Members of the SLC4A family couple the transport of bicarbonate (HCO3-) with sodium ions (Na+); they being members 4, 5, 7 and 9. SLC4A5 encodes a protein which is expressed in liver, spleen and testes, with lower levels expressed in parts of the brain and kidney (Sassani et al. 2002). It may have a housekeeping function in regulating the pH of these tissues (Pushkin et al. 2000).
SLC4A7 (aka NBC3, NBCn1) encodes a protein which performs electroneutral cotransport of Na+ and HCO3- with a 1:1 stoichiometry. It is highly expressed in testes and spleen and, to a lesser extent, in many other tissues including heart, muscle, kidney and GI tract (Pushkin et al. 1999).
SLC4A9 (aka AE4) was originally thought to exchange Cl- with HCO3- (hence the name AE4) but this has not been reported. Consensus has emerged that it is indeed a Na+/HCO3- co-transporter (Lipovich et al. 2001). It is predominantly expressed in the kidney, salivary glands, testes, thyroid glands and trachea (Parker et al. 2001).