Factors influencing chloride reabsorption in the collecting duct segment of the rat.

To study factors influencing Cl reabsorption in the collecting duct segment of the rat, excretion of Cl and of 36Cl injected into late distal tubule segments of superficial nephrons were examined by microinjection and free-flow micropuncture techniques. Selective dietary Cl restriction for 7-10 days decreased urinary 36Cl recovery for microinjection [74 +/- 5% (SE)] compared with normal rats (90 +/- 3%, P < 0.01). Infusion of 0.15 M NaHCO3 (10% body wt) in the Cl-restricted rat led to marked increases in urinary flow rates and sodium excretion but did not increase 36Cl recovery. In these NaHCO3-infused rats, recovery of 36Cl was unaffected by increasing Cl concentration in the injectate from 40 to 140 peq/nl, but increased to levels not different from control (89 +/- 3%) when amphotericin B was included in the injectate. Equivalent infusion of 0.15 M NaCl in the Cl-restricted rats also increased 36Cl recovery to control levels (89 +/- 3%). This response was not altered by administration of DOCA. Absolute chloride delivery to late distal tubule sites was not different in NaCl- and NaHCO3-expanded groups, and (TF/UF)Cl was less in the NaHCO3 group (0.27 +/- 0.03 vs. 0.37 +/- 0.02, P < 0.05); therefore, the chemical gradient for Cl did not favor Cl efflux. There was a significant correlation between urinary recovery of 36Cl and urinary Cl excretion in the NaCl- and NaHCO3-expanded animals. It is suggested that Cl restriction alters, by an as yet unknown mechanism that is independent of changes in Na balance, the intrinsic Cl-absorbing capacity of the collecting duct.