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

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 α 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.