Effect of hypochloremia on loop segment chloride and solute reabsorption in the rat during volume expansion.

Hypochloremia has been associated by inference from clearance studies with disturbances in free water reabsorption and renal renin release by a macula densa mechanism. To examine directly these relationships, we studied chloride, fluid, and solute transport in the loop segment (LS) of rat superficial nephrons by micropuncture techniques following acute volume expansion with 0.15 m sodium chloride (CVE) or sodium bicarbonate (BVE). Plasma renin activity was suppressed in group CVE but not in group BVE. Blood pressure, plasma volume expansion, whole kidney GFR, urinary sodium excretion, late proximal and early distal SNGFR, and fluid delivery to the LS were no different between groups CVE and BVE despite different plasma chloride concentrations (102 +/- 5 mEq/liter for CVE vs. 74 +/- 3 for BVE; P less than 0.001). Absolute LS chloride reabsorption was less in BVE than it was in CVE (2030 +/- 120 pEq/min vs. 2454 +/- 136; P less than 0.025). This was associated with significantly lower fractional absolute LS solute reabsorption and higher early distal tubule fluid osmolality. These data demonstrate directly and in vivo that a decrease in chloride reabsorption in the LS of superficial nephrons is associated with impaired solute reabsorption in that segment and a failure to suppress plasma renin activity by sodium loading.