In vivo quantitation of carbonic anhydrase and band 3 protein contributions to pulmonary gas exchange

Abstract

The contributions to pulmonary gas exchange of red blood cell (RBC) membrane band 3 protein HCO3(-)-Cl- exchange and carbonic anhydrase- (CA) catalyzed HCO3- dehydration have never been determined directly in the whole animal. We utilized an experimental and model approach to measure these by analysis of phase III exhaled CO2 and O2 profiles in anesthetized dogs. In this method, we inhibit RBC membrane band 3 protein and cytoplasmic CA in RBCs passing the pulmonary capillaries and lung vascular luminal membrane-bound CA during a single ventilatory cycle. This is achieved with appropriately timed right atrial infusions of 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS) to inhibit band 3 protein, ethoxzolamide (a lipophilic CA inhibitor with rapid membrane penetrance) to inhibit RBC and lung tissue CA, and benzolamide (an extremely hydrophilic CA inhibitor with virtually no penetrance into RBC cytoplasm) to inhibit only lung vascular luminal membrane CA. DNDS caused a 15% reduction in CO2 production (VCO2) without any change in O2 consumption (VO2). The addition of benzolamide to DNDS did not cause any further decrease in VCO2. Inhibition of RBC CA by ethoxzolamide caused a 67% reduction in VCO2 and a 11.5% reduction in VO2. Inhibition of lung vascular CA by benzolamide alone caused no statistically significant changes in either VCO2 or VO2. These results are in general agreement with in vitro data and model calculations. The only exceptions are the higher than predicted effect of RBC CA inhibition on VO2 (Bohr effect) and the lack of any contribution to CO2 transfer in the dog by lung vascular CA with access to plasma as a possible consequence of an endogenous plasma CA inhibitor.