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The adaptation of newborn mammals to extrauterine life is conditioned by increased expression of enzymes that are of importance for specialized tissue function. In the rat, maturation is accelerated around birth in the lung and around weaning in the kidney. The Na+, K+-ATPase enzyme is responsible for the active transport of ions across the membrane of each cell at the expense of 30% of total body O2 consumption. We have reported that in the infant rat renal cortex, glucocorticoids (GC) rapidly increases the abundance of the Na+, K+-ATPase mRNA, suggesting a direct transcriptional regulation. We have now compared the effect of a single injection of betamethasone, a synthetic highly specific GC, on the abundance of mRNA for the catalytic α and the regulatory β subunits in lung and kidney from fetal, infant and young rats. In the kidney the most prominent effects on the a mRNA was found at 10 days of age (5.3±0.9 fold). A significant increase was also found in 20-day-old rats (1.6±0.2 fold), but no effect was found in fetal and 5-day-old rats. In the lung the most prominent effect was found around birth, but no effect on α mRNA was found at 10 and 20 days. In the kidney there was a coordinate increase in α and β mRNA subunits, while in the lung, the β subunit was stimulated alone, in 10- and 20-day-old rats.

Conclusion. GC induction of Na+, K+-ATPase genes is age- and tissue dependent. The GC sensitive period coincides with the physiological need for organ maturation.

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