Microelectrode determination of pH and PCO₂ in rat proximal tubule after benzolamide: Evidence for hydrogen ion secretion

Previous micropuncture studies supporting hydrogen secretion as the mechanism of bicarbonate reabsorption have relied on the demonstration of an acid disequilibrium pH in the proximal tubule after systemic administration of a carbonic anhydrase inhibitor. Previous calculations of disequilibrium pH, however, have involved the necessary assumption that PCO₂ in the proximal convoluted tubule was equal to arterial blood PCO₂. This assumption can no longer be supported in view of the recent demonstration that the PCO₂ in proximal and distal tubular fluid exceeded arterial blood by approximately 25 mm Hg. The purpose of the present study was to determine directly pH and PCO₂ with microelectrodes in both the early and late segments of the accessible proximal tubule of nine Sprague-Dawley rats before and after administration of benzolamide (2.0 mg/kg/hr, i.v.). In the early proximal tubule, pH decreased significantly after benzolamide administration from 6.98 ± 0.03 to 6.62 ± 0.03 pH U (P < 0.001), and PCO₂ also decreased from 65.1 ± 1.2 to 59.3 ± 1.4 mm Hg (P < 0.005). In the late proximal convoluted tubule, pH did not change after benzolamide (6.75 ± 0.02 to 6.77 ± 0.02), but PCO₂ decreased significantly (64.3 ± 1.5 to 57.7 ± 1.6) (P < 0.01). We conclude: (1) the fall in both pH and PCO₂ in the early proximal tubule indicates that carbonic acid, not carbon dioxide accumulates after inhibition of luminal carbonic anhydrase; (2) although PCO₂ also decreased in the late proximal tubule, unlike the early segment, pH was unchanged after benzolamide administration, perhaps as a result of increased bicarbonate delivery; and (3) PCO₂ in vivo was significantly greater than was systemic arterial PCO₂ before and after benzolamide administration in both the early and late proximal convoluted tubule. These findings lend support to the view that bicarbonate reabsorption in the proximal convoluted tubule occurs, in part, by hydrogen secretion.