PCO2 in renal cortex.

In a number of recent investigations a renal cortical PCO2 higher than that of systemic blood was reported. We have studied this problem with the use of micro-Severinghaus electrodes based on antimony, H+ liquid ion exchange, and glass pH electrodes with an inner buffer solution containing 0.5 mg/ml carbonic anhydrase (CA). Measurements in renal cortical structures (renal tubules, star vessels, capillaries, and glomeruli in Munich-Wistar rats) were compared with determinations in renal vein or artery performed with the same electrode in sequence. No significant differences in PCO2 were found between cortical structures and renal vein in control rats, in metabolic alkalosis, respiratory acidosis and alkalosis, and after CA inhibition. Nevertheless, absolute PCO2 levels, which followed the PCO2 of systemic blood, were markedly different in these groups. Measurements of pH and PCO2 at the same tubule site were compatible with HCO3- determinations in tubule fluid in vitro (made with use of the Henderson-Hasselbalch equation) in control rats. When proximal tubules were pump-perfused in vivo with a solution containing 30 mM NaHCO3, measured PCO2 approached that of the perfusing solution at high pump rates, and approached the free-flow value as rates were reduced to zero, indicating that the CO2 generated in the lumen equilibrated rapidly across the epithelium. Reducing renal blood flow by aortic clamping reduced renal cortical PCO2. In conclusion, in a large number of experimental conditions renal cortical PCO2 was never higher than that measured in systemic blood.