CO2 permeability of cell membranes is regulated by membrane cholesterol and protein gas channels

Recent observations that some membrane proteins act as gas channels seem surprising in view of the classical concept that membranes generally are highly permeable to gases. Here, we study the gas permeability of membranes for the case of CO2, using a previously established mass spectrometric technique. We first show that biological membranes lacking protein gas channels but containing normal amounts of cholesterol (30-50 mol% of total lipid), e.g., MDCK and tsA201 cells, in fact possess an unexpectedly low CO2 permeability (PCO2) of similar to 0.01 cm/s, which is 2 orders of magnitude lower than the PCO2 of pure planar phospholipid bilayers (similar to 1 cm/s). Phospholipid vesicles enriched with similar amounts of cholesterol also exhibit PCO2 approximate to 0.01 cm/s, identifying cholesterol as the major determinant of membrane PCO2. This is confirmed by the demonstration that MDCK cells depleted of or enriched with membrane cholesterol show dramatic increases or decreases in PCO2, respectively. We demonstrate, furthermore, that reconstitution of human AQP-1 into cholesterol-containing vesicles, as well as expression of human AQP-1 in MDCK cells, leads to drastic increases in PCO2, indicating that gas channels are of high functional significance for gas transfer across membranes of low intrinsic gas permeability.-Itel, F., Al-Samir, S., Oberg, F., Chami, M., Kumar, M., Supuran, C. T., Deen, P. M. T., Meier, W., Hedfalk, K., Gros, G., Endeward, V. CO2 permeability of cell membranes is regulated by membrane cholesterol and protein gas channels. FASEB J. 26, 5182-5191 (2012). www.fasebj.or

Rapid CO 2

The degree to which cell membranes are barriers to CO2 transport remains controversial. Proteins, such as aquaporins and Rh complex, have been proposed to facilitate CO2 transport, implying that the nonchannel component of membranes must have greatly reduced CO2 permeability. To determine whether membrane CO2 permeation is rate limiting for gas transport, the spread of CO2 across multicellular tissue growths (spheroids) was measured using intracellular pH as a spatial readout. Colorectal HCT116 cells have basal water and NH3 permeability, indicating the functional absence of aquaporins and gas channels. However, CO2 diffusivity in HCT116 spheroids was only 24 ± 4% lower than in pure water, which can be accounted for fully by volume exclusion due to proteins. Diffusivity was unaffected by blockers of aquaporins and Rh complex (Hg(2+), p-chloromercuribenzoic acid, and 4,4'-diisothiocyano-2,2'-stilbene-disulfonic acid) but decreased under hypertonic conditions (by addition of 300 mOsm mannitol), which increases intracellular protein crowding. Similar CO2 diffusivity was measured in spheroids of T47D breast cells (basal water permeability) and NHDF-Ad fibroblasts (aquaporin-facilitated water permeability). In contrast, diffusivity of NH3, a smaller but less lipophilic gas, was considerably slower than in pure water, as expected from rate-limiting membrane permeation. In conclusion, membranes, even in the functional absence of proposed gas channels, do not restrict CO2 venting from tissue growths.-Hulikova, A., Swietach, P. Rapid CO2 permeation across biological membranes: implications for CO2 venting from tissue

Red blood cell thickness is evolutionarily constrained by slow, hemoglobin-restricted diffusion in cytoplasm

During capillary transit, red blood cells (RBCs) must exchange large quantities of CO2 and O2 in typically less than one second, but the degree to which this is rate-limited by diffusion through cytoplasm is not known. Gas diffusivity is intuitively assumed to be fast and this would imply that the intracellular pathlength, defined by RBC shape, is not a factor that could meaningfully compromise physiology. Here, we evaluated CO2 diffusivity (DCO2) in RBCs and related our results to cell shape. DCO2 inside RBCs was determined by fluorescence imaging of [H+] dynamics in cells under superfusion. This method is based on the principle that H+diffusion is facilitated by CO2/HCO3− buffer and thus provides a read-out of DCO2. By imaging the spread of H+ions from a photochemically-activated source (6-nitroveratraldehyde), DCO2 in human RBCs was calculated to be only 5% of the rate in water. Measurements on RBCs containing different hemoglobin concentrations demonstrated a halving of DCO2 with every 75 g/L increase in mean corpuscular hemoglobin concentration (MCHC). Thus, to compensate for highly-restricted cytoplasmic diffusion, RBC thickness must be reduced as appropriate for its MCHC. This can explain the inverse relationship between MCHC and RBC thickness determined from >250 animal species

Invertebrate aquaporins: a review

Ewan M. Campbell, Andrew Ball, Stefan Hoppler, Alan S. Bowma