Regional differences in rat conjunctival ion transport activities

Abstract

Active ion transport and coupled osmotic water flow are essential to maintain ocular surface health. We investigated regional differences in the ion transport activities of the rat conjunctivas and compared these activities with those of cornea and lacrimal gland. The epithelial sodium channel (ENaC), sodium/glucose cotransporter 1 (Slc5a1), transmembrane protein 16 (Tmem16a, b, f, and g), cystic fibrosis transmembrane conductance regulator (Cftr), and mucin (Muc4, 5ac, and 5b) mRNA expression was characterized by RT-PCR. ENaC proteins were measured by Western blot. Prespecified regions (palpebral, fornical, and bulbar) of freshly isolated conjunctival tissues and cell cultures were studied electrophysiologically with Ussing chambers. The transepithelial electrical potential difference (PD) of the ocular surface was also measured in vivo. The effect of amiloride and UTP on the tear volume was evaluated in lacrimal gland excised rats. All selected genes were detected but with different expression patterns. We detected αENaC protein in all tissues, βENaC in palpebral and fornical conjunctiva, and γENaC in all tissues except lacrimal glands. Electrophysiological studies of conjunctival tissues and cell cultures identified functional ENaC, SLC5A1, CFTR, and TMEM16. Fornical conjunctiva exhibited the most active ion transport under basal conditions amongst conjunctival regions. PD measurements confirmed functional ENaC-mediated Na(+) transport on the ocular surface. Amiloride and UTP increased tear volume in lacrimal gland excised rats. This study demonstrated that the different regions of the conjunctiva exhibited a spectrum of ion transport activities. Understanding the specific functions of distinct regions of the conjunctiva may foster a better understanding of the physiology maintaining hydration of the ocular surface.

Small‐molecule CFTR activators increase tear secretion and prevent experimental dry eye disease

“…However, we found substantial capacity for CFTRfacilitated Cl 2 transport at the ocular surface in mice (11,23), as subsequently shown in rat conjunctiva (24), providing a rational basis for investigation of CFTR activators as a prosecretory strategy for dry eye. The only clinically approved CFTR activator, VX-770 (ivacaftor), is indicated for potentiating the channel gating of certain CFTR mutants causing CF, but only weakly activates wild-type (WT) CFTR (25,26).…”

TRPV4 heats up ANO1‐dependent exocrine gland fluid secretion

“…Although Marty et al (44) described the main electro‐physiological characteristics of CaCCs in lacrimal glands, the molecular nature of these channels was never investigated. Later, the detection of abundant Tmem16a mRNA in LG tissue raised the possibility that ANO1 is involved in LG fluid secretion, but this possibility was also not confirmed (45). Our data show that the inhibition of ANO1 significantly reduced tear secretion stimulated by CCh.…”

The function and regulation of acid‐sensing ion channels (ASICs) and the epithelial Na+ channel (ENaC): IUPHAR Review 19

“…The β subunit is present in basal regions of the limbal epithelium, while the γ subunit is present throughout all layers of the corneal epithelium but not in the basal regions of the limbal epithelium. Measurements of electrical potential difference confirmed functional ENaC‐mediated Na + ‐transport, which is probably involved in maintaining hydration of the ocular surface (Yu et al , ). ENaC is also expressed in endothelial and vascular smooth muscle cells, where its increased surface expression can lead to membrane stiffening and reduced release of nitric oxide (Jeggle et al , ).…”