Nitric Oxide Inhibits TRPV4‐mediated Hemichannel Opening in Porcine Nonpigmented Ciliary Epithelium

Abstract

Previous studies showed the presence of connexin 43 (Cx43), connexin 50 (Cx50), and pannexin 1 (Panx1) in the nonpigmented ciliary epithelium (NPE) of the eye, as well as evidence of hemichannel (HC) activity. In the lens, mechanosensitive Transient Receptor Potential Vanilloid 4 (TRPV4)‐mediated calcium entry stimulates hemichannel activity. The aim here is to test for TRPV4 expression in the NPE, determine whether TRPV4 activation causes HC opening, and study the influence of nitric oxide.Immunohistochemistry was used to probe for TRPV4 in porcine ciliary body. Propidium iodide (PI) uptake and ATP release by primary cultured NPE was used to assess HC opening. PI and ATP are large molecules (MW 668 and 507, respectively) that pass through open hemichannels but not the plasma membrane. ATP in the bathing medium was measured by luciferase luminometry and PI uptake by fluorimetry.Abundant expression of TRPV4 was detected in the NPE cell layer of the ciliary body. Cultured NPE cells displayed an increase in PI uptake when exposed to a TRPV4 agonist GSK 1016790A (10 nM) or to hypoosmotic solution (200 mOsm) (Control 4.65 ± 0.24 vs GSK 10.26 ± 0.32 and Hypo 8.26 ± 0.42 fluorescence units per mg protein; n=6, p<0.001). PI uptake also was increased in NPE cells cultured on a flexible membrane and subjected to cyclic stretch for 1‐10 min (10%, 0.5 Hz), the peak response was at at 2 min (Control 1.5 ± 0.03 vs stretch 2.5 ± 0.10; n=6, p<0.001). The increase in PI uptake caused by hypoosmotic solution or by cyclic stretch was abolished by a selective TRPV4 antagonist HC 067047 (10 μM). Hyposmotic solution increased ATP release from 1.12 ± 0.24 to 2.81± 0.27 (n=12, p<0.001) while cyclic stretch increased ATP release from 0.20 ± 0.03 to 0.40 ± 0.03 pmoles/mg protein (n=6, p<0.001). ATP release responses caused by hypoosmotic solution and cyclic stretch were inhibited by HC 067047. Hypoosmotic solution‐induced ATP release was also inhibited by connexin inhibitory peptide, Gap‐27 (200 μM) and nitric oxide (NO) donors, sodium nitroprusside (SNP) and S‐nitroso‐N‐acetylpenicillamine (SNAP).TRPV4 channels are expressed by the NPE. The ability of HC 067047 and Gap‐27 to prevent ATP release and PI uptake points to TRPV4‐dependent opening of connexin HCs in NPE cells. Suppression of ATP release by SNP and SNAP indicates the hemichannel opening mechanism is inhibited by NO.