We have measured the amounts of Na+, K+ and C− in preimplantation mouse embryos (1-cell, 2-cell and morula) using electron probe X-ray microanalysis. The levels of these ions do not vary much over this period, and are approximately the same as those found in other mammalian cells, contrary to previous reports. We have confirmed that preimplantation embryos exhibit Na+/K+-ATPase activity at all stages examined, and have shown that the ATPase maintains high K+/Na+ ratios (12–16) in all these embryonic stages, comparable to those seen in other healthy cells; this is in contrast to the low ratios reported in earlier work. Inhibition of the Na+/K+-ATPase results in the slow exchange of intracellular K+ for extracellular Na+ (half-time approximately 5 h), indicating that Na+/K+-ATPase activity maintains steep Na+ and K+ gradients in preimplantation mouse embryos as it does in most other cells.
