Resting trabecular meshwork cells experience constitutive cation influx

Abstract

A quintessential sentinel of cell health, the membrane potential in nonexcitable cells integrates biochemical and biomechanical inputs, determines the driving force for ionic currents activated by input signals and plays critical functions in cellular differentiation, signaling, and pathology. The identity and properties of ion channels that subserve the resting potential in trabecular meshwork (TM) cells is poorly understood, which impairs our understanding of intraocular pressure regulation in healthy and diseased eyes. Here, we identified a powerful cationic conductance that subserves the TM resting potential. It disappears following Na⁺ removal or substitution with choline or NMDG⁺, is insensitive to TTX, verapamil, phenamil methanesulfonate, amiloride and GsMTx4, is substituted by Li⁺ and Cs⁺, and inhibited by Gd³⁺ and Ruthenium Red. Constitutive cation influx is thus not mediated by voltage-operated Na⁺, Ca²⁺, epithelial Na⁺ (ENaC) channels, Piezo channels or Na⁺/H⁺ exchange but may involve TRP-like channels. Transcriptional analysis detected expression of many TRP genes, with the transcriptome pool dominated by TRPC1 followed by expression of TRPV1, TRPC3, TRPV4 and TRPC5. Pyr3 and Pico1,4,5 did not affect the standing current whereas SKF96365 promoted rather than suppressed, Na⁺ influx. SEA-0400 induced a modest hyperpolarization, indicating residual contribution from Na⁺/Ca²⁺ exchange. The resting membrane potential in human TM cells is thus maintained by a constitutive monovalent cation leak current with properties not unlike those of TRP channels. This conductance is likely to influence conventional outflow by setting the homeostatic steady-state and by regulating the magnitude of pressure-induced currents in normotensive and hypertensive eyes.