRPV1/4 channels which in other studies have been shown to regulate lens water transport and lens power. These findings suggest that lens power may be influenced by extrinsic feedback regulation from other tissues in the eye.