CO 2 and HCO 3- Permeability of the Rat Liver Mitochondrial Membrane

Abstract

Background/Aims: Across the mitochondrial membrane an exceptionally intense exchange of O 2 and CO 2 occurs. We have asked, 1) whether the CO 2 permeability, P M,CO2 , of this membrane is also exceptionally high, and 2) whether the mitochondrial membrane is sufficiently permeable to HCO 3- to make passage of this ion an alternative pathway for exit of metabolically produced CO 2 . Methods: The two permeabilities were measured using the previously published mass spectrometric 18 O 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 HCO 3- but extremely permeable to CO 2 . 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.