TRPV4 Channels Enable Na,K‐ATPase Activity in the Lens Epithelium to Respond to a Remote Stimulus

Abstract

Na,K-ATPase activity is low or absent in the fiber cells that make up most of the eye lens. Ion and water homeostasis of the entire structure is made possible by Na,K-ATPase activity in a single epithelial cell layer at the anterior lens surface. Previously we have shown Src family tyrosine kinase-dependent changes of Na,K-ATPase activity in the epithelium. Here we provide evidence that the epithelium is able to increase Na,K-ATPase activity in response to a remote stimulus in the fiber mass. The posterior pole of intact pig lenses was subjected to localized damage either by a 2.5 mm diameter freeze-thaw wound or by injection of 5 µl of hyperosmotic saline just beneath the surface. After 2-30 min the epithelium was removed from the lens, homogenized and the rate of ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was determined. Src family tyrosine kinase (SFK) activation (phosphorylation) was assessed by Western blot. Localized damage to lens fibers at posterior pole caused SFK activation and a significant increase in Na,K-ATPase activity in the epithelium that occurred within minutes. Na,K-ATPase activity almost doubled. The increase in Na,K-ATPase activity was prevented by the SFK inhibitor PP2 (10 µM). Importantly, the SFK phosphorylation response as well as the increase in Na,K-ATPase activity were suppressed by a TRPV4 channel antagonist, HC067947 (10 µM). By Western blot, TRPV4 was detected in the epithelium but not fiber cells. The findings point to a TRPV4-dependent mechanism in the anterior epithelium that enables the cells to sense remote damage in the fiber mass and respond by activating SFK and increasing Na,K-ATPase activity.