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

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 across 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 HCO-3/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 HCO-3/Cl- exchange activity in collapsed blastocysts but little in intact blastocysts, suggesting predominantly basolateral HCO-3/Cl- exchange activity. This was corroborated by the immunolocalization of the AE2 isoform of HCO-3/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 HCO-3/Cl- exchanger.