Several membrane channels, like aquaporin-1 (AQP1) and the RhAG protein of the rhesus complex, were hypothesized to be of physiological relevance for CO(2) transport. However, the underlying assumption that the lipid matrix imposes a significant barrier to CO(2) diffusion was never confirmed experimentally. Here we have monitored transmembrane CO(2) flux (J(CO2)) by imposing a CO(2) concentration gradient across planar lipid bilayers and detecting the resulting small pH shift in the immediate membrane vicinity. An analytical model, which accounts for the presence of both carbonic anhydrase and buffer molecules, was fitted to the experimental pH profiles using inverse problems techniques. At pH 7.4, the model revealed that J(CO2) was entirely rate-limited by near-membrane unstirred layers (USL), which act as diffusional barriers in series with the membrane. Membrane tightening by sphingomyelin and cholesterol did not alter J(CO2) confirming that membrane resistance was comparatively small. In contrast, a pH-induced shift of the CO(2) hydration-dehydration equilibrium resulted in a relative membrane contribution of about 15% to the total resistance (pH 9.6). Under these conditions, a membrane CO(2) permeability (3.2 +/- 1.6 cm/s) was estimated. It indicates that cellular CO(2) uptake (pH 7.4) is always USL-limited, because the USL size always exceeds 1 mum. Consequently, facilitation of CO(2) transport by AQP1, RhAG, or any other protein is highly unlikely. The conclusion was confirmed by the observation that CO(2) permeability of epithelial cell monolayers was always the same whether AQP1 was overexpressed in both the apical and basolateral membranes or not.
