Bicarbonate transport in cortical and outer medullary collecting tubules

Abstract

The collecting ducts are thought to represent a low-capacity high-gradient acidification system. The inaccessibility of the various regions of the collecting duct system has prevented direct segmental analysis of its contribution to distal acidification. The present in vitro microperfusion studies compare HCO3- transport (in pmol/min per min) in rabbit cortical (CCT) and outer medullary collecting tubules (MCT) perfused and bathed with symmetrical Ringer HCO3- solution at pH 7.4. Cortical segments from normal animals exhibited no net bicarbonate transport (-2.15 .+-. 1.93); MCT from normal animals reabsorbed HCO3- at a rate of 11.3 .+-. 1.4. Both HCO3- reabsorption and the lumen-positive voltage (+9.4 .+-. 1.1 mV) in MCT were totally inhibited by 10-4 M acetazolamide. CCT from NH4Cl-treated rabbits demonstrated significant HCO3- reabsorption (1.8 .+-. 0.7) when perfused at slow rates. CCT harvested from animals given a NaHCO3 load for 48 h prior to death secreted HCO3- (-6.2 .+-. 2.5). The ability of the CCT to reabsorb or secrete HCO3- was confirmed. They demonstrate significant axial heterogeneity in acidification in the collecting duct system and identify the outer medullary collecting tubule from inner stripe of outer medulla as a segment of major capacity.