Novel bumetanide-sensitive K+ transport in preimplantation mouse conceptuses.

Ouabain-resistant K+ transport activity was characterized primarily by measuring Rb+ uptake because 86Rb+ has a more convenient half-life than 42K+. Ouabain-resistant 86Rb+ uptake by mouse two-cell conceptuses and blastocysts was slowed by the K(+)-Na(+)-2Cl- cotransporter inhibitors bumetanide [inhibitory constant (Ki) = 400 nM] and furosemide (Ki approximately 10 microM), but it was insensitive to a variety of K+ channel blockers. This component of 86Rb+ transport was also inhibited by K+ and nonradioactive Rb+ and it was stimulated by Cl-. Nevertheless, neither 36Cl- nor 22Na+ uptake was inhibited by bumetanide, whereas 42K+ uptake was inhibited by both bumetanide and furosemide. Bumetanide-sensitive Rb+ transport in blastocysts had a Hill coefficient of 1.0 and a Michaelis constant value of 3.0 mM. By these criteria, preimplantation conceptuses contain a novel, bumetanide-sensitive K+ transport system that does not cotransport Cl- or Na+. Moreover, bumetanide-sensitive Rb+ uptake was 10 times faster in blastocysts when they were collapsed to expose the basal membrane of the trophectoderm to 86Rb+ in the medium. Therefore, the novel system may be located predominantly in the basal rather than in the apical membrane of the trophectoderm.