Postcapillary changes in blood pH in vivo during carbonic anhydrase inhibition.

A rapidly responding stopped-flow glass pH electrode apparatus was used to investigate pH changes in blood in vivo after it exits from an exchange capillary. Arterial blood was drawn from anesthetized animals through the apparatus. Temperature and pH of the blood in the electrode chamber were continuously recorded, both during withdrawal and after flow was stopped. Blood pH did not change after stopping flow in control experiments. When benzolamide (2 mg/kg) was given to inhibit carbonic anhydrase activity available to plasma (e.g., due to lysis) while having less effect on intracellular activity, pH increased 0.02-0.04 (t1/2 approximately 8 s) after stopping flow. Administration of acetazolamide (50 mg/kg) resulted in pH decreasing 0.07-0.10 (t1/2 approximately 15 s) after stopping flow. Ventilation for 1 min with N2 resulted in an increased rise in pH for the benzolamide-treated animals but a decreased fall in pH for the acetazolamide-treated animals. These shifts in arterial blood pH after gas exchange are largely due to disequilibrium of [H+] between red cells and plasma at the end of the pulmonary capillary.