Carbonic anhydrase activity of the cat hind leg.

The accessibility of tissue carbonic anhydrase to plasma was studied in five surgically isolated cat hind legs. After the leg was skinned and the paw circulation occluded with a tourniquet, it was perfused with a solution that contained neither red cells nor carbonic anhydrase. Solutions containing either H14CO3- or 14CO2 were injected with 125I-albumin, 22Na+, and 3H2O into the femoral artery and the concentrations of each were measured in the femoral venous outflow. Under control circumstances the outflow patterns of H14CO3- and 14CO2 were very similar. However, after carbonic anhydrase inhibition with 20 mg/l acetazolamide in the perfusion solution, the initial exchange of H14CO3- ("extraction") was greatly decreased, whereas the extraction of 14CO2 was slightly increased. Because there was insignificant carbonic anhydrase activity in the venous outflow, these data suggest the presence of carbonic anhydrase at a readily accessible site, possibly bound to the endothelial surface. In this location it would promote CO2 exchange and minimize disequilibrium between plasma HCO3- and CO2.