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Département de Physiologie, Université Catholique de Louvain, Bruxelles, Belgium.

1. Transepithelial Na+ transport, Na(+)-K(+)-ATPase activity and ouabain binding were measured in cells originating from the distal part of amphibian nephron (A6) which form 'tight' monolayers in culture, under standard (control) incubation conditions and after various manoeuvres designed to reversibly interfere with Na+ transport. 2. At spontaneous transport rate, the short-circuit current (which reflects transepithelial Na+ transport) and the Na(+)-K(+)-ATPase activity averaged 7.0 microA/cm2 and 5.9 mumol Pi/(mg protein.h), respectively (n = 53). Short-circuit current and Na(+)-K(+)-ATPase activity appeared to be directly related over a wide range. 3. Suppression of Na+ transport led to a progressive decrease in Na(+)-K(+)-ATPase activity over several hours, with an apparent half-life of approximately 6 h after subtraction of baseline enzyme activity. 4. When A6 cells were allowed to resume sodium transport, the short-circuit current and Na(+)-K(+)-ATPase activity returned to control levels within 12-24 h, the former recovering somewhat faster. 5. When apical sodium concentration was reduced, a decrease in enzyme level occurred inasmuch as short-circuit current decreased. 6. There was good agreement between the measured enzyme activity and ouabain binding onto dispersed A6 cells, which suggests that it is unlikely that the changes observed result from internalization vs. insertion in the plasma membrane of sodium pumps.

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