What transport method is used by the respiratory gases to cross cell membranes?

CAN GASES CROSS BIOLOGICAL MEMBRANES THROUGH CHANNELS?

One of the bedrock beliefs of membrane transport physiology is that gas molecules cross membranes by diffusing freely through the lipid bilayer. Here Gordon Cooper argues that this view is in need of a reappraisal.

Features

Gordon Cooper
Department of Biomedical Sciences, University of Sheffield

https://doi.org/10.36866/pn.43..14

It is commonly believed that gases diffuse freely through the lipid phase of cell membranes. However, in the last several years, a number of experimental observations have challenged this dogma, as summarised below. It is now becoming clear, not only that membranes can have an extremely low gas permeability, but that in some cases “gas channels” appear to augment membrane gas permeability.

Membranes with low gas permeability

Although hints were available earlier, the conclusive evidence that membranes might have extremely low permeabilities to gases came from Walter Boron’s laboratory at Yale in 1994 (Waisbren et al., 1994). The chief and parietal cells of the gastric gland are presented with a very hostile environment, namely a pH as low as 0_7 and the presence of the protease pepsin. The apical membrane of these gland cells provides the barrier layer, which prevents the cells from digesting themselves. Boron’s group studied single micro-dissected gastric glands using the microperfusion techniques first described for renal tubules (Burg et al, l 966). When the peritubular (blood-facing) side of the glands were exposed to a solution containing 1% CO2/5mM HCO3 there was the expected decrease in intracellular pH (pH)* as CO2 entered the cell and formed H+ and HCO3, indicating that the basolateral membrane of the cells is CO2 -permeable. However, when the gland lumen was exposed to a solution containing as much as 100% CO2/22 mM HCO3 there was no change in pH,. Moreover, exposing the bath to 20 mM NH3/NH4+at pH 7.4 caused a significant pH, increase, as NH3 enters the cell across the basolateral membrane and combines with H+ to give NH!, whereas exposing the lumen to a 25 times greater concentration of NH, elicited no change in pH,_ Thus, the apical membranes of chief and parietal cells have no demonstrable permeability to CO2 or HCO3, nor to NH3 or NH+4 (Waisbren et al, 1994).

*The changes in pH; associated with the movement of NH3 and C02 are reviewed by Boron (1992).

Contribution of Channels to Membrane Gas Permeability

It is clear that specialised cells can construct membranes impermeable to gases. A question that then arises is whether cells can use channels to increase gas permeability. Three recent studies indicate that the movement of gases through channels can contribute to membrane gas permeability.

The water channel aquaporin-1 is permeable to CO2

Cooper and Boron (1998) examined the effect of expressing the aquaporin-1 water channel protein AQPl on the CO2 permeability of oocytes. Figure 1 shows experimental traces from three oocytes expressing different amounts of AQP l. The rate of CO2 -induced acidification is an index of CO2 permeability – the greater the acidification rate, the higher the CO2 permeability. The level of AQPl expression was judged from the time taken for the oocytes to Jyse when exposed to de­ionised water. Under these conditions the oocyte gains water osmotically, swelling and eventually lysing. Expression of AQP I introduces a pathway for this osmotic water movement, so the oocytes lyse faster the more AQP I they express. Figure I shows that an increase in AQP I expression is paralleled by an increase in CO2 permeability. These results are consistent with AQP I being permeable to CO2. However, they cannot exclude other possibilities, particularly that: (i) the oocyte contains a native gas channel that is induced by AQP 1; and/or (ii) that over-expression of AQPl alters the membrane lipid composition, making the membrane leakier and thus more permeable to CO2.

To distinguish these three options, we examined the effect of the mercurial agent pCMBS on the AQPl-dependent increase in CO2 permeability. Mercurial compounds such as HgCl2 or pCMBS prevent the movement of H2O through AQP I (Preston et al, 1992) by interacting with a critical cysteine residue (C 189). When C 189 is mutated to a serine (the Cl89S mutant), the movement of H2O through the resulting channel is no longer prevented by mercurials (Preston et al, 1993). In a similar way, pCMBS inhibited the AQPl­dependent increase in CO2 permeability (Figure 2A). As pCMBS does not interact with membrane lipids, the third option – that AQP I increases CO2 permeability by altering lipid composition – can be discounted. In order to address the second possibility, that AQP I induces a native gas channel, we used the C 189S mutant of AQPI. Expressing C 189S increased oocyte CO2 permeability to the same degree as expressing wild type AQPI. However, the C 189S-dependent increase in CO2 permeability was no longer inhibited by pCMBS (summarised in Figure 2B). Thus, as well as being permeable to H2O, the AQPI channel can act as a conduit for the entry of CO2.

