The effect of potassium and extracellular volume on renal bicarbonate reabsorption

Bicarbonate reabsorption was measured in dogs infused with either KCl or KHCO₃. As has been previously reported, potassium loading depressed bicarbonate reabsorption. Similar studies in dogs with partial obstruction of the thoracic inferior vena cava failed to demonstrate an effect of potassium loading on bicarbonate reabsorption. Extracellular volume was expanded with isotonic saline in three groups of dogs: one with potassium depletion, a second with hyperkalemia, and a third normal group. Bicarbonate reabsorption varied inversely with fractional chloride excretion in all three groups. At any one level of fractional chloride excretion, however, bicarbonate reabsorption was higher in the potassium depleted animals than in the normal dogs, and higher in the normal dogs than in those subjected to potassium loading. This study demonstrates a significant regulatory role of potassium over renal bicarbonate reabsorption. This role can only clearly be defined, however, when the precise state of effective extracellular volume is delineated.