Carbonic anhydrase (CA) cytoplasmic isozymes CA I and CA II were found in human erythrocyte membrane ghosts, when prepared at pH 5.4 and pH 7.4, but not in ghosts prepared at pH 8.2. These findings could indicate that previously reported CA activity of ghosts was owing to contamination by CA I and CA II during the preparation of the ghosts. However, using a sensitive micro-assay, CA activity was also found in ghosts prepared at pH 8.2. This activity constitutes 0.2% of the erythrocytes' total CA-activity, and originates from a membrane-associated isoform of CA, located at the exterior membrane surface. It has immunochemical and kinetic properties like those of the membrane-bound CA IV, previously isolated from kidney, lung and small blood vessels. Its function is possibly to interact with the red cell membrane anionic transport protein, band 3, for the bicarbonate/chloride exchange.

Skeletal muscle extracellular carbonic anhydrase III was investigated in anesthetized rats by a microdialysis technique. A small dialysis probe was inserted into the tibialis anterior (TA) muscle and perfused continuously. Perfusates were collected before and during muscle contraction, induced by electrical stimulation of the muscle or of the sciatic nerve. In the perfusate of resting normal and denervated muscle, the concentration of CA III was 10 to 12 ng/mL, as measured by a radioimmunosorbent technique. During contractile activity, the concentrations of CA III increased markedly in the normal and denervated muscle. A TA muscle suspended in physiological saline behaved similarly, even though the leakage before and during contraction was higher than in vivo. The results show that skeletal muscle leaks CA III both in vivo and in vitro, a leakage which was markedly increased by contractile activity. The microdialysis technique should also be useful in humans to study the efflux of various proteins from different kinds of diseased or fatigued muscles.