CO2 and HCO3- Permeability of the Rat Liver Mitochondrial Membrane

Abstract

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.

CO2permeability and carbonic anhydrase activity of rat cardiomyocytes

“…It is essential that the extracellular CA activity is suppressed in order to obtain correct estimates of P CO2. 2 Mechanism of high CO 2 permeability of cardiac sarcolemma. This P CO2 value is similar to the P CO2 of about 0.15 cm/s reported for human red blood cells.…”

“…1,2 Itel et al, 3 Hub et al, 4 Values reported so far vary between 0.3 cm s À1 for the mitochondrial membrane and 0.001 cm s À1 for the apical membrane of colon epithelium.…”

“…Values reported so far vary between 0.3 cm s À1 for the mitochondrial membrane and 0.001 cm s À1 for the apical membrane of colon epithelium. 1,2 Itel et al, 3 Hub et al, 4 and Kai and Kaldenhoff 5 have shown that the modulation of P CO2 is achieved by variations in membrane cholesterol content and absence/presence of protein gas channels in the membrane. What is the biological purpose of the adaptability of P CO2 in such a wide range?…”

CO2 Permeability of Rat Hepatocytes and Relation of CO2 Permeability to CO2 Production

“…Membranes from various cells and organelles have shown P CO2 values varying between 0.001 cm/s for the apical membrane of colon ephitelium [1] to 0.3 cm/s for the mitochondrial membrane [2]. These differences in P CO2 have been shown to be associated with two main factors: the cholesterol content of the membrane [3] and the presence/absence of CO 2 channels in the membrane [3-5].…”

“…Another observation has been that cultured MDCK and tsA201 cells, with very low oxygen consumptions of ∼12 nmol/s/ml [2], exhibit very low P CO2 values of 0.017 and 0.007 cm/s, respectively [3], whereas heart cells with a maximal oxygen consumption of around 300 nmol/s/ml have a permeability of 0.1 cm/s, and mitochondria with an even higher maximal oxygen consumption of ∼ 1000 nmol/s/ml have the highest CO 2 permeability ever measured for a biological membrane of 0.3 cm/s [2]. …”