Coordinate development of oxidative enzymes and Na-K-ATPase in thick ascending limb: role of corticosteroids. 
The postnatal development of mitochondrial ATP-producing pathways and Na-K-adenosinetriphosphatase (ATPase) in the rat medullary thick ascending limb of Henle (MTAL) was studied by measuring the activities of 3-ketoacid-CoA transferase, fumarase, citrate synthase, and Na-K-ATPase in microdissected MTAL of 16, 21, and 30-day-old pups and in adults.  The role of adrenal steroids in the development of these four markers was also investigated by studying 21-day-old rats adrenalectomized on day 16 and given dexamethasone or aldosterone or NaCl injections from day 16 to day 21.  There were large and correlated increases in the activities of the oxidative enzymes in the MTAL of control rat kidneys between 16 and 30 days after birth; Na-K-ATPase activity in the MTAL also greatly increased during the same period.  Adrenalectomy completely prevented the developmental increases in MTAL oxidative enzymes and Na-K-ATPase; dexamethasone restored the development of all four enzymes, whereas aldosterone had no effect.  We conclude that the postnatal maturation of Na+ reabsorption functions in MTAL cells involves coordinated increases in the capacity to produce ATP by oxidative metabolism and in Na-K-ATPase activity.  This maturation process is probably triggered by the rise in circulating glucocorticoids that occurs during the weaning period.