Relief from alkaline load in two-cell stage mouse embryos by bicarbonate/chloride exchange

Mouse embryos at the two-cell stage are able to recover from an alkaline load. We found that this recovery is mediated by sodium-independent bicarbonate/chloride exchange: intracellular pH (pHi) recovery from alkaline load is inhibited by the anion exchange inhibitor 4,4'-diisothiocyanostilbene disulfonic acid, lack of bicarbonate, or lack of chloride. The dependence of the pHi recovery on extracellular chloride concentration exhibits Michaelis-Menten kinetics. Furthermore, uptake of chloride is inhibited in a dose-dependent manner by extracellular bicarbonate. The Km for external chloride was found to be about 3 mM, with a Ki for external bicarbonate of about 2 mM. The exchanger is active above approximately pH 7.15. These results demonstrate that mouse embryos at the two-cell stage possess a sodium-independent bicarbonate/chloride exchange mechanism that is similar to that found in other mammalian cells. This bicarbonate/chloride exchanger appears to be the sole pHi-regulatory mechanism in the two-cell stage mouse embryo, since our previous results have shown that there are apparently no specific mechanisms active in these cells for relieving acid loads.