Load Dependence of Proximal Tubular Bicarbonate

Studies were undertaken in Munich-Wistar rats to determine whether maintenance of chronic metabolic alkalosis (CMA) is associated with an increase in proximal HCO3 reabsorption, or whether a reduction in glomerular filtration rate (GFR) is required to sustain the elevated plasma HCO3 concentration. Superficial single nephron glomerular filtration rate (SNGFR), and absolute proximal HCO3 (APRHCo3) and water (APRH2O) reabsorption were measured 20±3 d after the induction of CMA in eight rats and the results compared with seven age-matched control animals. Plasma [HCO31 was 39.1±1.8 mM in CMA rats compared with 26.0±0.4 mM in controls (P < 0.001). In the CMA rats, SNGFR was 44.8±1.1 vs. 38.2±2.1 nl/min in controls (P < 0.025). As a result, the single nephron filtered load of HCO3 (FLHco3) increased from 1,147±61 pmol/min in control to 2,040±108 pmol/min in CMA (P < 0.001). APRHcO3 increased by >65%, from 970±65 pmol/min in control to 1,624±86 pmol/min inCMA (P < 0.001). APRH1o increased from 18.4±1.6 nl/min in control to 24.0±0.8 nl/min in CMA (P < 0.005). Tubular hypertrophy resulted in an increase in the length of the proximal convoluted tubule from 5.6±0.2 to 6.5±0.2 mm (P <0.005). The pattern of HCO3 reabsorption along the length of the proximal convoluted tubule inCMA was indistinguishable from that found in normal rats in which FLHCo3 was varied acutely by altering SNGFR. The increase in tubular length accounted for only 30% of the increase in APRH2o and 15% of the increase in APRHCo3. We conclude that a sustained reduction in GFR is not required for maintenance ofCMA in the rat. IfGFR is chronically restored to normal levels, the alkalosis is maintained by an increase in APRHCO3. The increase in reabsorption is accounted for by (1) tubular hypertrophy, a chronic adaptive response, and (2) a load-dependent response that is indistinguishable from that seen in normal rats when FLHCO3 is increased acutely by increasing SNGFR.