Angiotensin II clamp prevents the second step in renal apical NHE3 internalization during acute hypertension

Abstract

Acute hypertension inhibits proximal tubule (PT) sodium reabsorption. The resultant increase in NaCl delivery to the macula densa suppresses renin release. We tested whether the sustained pressure-induced inhibition of PT sodium reabsorption requires a renin-mediated decrease in ANG II levels. Plasma ANG II concentration of anesthesized Sprague-Dawley rats was clamped by simultaneous infusion of the ANG I-converting enzyme inhibitor captopril (12 microg/min) and ANG II (20 ng. kg(-1). min(-1)). Blood pressure was increased 50 mmHg for 20 min by arterial constriction +/- ANG II clamp or by sham operation. This acute hypertension increased urine output and endogenous Li(+) clearance, and these responses were blunted 40-50% in ANG II clamped rats. Acute hypertension provoked a rapid redistribution of Na(+)/H(+) exchanger isoform 3 (NHE3) out of apical brush-border membranes (21 +/- 4% decrease of total NHE3 abundance) to endosomal/lysosomal membranes (16 +/- 6% increase of total). In ANG II-clamped rats, acute hypertension also provoked disappearance of NHE3 from the apical membranes (27 +/- 2% decrease of total), but NHE3 was shifted to membranes enriched in intermicrovillar cleft and dense apical tubules (step 1) rather than endosomal/lysosomal membranes (step 2). This difference was independently confirmed by confocal analysis. In contrast, the pressure-induced redistribution of Na(+)-P(i) cotransporter type 2 was not altered by ANG II clamp. We conclude that the responses to acute hypertension, including diuresis and redistribution of PT NHE3 into intracellular membranes, require a responsive renin-angiotensin system and that the responses may be induced by the sustained increase in NaCl delivery to the macula densa during acute hypertension.