Renal function studies and tissue enzyme analysis were carried out in rats with bilateral ureteral ligation (24 h) and after relief of obstruction at intervals of 2 h, 24 h, 3 days, and 7 days. A persistent decrease in glomerular filtration rate and in tubular reabsorption of sodium, solute, and water was noted at each interval after relief of obstruction. Renal sodium- and potassium-activated adenosine triphosphatase (Na–K-ATPase) activity was unchanged in kidneys obstructed for 24 h, but enzyme activity was decreased in the outer medulla of the kidney and, less often, in the renal cortex, at each interval studied during the post-obstructive period. Magnesium-dependent adenosine triphosphatase (Mg-ATPase) activity was not changed in any of the groups. In the post-obstructive kidney (24 h, 3 days, and 7 days) there was a significant correlation between the decrease in outer medullary Na–K-ATPase activity and the changes in filtered sodium load, tubular reabsorption of sodium, and urine osmolality. A deficiency of renal Na–K-ATPase could be important in the pathogenesis of post-obstructive natriuresis and diuresis.

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The aim of this study was to determine the effect of changes in osmolality on the reduced renal medullary Na-K-ATPase (EC 3.6.1.3) activity of the postobstructive kidney. The effect of osmolality on renal medullary Na-K-ATPase activity was studied by incubating tissue slices from sham-operated and bilaterally obstructed rats in media with osmolality varied before enzyme isolation using sodium chloride, choline chloride, or sucrose. Both sham-operated and bilaterally obstructed rat renal medullary enzyme showed a similar increase in activity with increased osmolality due to sodium chloride. Medullary Na-K-ATPase from the postobstructive kidney also showed increased activity with osmotic changes induced by choline chloride or sucrose. It is proposed that the decrease of Na-K-ATPase activity observed after bilateral ureteral obstruction is due, at least in part, to the loss of the solute concentration gradient in the kidney.

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Na-K-ATPase activity and renal function were compared in rats studied after relief of 24 h of unilateral or bilateral ureteral ligation (UUL or BUL), that is, in the absence or presence of post-obstructive diuresis. Na-K-ATPase activity in the outer medulla of the rat kidney after relief of UUL was not significantly altered immediately but was markedly reduced 1 and 3 days post-obstruction. The decrease in medullary Na-K-ATPase activity was not significantly different from that observed after relief of BUL. These results indicate that decreased Na-K-ATPase activity in the post-obstructive kindney is not responsible for post-obstructive diuresis and is not due to uremia, but is a local phenomenon which is probably secondary to altered renal structure or function. It may be due to decreased filtered sodium load or direct tubular damage, but other data suggests that the decreased medullary solute concentration gradient in the post-obstructive kidney (UUL or BUL) may influence Na-K-ATPase activity which, in turn, contributes to the decreased ability to conserve sodium and water.