Abstract
NaK‐ATPase is a plasma membrane enzyme that plays a critical role in eutherian blastocoel formation (cavitation) by pumping Na^+^ into the extracellular space enclosed by the trophectoderm. Previous experiments with the mouse had shown that the α (catalytic) subunit of the enzyme becomes detectable by immunocyto‐chemistry in the late morula, just prior to the onset of cavitation. In the present study we have used cDNAs corresponding to three mRNA isoforms of the α subunit and a β subunit to determine which genes are expressed during preimplantation development and to explore the timing of their expression. Of the three α subunit cDNAs tested by Northern blot hybridization with blastocyst RNA, only α1 produced a hybridization signal, recognizing a single mRNA about 4 kb in length. This mRNA is relatively abundant in zygotes but barely detectable by the 2‐cell stage and then accumulates steadily thereafter to reach its preimplantation maximum in blastocysts. The β1 cDNA detected mRNA of about 2.6‐2.8 kb. This mRNA is present in zygotes but could not be detected in 2‐, 4‐, or 8‐ cell stages; it is present at a low level in late morulae and is abundant in blastocysts. The temporal profile of accumulation of β1 mRNA thus matches more closely than does α1 the timing of appearance of the catalytic subunit. This suggests that the β subunit may regulate production of the holoenzyme and hence the timing of cavitation.