Routes of Cl- transport across the trophectoderm of the mouse blastocyst

Abstract

The blastocyst stage of embryo development is characterized by a fluid- filled cavity called the blastocoel. Blastocoel formation requires vectorial Na+ and Cl- transport and the accompanying osmotic accumulation of fluid. We found under conditions of low external Cl- that inhibitors of Cl- transport mechanisms inhibited blastocoel expansion, indicating a possible transcellular route for Cl- uptake cross the outer epithelial layer (the trophectoderm). Using the Cl-sensitive fluorophore, N-(6- methoxyquinolyl)acetoethyl ester, we found that Cl- efflux from the blastocoel can occur via pathways with properties that resemble both HCO3/- /Cl- exchange and Cl- channels, as well as by another yet uncharacterized pathway. In contrast, Cl- re-uptake into Cl- depleted blastocoels (the physiologically relevant direction for Cl- transport during blastocoel expansion) occurred only via the channel-like mechanism. Patch-clamp recordings detected a component of current carried by apical Cl- channels. Intracellular pH measurements during external Cl- removal detected HCO3/- /Cl- exchange activity in collapsed blastocysts but little in intact blastocysts, suggesting predominantly basolateral HCO3/-/Cl- exchange activity. This was corroborated by the immunolocalization of the AE2 isoform of HCO3/-/Cl- exchanger to the basolateral surface of the trophectoderm. Thus, it appears that Cl- transport into the blastocoel may occur via apical Cl- channels, while efflux also involves a basolateral HCO3/-/Cl- exchanger.