CO2 Permeability of Biological Membranes and Role of CO2 Channels

Figure 1. Influence of membrane CO2 permeability on CO2 release from a maximally metabolizing cardiomyocyte at a given partial pressure difference of 0.35 mmHg over the half-thickness of the cell. From: Endeward et al. [1].

Figure 2. CO2 permeability (PCO2) of phospholipid vesicles with variable concentration of cholesterol as a function of cholesterol content in mol % cholesterol per mol total membrane lipids. PCO2 determination by the mass spectrometric 18O exchange method. From: Itel et al. [3].

Figure 3. Cholesterol dependency of the CO2 permeability of phospholipid vesicles. Upper curve: calculated from the molecular dynamics simulation data of Hub et al. [22], lower curve: experimental curve of Itel et al. [3] as shown in Figure 2.

Figure 4. CO2 permeability of biological membranes as a function of their cholesterol content (% mol cholesterol/mol total membrane lipids). From left to right: rat liver mitochondria [17], cardiomyocytes [18], basolateral membrane of proximal guinea pig colon [8], and apical membrane of proximal guinea pig colon [8].

Figure 5. Relative changes in the combined diffusion resistances of 7 μm of cytoplasm and the cell membrane (Y-axis), versus varying values of CO2 permeability (X-axis). Relative changes in total resistance are defined as total resistance per resistance of the cytoplasm alone. Note that PCO2 on the X-axis is plotted logarithmically and varies between 10 and 0.003 cm/s. The upper curve (black) is calculated for an intracellular CO2 diffusion coefficient of 1.3 × 10−5 cm2/s, the lower curve (blue) for a more than 10 times lower DCO2 of 0.104 × 10−5 cm2/s as it has been postulated by Richardson and Swietach [2]. The horizontal red line represents total diffusion resistance for infinitely high PCO2, i.e., the normalized cytoplasmic resistance alone. It is apparent that, for “normal” PCO2 values between 0.02 and 0.3 cm/s (Table 1), the relative contribution of membrane CO2 permeability to the total cellular diffusion resistance becomes almost negligible for this very low cytoplasmic CO2 diffusivity (blue curve), whereas for the “classical” diffusion coefficient of 1.3 × 10−5 cm2/s (black curve) this contribution rises enormously with lower PCO2 values.