Tubular handling of bicarbonate and chloride in the dog

Bicarbonate reabsorption was assessed in the superficial proximal and distal tubule of the hydropenic dog. Tubule fluid bicarbonate concentration was measured by determination of total CO2 with microcalorimetry. Total CO2 declined from 21 to 14 mM along the proximal tubule, which reabsorbed 56% of the filtered bicarbonate prior to the late proximal sampling site. Absolute bicarbonate reabsorption was 124 pmol . min-1 . mm tubule length-1. Concurrent water reabsorption was 34% of the glomerular filtration rate. Tubule fluid-to-ultrafilterable chloride concentration ratio rose to 1.10 as the tubule fluid-to-blood bicarbonate ratio fell to 0.69 at the late proximal collection site. The total CO2 in distal tubule fluid was 6 mM, indicating that 94% of the filtered bicarbonate was reabsorbed prior to the distal tubule. It is concluded that bicarbonate is reabsorbed in preference to chloride in the superficial proximal tubule of the dog, which, like that of the rat, may provide a potential driving force for water and salt reabsorption.