How bicarbonate loading inhibits tubular reabsorption of NaCI in dog kidneys

During continuous infusion of ethacrynic acid in dogs, changes in glomerular filtration rate (GFR) and PCO2 at constant plasma bicarbonate concentration (PHCO3) alter bicarbonate and chloride reabsorption in a ratio of 1:2. This ratio did not apply when PHCO3 was raised by bicarbonate loading in 11 anaesthetized volume-expanded dogs. A rise in PHCO3 from 30 to 54 mM at constant PCO2 and GFR reduced sodium reabsorption during ethacrynic acid infusion from 3586 +/- 725 to 2449 +/- 403 mumol min-1. Bicarbonate and chloride reabsorption were reduced in a ratio of 1:10. When plasma pH was restored from 7.8 to 7.5 by raising PCO2, the inhibitory effect on chloride reabsorption was halved. At constant plasma pH 7.5 a rise in PHCO3 from 20 to 30 mM reduced chloride reabsorption by 20%. A further 30% inhibition was caused by raising PHCO3 from 30 to 54 mM. Bicarbonate reabsorption was highest at PHCO3 54 mM, suggesting a large capacity for bicarbonate reabsorption if PHCO3 is raised at constant plasma pH 7.5. Water and NaCl reabsorption remaining during ethacrynic acid infusion is almost equally inhibited by alkalosis and by an osmotic effect of unreabsorbed NaHCO3.