POSTTRANSCRIPTIONAL UP-REGULATION OF NA-K+-ATPASE ACTIVITY IN NEWBORNGUINEA-PIG RENAL-CORTEX()

We measured Na+-K+-adenosinetriphosphatase (Na+-K+-ATPase) activity an d subunit abundance in renal cortical homogenates and basolateral memb ranes (BLM) from fetal, newborn, and adult guinea pigs. Pump specific activity increased four-to fivefold in cortical homogenates and BLM du ring the transition from fetus to newborn. Immunoblots of BLM showed t hat alpha- and beta-subunit abundance increased four-to seven-and four fold, respectively, during the transition from fetus to newborn. Immun oblots of cortical homogenates revealed similar developmental patterns , with newborns having 3.5-fold (alpha) and 2.3-fold (beta) greater su bunit abundances than fetuses. Therefore, the bulk of the postnatal in crease in BLM Na+-K+-ATPase abundance resulted from increased pump pro duction or decreased pump degradation, rather than from redistribution of pumps from intracellular pools. Despite the developmental increase in alpha- and beta-subunit protein levels, newborn guinea pig kidneys had only 1.4- to 2.1-fold greater alpha(1)-subunit mRNA abundance and only a 1.5-fold greater beta(1)-subunit mRNA abundance than fetal kid neys. These results demonstrate large increases in renal cortical Na+- K+-ATPase specific activity and protein abundance immediately after bi rth. These increases, which appear to result largely from posttranscri ptional upregulation, may play an important role in mediating the rapi d postnatal increase in tubular NaCl reabsorption.