The CO2 permeability of red blood cells is inhibited by DIDS

The red blood cell membrane has an extremely high permeability to CO2, a property that goes hand in hand with the major role of the red cells (i.e., the transport of respiratory gases in the blood). The stilbene derivative DIDS, a compound commonly used as an inhibitor of HCO3 transporters, inhibits the CO2 permeability of red blood cells by about 90% (Forster et al, 1998). However the inhibitory action of DIDS on red blood cell CO2 permeability does not involve an interaction with Band 3 (i.e., the red-cell Cl”­HCO3- exchanger) or with intracellular carbonic anhydrase. Moreover, O1OS does not interfere with membrane lipid composition. Forster et al. thus concluded that the transport of CO2 into the red blood cell involves a protein-mediated pathway. The water channel AQP I is abundant in the red blood cell, and it is intriguing to speculate that this protein may account for pait of the protein-mediated transport of CO2 in these cells.

The NH3 permeability or the peribacterial membrane is protein-mediated

In legumes, biological nitrogen fixation (i.e., N2 + 3H2 –+ 2 NH3) occurs in specialised root nodules that play host to the bacterium rhizobia. The rhizobia are enclosed in a compartment bounded by the peribacteroid membrane. The symbiotic arrangement found in the root nodules of these plants is of particular interest with respect to gas transport. High levels of 02 inhibit the nitrogenase enzyme in rhizobia, which is responsible for nitrogen fixation. For efficient nitrogen fixation, the peribacteroid membrane needs to limit 02 influx whilst maintaining a high permeability to N2 and NH3. Recently it was demonstrated that about half of the movement of NH3 across the peribacteroid membrane is temperature-dependent and can be inhibited by pCMBS (Niemietz & Tyerman, 2000). Both of these properties are characteristics of a protein­mediated process. The authors suggested that a plant water channel called nodulin-26, found at high levels in the peribacteroid membrane, might account for the protein-mediated component of NH3 transport.

Conclusion

The studies discussed above illustrate how our commonly held ideas about the movement of gases across membranes need to be re-evaluated. On the one hand, as a matter of survival, cells can make membranes impermeable to gases. On the other hand, movement of gases across membranes can be protein-mediated. It is interesting to speculate that this protein-mediated gas transport may play an important role in biological functions, such as nitrogen fixation or renal bicarbonate reabsorption.

Reference

Boron, W. F. (1992). Control of intracellular pH. In The Kidney: Physiology and Pathophysiology, eds. Seldin, D. W. & Giebisch, G., pp. 219-263. Raven Press Ltd, New York.

Burg, M., Grantham, J., Abramow, M., & Orloff, J. (1966). Preparation and study of fragments of single rabbit nephrons. Am.J.Physiol. 210, 1293-I 298.

Cooper, G. J. & Boron, W. F. (1998). Effect of pCMBS on the CO2 permeability of Xenopus oocytes expressing aquaporin I or its C.I 89S mutant. AmJPhysiol. 215, Cl48 I-CJ486.

Forster, R. E., Gros, G., Lin, L., Ono, Y., & Wunder, M. (1998). The effect of 4,4′ -diisothiocyanato­stilbene-2,2′ -disulfonate on CO2 permeability of the red blood cell membrane. Proc.Natl.Acad.Sci.USA 95, 15815-15820.

Niemietz, C. M. & Tyerman, S. D. (2000). Channel­mediated permeation of ammonia gas through the peribacteroid membrane of soybean nodules. FEBS Lett. 465, 110-114.

Preston, G. M., Carroll, T. P., Guggino, W. B., & Agre, P. ( 1992). Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein. Science 256, 385-387.

Preston, G. M., Jung, J. S., Guggino, W. B., & Agre, P. (1993). The mercury-sensitive residue at cysteine 189 in the CHIP28 water channel. }.Biol.Chem. 268, 17-20.

Waisbren, S. J., Geibel, J., Modlin, I. M., & Boron, W. F. (1994). Unusual permeability properties of gastric gland cells. Nature 368, 332-335.

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What are the types and method of transport across the cell membrane?