The effect of 4,4′-diisothiocyanato-stilbene-2,2′-disulfonate on CO2 permeability of the red blood cell membrane

It has long been assumed that the red cell membrane is highly permeable to gases because the molecules of gases are small, uncharged, and soluble in lipids, such as those of a bilayer. The disappearance of ¹²C¹⁸O¹⁶O from a red cell suspension as the ¹⁸O exchanges between labeled CO₂ + HCO₃⁻ and unlabeled HOH provides a measure of the carbonic anhydrase (CA) activity (acceleration, or A) inside the cell and of the membrane self-exchange permeability to HCO₃⁻ (P_m,HCO⁻3). To test this technique, we added sufficient 4,4′-diisothiocyanato-stilbene-2,2′-disulfonate (DIDS) to inhibit all the HCO₃⁻/Cl⁻ transport protein (Band III or capnophorin) in a red cell suspension. We found that DIDS reduced P_m,HCO⁻3 as expected, but also appeared to reduce intracellular A, although separate experiments showed it has no effect on CA activity in homogenous solution. A decrease in P_m,CO2 would explain this finding. With a more advanced computational model, which solves for CA activity and membrane permeabilities to both CO₂ and HCO₃⁻, we found that DIDS inhibited both P_m,HCO⁻3 and P_m,CO2, whereas intracellular CA activity remained unchanged. The mechanism by which DIDS reduces CO₂ permeability may not be through an action on the lipid bilayer itself, but rather on a membrane transport protein, implying that this is a normal route for at least part of red cell CO₂ exchange.