Gas channels and CO ₂ transport across cell membranes: Mechanisms and synergy

The transport of gases, such as CO2, NH3 and O2, across cell membranes is critical for all forms of life. It was generally accepted that gases freely and passively cross the cell membrane barrier governed by the well-established solubility-diffusion model. As early as 1830 it was established that solubility across membranes increases with the lipophilic nature of the molecule. Shortly thereafter the principle of ‘diffusion’ was introduced to the solubility concept. In 1855 Fick's theory of diffusion established the parameters that govern the flow of solutes. Extending the solubility-diffusion model to include membrane barriers, the rate of transport of CO2 across a cell membrane can be expressed as JCO2 = PCO2. ([CO2]o – [CO2]i), where JCO2 is the flux of CO2 and PCO2 is the permeability of CO2 driven by the concentration gradient of CO2 across the membrane. PCO2 includes the diffusion coefficient of CO2 (DCO2) and the area of CO2 exposure and is inversely related to the barrier thickness (bulk solution and cell membrane). Around 1900 Meyer and Overton, working independently on anaesthetics, recognized an association between the lipid solubility of substances and their increased ability to cross the membrane. This association, known as the Meyer–Overton rule, is inaccurately often used to mean the solubility-diffusion model. Building on the principles of diffusion, a cell can influence membrane transport of CO2 by controlling membrane permeability or the CO2 concentration gradient across the cell membrane. Membrane permeability can be affected by altering the composition of the lipid bilayer or by the presence of membrane proteins. Abundance of the hydrophilic membrane proteins, which do not conduct CO2, and their arrangement can usually hinder or obstruct CO2 access to the membrane. On the other hand, maintaining the CO2 concentration gradient is dependent on CO2 build-up in unstirred layers adjacent to the membrane. When a cell is exposed to CO2-containing solution, it diffuses through the extracellular unstirred layer, through the membrane and then through an intracellular layer before moving to the intracellular compartment. Accumulation of CO2 in the membrane intracellular unstirred layer will reduce the concentration gradient and slow diffusion of CO2. Converting intracellular CO2 to HCO3− will keep low intracellular CO2 concentration adjacent to the membrane. Similarly CO2 in the extracellular unstirred layer needs to be replenished from HCO3− to maintain the gradient. Interconversion of CO2 and HCO3− is a slow process that is highly accelerated by carbonic anhydrases (CAs). The widespread acceptance of Overton's rule that all gases cross all cell membranes by dissolving in the lipid layer of the cell membrane was challenged by two important discoveries. Boron's group in 1994 (Waisbren et al., 1994) reported that specific cell membranes were relatively impermeable to CO2. This was followed by another seminal discovery that the water channel AQP1, expressed in oocytes, increased the permeability of the membrane to CO2, thus acting as a CO2 channel (Nakhoul et al., 1998). Using pH microelectrodes to measure intracellular pH, exposure of the oocyte to a solution containing CO2/HCO3− caused intracellular acidification due to the intracellular reaction: CO2 + H2O↔HCO3− + H+. As such CO2 transport was estimated from the pHi decrease and the rate of intracellular acidification. Because hydration of CO2 is slow, build-up of CO2 in the intracellular unstirred layer could decrease the concentration gradient of CO2. Indeed injecting the oocyte with CA increased CO2 influx significantly in oocytes expressing AQP1 compared to H2O-injected oocytes. This narrative briefly describes the important hallmarks of studying CO2 transport across cell membranes. The important points include (i) the permeability and diffusion of CO2, (ii) the role of CAs and (iii) the AQPs as CO2 channels. Boron's group has contributed significantly to elucidating the mechanisms of all these points. In those studies identifying cell membranes that are impermeable to CO2 led to the discovery of AQP1 as a CO2 channel. In a series of several elegant papers, they examined the role of CA in affecting CO2 transport. In the first paper (Musa-Aziz et al., 2014a) they addressed the role of intracellular CA. They showed that injecting oocytes with soluble CA-II accelerated CO2 influx and this influx was blocked by ethoxzolamide. In the second paper (Musa-Aziz et al., 2014b) they examined the effect of extracellular CA by expressing CA-IV at the cell membrane. The same group then used mathematical modelling to interpret the CO2 influx in the oocyte. The paper in this issue by Wang et al. (2025) examines all three factors that were previously studied independently to address CO2 membrane transport. They systematically assessed functional interactions among AQP5 (a water channel expressed in the lung), intracellular CA and extracellular CA. They used demanding simultaneous measurements of intracellular and membrane-surface pH to measure CO2 transport in presence and absence of AQP5. This approach made it possible to determine the response in CO2 transport when changes and measurements were made on the same side of the membrane (e.g. intracellular CA-II and intracellular pH) named ‘cis measurements’ or on opposite sides of the membrane (e.g. CA-II and surface pH) named ‘trans measurements’. The results of the study demonstrated independent cooperation among intracellular and extracellular CA and AQP5, which does not involve direct physical interaction but augmented CO2 transport to more than an additive effect in what is termed ‘synergistic’ effect. These findings are novel and provide important insights into the factors that govern CO2 transport across cell membranes. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. No competing interests declared. N.L.N.: conception or design of the work; drafting the work or revising it critically for important intellectual content; final approval of the version to be published; agreement to be accountable for all aspects of the work. No funding.