Relation Between Tubular Transport of Inorganic Phosphate and Bicarbonate in the Dog

¹ From the Department of Physiology, College of Medicine, University of Cincinnati, Cincinnati, Ohio

It has been demonstrated that the renal capacity to reabsorb inorganic phosphate is depressed if the plasma CO₂ concentration is increased to such a level that bicarbonate is excreted in the urine. That this situation was not the result of alkalosis per se was demonstrated by the infusion of ‘Diamox.’ This agent caused a mild acidosis, and yet at the same time depressed phosphate reabsorption. Moreover, since ‘Diamox’ also caused a decrease in the total reabsorption of bicarbonate these results appear to be explicable only on the assumption that bicarbonate may be reabsorbed by two different mechanisms. The first, and possibly major pathway is one involving carbonic anhydrase. The second pathway is one in which bicarbonate is reabsorbed as such. It is along this second pathway that bicarbonate competes with phosphate for reabsorption. This assumption was given more weight by the findings that Tm phosphate was depressed by respiratory alkalosis, or by raising the bicarbonate and CO₂ levels of the plasma in such a way as to maintain the blood ph at the control level.