Absorptive apical amiloride‐sensitive Na⁺ conductance in human endometrial epithelium

Abstract

Human endometrial epithelial cells cultured on porous tissue culture supports formed tight, polarized epithelial monolayers with features characteristic of tight epithelia. Endometrial epithelial layers generated significant transepithelial electrical resistance (750 Ω cm2) and potential difference (15.3 mV), with an inward short‐circuit current (Isc; 20.5 μA cm−2). The Isc was linearly proportional to the external Na+ concentration and was abolished in the absence of Na+. The Isc was sensitive to apical amiloride. Net 22Na+ flux was in the absorptive apical to basolateral direction and fully accounted for the inward Isc. In addition, apical to basolateral and net 22Na+ transport were reduced in the presence of amiloride. The Isc was also sensitive to addition of ouabain and Ba2+ to the basal solution, consistent with a role for basolateral Na+−K+‐ATPase and K+ channels in generation of the current. These data demonstrate that human endometrial epithelial cells in primary culture produce tight, functional monolayers on permeable supports. We provide the first evidence that human endometrial epithelial cells have an inward Isc accounted for by an amiloride‐sensitive Na+ conductance. The Na+‐absorptive function of the endometrium may provide an appropriate environment for sperm function and embryo growth.

Sodium and anion transport across the avian uterine (shell gland)epithelium

“…The observation that amiloride and benzamil produced inhibition of the basal I sc and that this decrease in current was associated with inhibition of the apical-tobasolateral and net Na + flux strongly suggested that ENaC Na + channels were involved in Na + absorption across this epithelium. This result was consistent with previous studies of Na + transport across the porcine, murine and human endometrial epithelium (Vetter and O'Grady, 1996;Deachapunya et al, 1999;Palmer-Densmore et al, 2002;Chan et al, 2001;Matthews et al, 1998). In native porcine endometrial tissues mounted in Ussing chambers, the surface epithelial cells exhibited a basal I sc that was blocked by amiloride with an IC 50 value (0.8·µmol·l -1 ) identical to that reported for the shell gland epithelium in this study (Vetter and O'Grady, 1996).…”

Altered cyclic expression of epithelial Na+ channel subunits and cystic fibrosis transmembrane conductance regulator in mouse endometrium by a low sodium diet

“…Previous studies have demonstrated a functional role of both ENaC and CFTR in mediating the absorptive and secretory activities of the endometrium in rodents and humans, respectively (Chan et al, 1997aec, 1999Deachapunya and O'Grady, 1998;Matthews et al, 1998). We have also demonstrated differential and cyclic changes in the expression of uterine CFTR and ENaC, indicating the ability of the uterus to secrete and absorb at different stages of the estrus cycle (Chan et al, 2002).…”

Differential expression and localization of CFTR and ENaC in mouse endometrium during pre‐implantation

“…Immunohistochemical studies solved the puzzle, showing that CFTR immunoreactivity was not found in the epithelia, neither in the lumen nor in the glands. As a cAMP-dependent Cl ÿ channel, CFTR has been shown to be involved in mediating a variety of neurohormonal secretory responses in the uterus (Chan et al, 1997a(Chan et al, , 1999Deachapunya and O'Grady, 1998;Fong and Chan, 1998;Matthews et al, 1998), and thus is considered to be mainly responsible for driving uterine fluid secretion. The observed down-regulation or absence of CFTR in uterine epithelia ensures that the uterine secretory activity is at a minimum during implantation.…”

“…Recent molecular and electrophysiological studies have shown the expression of an epithelial Na C channel (ENaC), which consists of three homologous subunits (a, b and g) and is inhibited by amiloride, as well as an amiloride-blockable Na C -dependent short-circuit current (Isc), in primary cultured mouse (Chan et al, 1997a(Chan et al, , 1997b and human uterine epithelia (Matthews et al, 1998). Amiloride-sensitive and Na C -dependent fluid absorption has recently been demonstrated in rat uterine glands by confocal microscopy, indicating the involvement of ENaC in mediating the fluid absorption responsible for the closure of the uterine lumen and immobilization of the blastocyst necessary for implantation (Naftalin et al, 2002).…”

“…Active Na C absorption drives Cl ÿ counter-ion and fluid out of the lumen into the blood, while active Cl ÿ secretion drives both Na C and fluid from the plasma into the lumen (Chan et al, 2000). Recent studies on cultured mouse endometrial epithelia have demonstrated the involvement of the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated Cl ÿ channel, in mediating a number of neurohormonal-regulated secretory responses in the uterus (Chan et al, 1997a(Chan et al, , 1999Deachapunya and O'Grady, 1998;Fong and Chan, 1998;Matthews et al, 1998). On the other hand, previous studies have shown that uterine fluid volume and its Na C concentration during pre-implantation are relatively low compared to the other stages, suggesting that Na C and water are reabsorbed, leading to the ''closure'' of the lumen to promote successful embryo implantation (Hoversland and Weitlauf, 1981;van Winkle et al, 1983).…”