Na⁺-independent transport of bipolar and cationic amino acids across the luminal membrane of the small intestine

The role of sodium in transport of bipolar and cationic amino acids and their interactions were examined in vitro by measuring unidirectional influx across the brush-border membrane of intact rat jejunal and rabbit ileal epithelia. The chloride-dependent and β-alanine inhibitable B^0,+ present in rabbit ileum was blocked by combining inhibition by β-alanine with Na⁺- or Cl^--free conditions. Under these conditions, lysine influx across the brush-border membrane is Na⁺ independent. All Na⁺-independent influx of cationic and bipolar amino acids is by a system b^0,+ equivalent in the brush-border membrane of both species, where a system y⁺ is not present. System b^0,+ is shown to be a potent exchanger of intracellular leucine for extracellular lysine and of intracellular lysine for extracellular leucine. The model used to explain leucine stimulation of mucosa to serosa lysine transport can explain Na⁺ dependence of net lysine absorption. On the assumption that b^0,+ in situ, like the transporter induced by retroperitoneal brown adipose tissue in Xenopus laevi oocytes, acts as an obligatory exchanger, this model can also explain the effects of lysine on short-circuit current and net transport of sodium and the effect on transport capacity by preincubation at Na⁺-free conditions.