Bicarbonate transfer across cell membranes

Remember that biological membranes contain a great number of different types of proteins, which typically account for about 60% of their total mass. There are about 20 prominent proteins found in erythrocyte membranes. The chloride-bicarbonate anion exchanger accounts for about 30% of the total protein content of the red blood cell membrane. As carbonic anhydrase converts C02 to the more water-soluble form of HCOj", this membrane protein will transfer the ion across the erythrocyte membrane. At the same time, chloride ions are transported in the opposite direction to prevent the... 

These movements of hydrogen and bicarbonate ions in opposite directions across the cell membrane of the tubular cells are of necessity accompanied by the movement of other charged particles to ensure that the net transfer of charge is zero. The principal contribution to provide this balance is by sodium ions, which move from tubular fluid to interstitial fluid as shown in Figure 1.4. 

The intracellular pH is about 0.2 units more acid than the extracellular pH. This corresponds to an equilibrium potential for H across the cell membrane of about — 20 mV. Bicarbonate and hydroxide ions also have equilibrium potentials of this value. The membrane potential is typically much more negative than this. Although these ions do not permeate the cell membrane readily, they are able to cross slowly and such passive movement results in a net transfer (H inwards, OH " and HCO3 outwards). There must therefore be active transfer across the cell membrane of one or more of these ions in the opposite direction in order to maintain the observed concentration gradients. 

For each hydrogen ion excreted, one bicarbonate ion is added to the intracellular fluid of the renal tubular cell. The intracellular concentration of bicarbonate rises. In Figure 7.1 B, the reaction of CO2 is omitted and only the products Fl and HCO, are reproduced from Figure 7.1 A. Bicarbonate, being charged, is insoluble in lipid and so diffuses extremely slowly across the lipid regions of the cell membrane. On the aspect of the tubular cell membrane facing the interstitial fluid, the lipid membrane is traversed by protein macromolecules which comprise a carrier mechanism for bicarbonate ions (section A.l). The rise in intracellular concentration of bicarbonate leads to transfer of bicarbonate from the renal tubular cell cytoplasm to the renal interstitial fluid. 

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