Glomerular-tubular balance for bicarbonate in the dog.

Previous work that apparently showed a functional relationship between GFR and maximum bicarbonate reabsorption was done at a time when the effects on the latter of several factors (PCO2, plasma potassium concentration, and extracellular fluid volume expansion) were not recognized. The present study re-examines this relationship, while controlling these factors. In 14 hydropenic dogs, bicarbonate reabsorption per unt GFR increased linearly with increases in plasma bicarbonate concentration. At any level of plasma bicarbonate concentration,the absolute rate of bicarbonate reabsorption was functionally related to the GFR. In six volume-expanded dogs, bicarbonate reabsorption remained stable at 20-22 mmol/liter GFR as plasma bicarbonate was raised to greater than 40mM. The absolute rate of bicarbonate reabsorption increased with large increases in GFR induced by methylprednisolone and high-protein diet. In a third group of dogs, bicarbonate reabsorption varied directly with increases in GFR, while plasma bicarbonate concentration was held relatively constant above the threshold. We conclude there is a close functional relationship between the absolute rate of bicarbonate reabsorption and GFR in individual dogs.

Glomerular-tubular balance for bicarbonate in man.

We recently showed in dogs that there is a close relationship between bicarbonate reabsorption and glomerular filtration rate (GFR). The present studies were designed to confirm these findings in five normal men. Bicarbonate titrations were done after ingesting a low protein diet for one week and were repeated following elevation of GFR 11-29 per cent by high protein diet and dexamethazone ingestion. In every casetubular reabsorption of bicarbonate increased in proportion to the increases in GFR. We conclude that glomerular-tubular balance for bicarbonate reabsorption is a characteristic of human, as well as canine kidneys.