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The outer 150-micrometers layer of the renal cortex of the rat consists mainly of proximal tubules (PT). In this layer Na-K-ATPase (mumol Pi . mg protein-1 . h-1) increases from 3.45 +/- 0.19 (SE) in 10-day-old rats (R10) to 5.90 +/- 0.28 in 20-day-old rats (R20) to 9.52 +/- 0.42 in 40-day-old rats (R40). Betamethasone in pharmacologic doses increases Na-K-ATPase in R10, R20, and R40 to 11.5 +/- 1.19, 13.4 +/- 0.64, and 13.3 +/- 0.60, respectively. Estrogen in pharmacologic doses increases Na-K-ATPase to 9.1 +/- 0.46 in R20 and decreases Na-K-ATPase to 7.6 +/- 0.39 in R40. Aldosterone in a dose of 10 micrograms/100 g BW increases Na-K-ATPase to 8.7 +/- 0.26 in R20; in a dose of 40 micrograms/100 g BW it increases Na-K-ATPase to 6.9 +/- 0.35 in R10. However, aldosterone in a dose of 80 micrograms/100 g BW is needed to cause an increase in R40 to 13.4 +/- 0.37. Treatment with canrenone from the 10th to the 20th day increases Na-K-ATPase. It is concluded that the immature PT cells are particularly sensitive to hormone induction of Na-K-ATPase and that the sensitivity is maximal during fairly late stages of cellular differentiation. Moreover, the inductive effect is probably mediated via glucocorticoid receptors.

In obese children, plasma LC-PUFA profile may be associated with the degree of obesity.

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