Segmental chloride reabsorption in the rat nephron as a function of load

Segmental chloride (Cl) transport was examined during hydropenia (H) and under conditions in which Cl delivery was increased by either acute isotonic volume expansion (10% and 15% VE) or with low (LF) and high (HF) furosemide dosage in the absence of VE. Absolute chloride reabsorption in the proximal tubule was 2,062 ± 282 peq/min during hydropenia and decreased 31.2% after 10% VE and 85.3% after 15% VE, but was unchanged after furosemide. The fraction of chloride remaining in the late proximal tubule increased from 50.2 ± 0.02% (H) to 68.2 ± 0.04% (10% VE) and to 102.90 ± 4.9% (15% VE), but was unaltered by furosemide. The fraction of the filtered chloride remaining in the late distal tubule increased from 3.2 ± 0.4% (H) to 9.1 ± 0.7% (10% VE), 12.8 ± 0.7% (15% VE), 12.9 ± 1.2% (LF), and 33.6 ± 2.7% (HF). This was associated with an increase in fractional chloride excretion from 0.26 ± 0.07% (H) to 3.86 ± 0.7% (10% VE), 7.11 ± 1.0% (15% VE), 8.90 ± .68% (LF), and 19.5 ± 1.8% (HF). As absolute Cl delivery out of the late distal tubule was progressively increased, absolute urinary excretion of chloride increased as a function of load. This relationship held when chloride excretion was augmented by either systemic volume expansion or furosemide. It is concluded that 1) Cl reabsorption beyond the last accessible distal tubule increases as a function of chloride load irrespective of systemic VE suggesting that VE per se has no selective effect on this segment; 2) there is no evidence of net Cl secretion beyond the superficial late distal tubule; 3) massive VE may almost completely inhibit superficial proximal Cl reabsorption resulting in a marked increase in delivery to and thus reabsorption of Cl in the loop of Henle.