PCO2 measurements in surface proximal tubules and peritubular capillaries of the rat kidney.

PCO2 was measured in surface proximal tubules and peritubular capillaries in the rat under normal acid-base conditions and in three settings with decreased HCO3(-) reabsorption: benzolamide administration, respiratory alkalosis, and metabolic acidosis. Under normal conditions, PCO2 in the early proximal tubule (EP) was 10.5 mmHg higher than PaCO2 (P less than 0.001) and 3-4 mmHg higher than late proximal (LP) and peritubular capillary (PC) PCO2 (P less than 0.001). PCO2 in LP and PC was 7 mmHg higher than PaCO2 (P less than 0.001). Benzolamide (3 mg/kg) had no effect on the difference between PC and arterial PCO2 or between EP and PC PCO2. Increasing benzolamide to 8 mg/kg increased PCO2 in the surface structures relative to arterial PCO2 by 3-5 mmHg (P less than 0.01). Metabolic acidosis did not alter the relationships between cortical and arterial PCO2. By contrast, respiratory alkalosis decreased cortical PCO2 relative to PaCO2 by over 50%. Nonetheless, EP PCO2 was still higher than LP or PC PCO2 (P less than 0.01). Thus, reducing HCO3(-) reabsorption does not obliterate the difference between EP and LP or PC PCO2 nor does it invariably reduce PCO2 in the surface structures of the kidney relative to arterial PCO2.