We summarize here, mainly for mammalian systems, the present knowledge of (a) the membrane CO2 permeabilities in various tissues; (b) the physiological significance of the value of the CO2 permeability; (c) the mechanisms by which membrane CO2 permeability is modulated; (d) the role of the intracellular diffusivity of CO2 for the quantitative significance of cell membrane CO2 permeability; (e) the available evidence for the existence of CO2 channels in mammalian and artificial systems, with a brief view on CO2 channels in fishes and plants; and, (f) the possible significance of CO2 channels in mammalian systems.

In comparison with the P of other cells, this value is quite high and about identical to that of the human red cell membrane. As no major protein CO channels such as aquaporins 1 and 4 are present in rat cardiac sarcolemma, the high P of this membrane is likely due to its low cholesterol content of about 0.2 (mol cholesterol)·(mol total membrane lipids) . Previous work predicted a P of ≥0.1 cm s from this level of cholesterol. We conclude that the low cholesterol establishes a P high enough to render the membrane resistance to CO diffusion almost negligible, even under conditions of maximal O consumption of the heart.

Background/Aims: Across the mitochondrial membrane an exceptionally intense exchange of O₂ and CO₂ occurs. We have asked, 1) whether the CO₂ permeability, P_M,CO2, of this membrane is also exceptionally high, and 2) whether the mitochondrial membrane is sufficiently permeable to HCO₃^- to make passage of this ion an alternative pathway for exit of metabolically produced CO₂. Methods: The two permeabilities were measured using the previously published mass spectrometric ¹⁸O exchange technique to study suspensions of mitochondria freshly isolated from rat livers. The mitochondria were functionally and morphologically in excellent condition. Results: The intramitochondrial CA activity was exclusively localized in the matrix. P_M,CO2 of the inner mitochondrial membrane was 0.33 (SD ± 0.03) cm/s, which is the highest value reported for any biological membrane, even two times higher than P_M,CO2 of the red cell membrane. P_M,HCO3- was 2· 10^-6 (SD ± 2· 10^-6) cm/s and thus extremely low, almost 3 orders of magnitude lower than P_M,HCO3- of the red cell membrane. Conclusion: The inner mitochondrial membrane is almost impermeable to HCO₃^- but extremely permeable to CO₂. Since gas channels are absent, this membrane constitutes a unique example of a membrane of very high gas permeability due to its extremely low content of cholesterol.