Effect of carbonic anhydrase on CO₂ tension of urine and plasma during respiratory acidosis

¹ Departments of Physiology and Chemistry, University of Louisville School of Medicine, Louisville, Kentucky

Urine-plasma gradients of pCO₂ were found in 20 dogs under conditions of respiratory acidosis induced by inhalation of 15–20% CO₂. CO₂ tension of plasma varied from 100 to 120 mm Hg in some dogs and from 170 to 190 mm Hg in others, while that of urine varied from 100 to 250 mm Hg. In 25% of periods, pCO₂ of urine was less than that of plasma by 10–35 mm Hg In 40% of the periods, pCO₂ of urine exceeded that of plasma by 10–35 mm Hg. Injection of carbonic anhydrase in amounts of 60,000–100,000 Brinkman u failed to obliterate gradients of pCO₂ in acidic urine, even though abundant quantities of enzyme were detected in urine. On the other hand, carbonic anhydrase did obliterate pCO₂ gradients between alkaline urine and plasma. Tonometric determinations of and pKa were made on several specimens of urine and of plasma, especially those wherein pCO₂ of urine was less than that of plasma. This made it possible to estimate the pCO₂ of biological fluids to within 2–3% of the true value. The usual magnitude of urine-plasma difference of pCO₂ was of the order of 15–20%. Therefore, we inferred that gradients of pCO₂ between acidic urine and plasma are not experimental artifacts. Relationships between mechanisms of urinary acidification and urine-plasma gradients of pCO₂ are discussed.