RENAL CORTICAL NA-K+-ATPASE ACTIVITY AND ABUNDANCE IS DECREASED IN NORMAL PREGNANT RATS()

During late pregnancy, the rat undergoes massive plasma volume expansi on due to cumulative renal sodium retention. In the present study, con ducted in virgin, mid- (days 11-13), and late-pregnant (days 18-20) ra ts, we measured both Na+-K+-ATPase activity (by coupled enzyme assay) and abundance of the ol-subunits of the Na+-K+-ATPase (by Western and slot blot analyses) in renal cortex, medulla, and brain stem. Unexpect edly, Na+-K+-ATPase in renal cortex, in both stages of pregnancy, is r educed versus the virgin, consistent with our finding of a reduced qua ntity of the alpha 1-subunit. In renal medulla, there is a small rise in activity at midterm, but there is no difference in either activity or abundance of the alpha 1-subunit in late pregnancy, when renal Na r etention is maximal. In brain stem, where only alpha 2- and alpha 3-su bunits are evident, pregnancy has no impact on enzyme activity or abun dance of either isoform. In conclusion, the outcome of these experimen ts was unexpected in that we did not observe increased renal Na+-K+-AT Pase activity in late pregnancy in the rat. In fact, in renal cortex, Na+-K+-ATPase activity and abundance are reduced. Whatever promotes ne t sodium retention in pregnancy must be capable of overwhelming this a nd several other strong natriuretic signals.