Reversible Effects of Acute Hypertension on Proximal Tubule Sodium Transporters

Acute hypertension provokes a rapid decrease in proximal tubule sodium reabsorption with a decrease in basolateral membrane sodium-potassium-ATPase activity and an increase in the density of membranes containing apical membrane sodium/hydrogen exchangers (NHE3) [Y. Zhang, A. K. Mircheff, C. B. Hensley, C. E. Magyar, D. G. Warnock, R. Chambrey, K.-P. Yip, D. J. Marsh, N.-H. Holstein-Rathlou, and A. A. McDonough. Am. J. Physiol. 270 (Renal Fluid Electrolyte Physiol. 39): F1004-F1014, 1996]. To determine the reversibility and specificity of these responses, rats were subjected to 1) elevation of blood pressure (BP) of 50 mmHg for 5 min, 2) restoration of normotension after the first protocol, or 3) sham operation. Systolic hypertension increased urine output and endogenous lithium clearance three- to fivefold within 5 min, but these returned to basal levels only 15 min after BP was restored. Renal cortex lysate was fractionated on sorbitol gradients. Basolateral membrane sodium-potassium-ATPase activity (but not subunit immunoreactivity) decreased one-third to one-half after BP was elevated and recovered after BP was normalized. After BP was elevated, 55% of the apical NHE3 immunoreactivity, smaller fractions of sodium-phosphate cotransporter immunoreactivity, and apical alkaline phosphatase and dipeptidyl-peptidase redistributed to membranes of higher density enriched in markers of the intermicrovillar cleft (megalin) and endosomes (Rab 4 and Rab 5), whereas density distributions of the apical cytoskeleton protein villin were unaltered. After 20 min of normalized BP, all the NHE3 and smaller fractions of the other apical membrane proteins returned to their original distributions. These findings suggest that the dynamic regulation of proximal tubule sodium transport by acute changes in BP may be mediated by rapid reversible regulation of sodium pump activity and relocation of apical sodium transporters.