To examine if
bicarbonate reabsorption varies with filtered
bicarbonate and plasma pH, we infused anesthetized dogs i.v. with
sodium chloride and
sodium bicarbonate to alter plasma
bicarbonate concentration (PHCO3) without changing hematocrit. Examinations in five dogs over a wide range of glomerular filtration rates (GFR) during
ethacrynic acid infusion showed that
bicarbonate reabsorption at equal filtered load and equal plasma pH of 7.5 was not significantly changed by increasing PHCO3 from 30.2 +/- 0.4 to 55.2 +/- 0.6 mM and PCO2 from 33.8 +/- 0.7 to 74.1 +/- 2.1 mm Hg. Examinations during respiratory and metabolic
alkalosis in five dogs at plasma pH of 7.8 showed that
bicarbonate reabsorption at equal filtered load was not significantly different at a PCO2 of 20.2 +/- 0.8 and 36.8 +/- 0.8 mm Hg. Finally, in five dogs that did not receive
ethacrynic acid, plasma pH was lowered by inducing
respiratory acidosis at a PHCO3 of 30 mM and raised during progressive respiratory and metabolic
alkalosis,
Bicarbonate reabsorption was linearly related to plasma pH within the range 7.1 to 7.85 (r = 0.92). By altering plasma pH by 0.1 unit,
bicarbonate reabsorption was altered by 10 +/- 1%. Thus, filtered
bicarbonate rather than GFR and plasma pH rather than PCO2 are important acute regulators of
bicarbonate reabsorption. This regulation may be achieved by determining pH and
bicarbonate concentration in the
luminal fluid along the proximal tubules.