ULTRACYTOCHEMICAL STUDY OF THE OUABAIN-SENSITIVE, K-DEPENDENT P-NITROPHENYLPHOSPHATASE (NA-K-ATPASE) ACTIVITY IN THE DEVELOPING RAT KIDNEY

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The ultracytochemical localization of ouabain-sensitive, K-dependent p-nitrophenylphosphatase (K-NPPase) activity of Na-K-ATPase was investigated in the fetal and neonatal rat kidneys with the one-step lead citrate method (17) in order to study the developmental changes in the site of the Na-pump. In fetal rat kidneys, the K-NPPase positive renal tubules first appeared in the inner cortex at 18 gestation days and increased with the maturation of the fetus. Electron microscopically, the reaction products were found on the basolateral plasma membranes of distal convoluted tubule cells at various levels of differentiation. In the kidney of 19 and more gestation days, K-NPPase positive tubules were found also in the ascending thick tubules of the loop of Henle in the medulla. In newborn rats, some of the ascending thick tubules with intense K-NPPase activity reached to the tip of the papilla. These tubules in the papilla gradually lost K-NPPase activity and redifferentiated into ascending thin tubules during the period of lactation. The clear separation of K-NPPase positive outer medulla and K-NPPase negative inner medulla was completed around 3 weeks after birth. The above results indicate that suckling rats have Na-pumps even in the inner medulla and thus have different urine concentration mechanisms from those in the adults.
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