CO2 and HCO3- Permeability of the Rat Liver Mitochondrial Membrane

BACKGROUND/AIMS: Across the mitochondrial membrane an exceptionally intense exchange of O2 and CO2 occurs. We have asked, 1) whether the CO2 permeability, PM,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. PM,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 PM,CO2 of the red cell membrane. PM,HCO3- was 2· 10-6 (SD ± 2· 10-6) cm/s and thus extremely low, almost 3 orders of magnitude lower than PM,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.