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 an d their interactions were examined in vitro by measuring unidirectiona l influx across the brush-border membrane of intact rat jejunal and ra bbit ileal epithelia. The chloride-dependent and beta-alanine inhibita ble B-0,B-+ present in rabbit ileum was blocked by combining inhibitio n by beta-alanine with Na+- or Cl--free conditions. Under these condit ions, lysine influx across the brush-border membrane is Na+ independen t. All Na+-independent influx of cationic and bipolar amino acids is b y a system b(0,+) equivalent in the brush-border membrane of both spec ies, where a system y(+) is not present. System b(0,+) is shown to be a potent exchanger of intracellular leucine for extracellular lysine a nd of intracellular lysine for extracellular leucine. The model used t o explain leucine stimulation of mucosa to serosa lysine transport can explain Na+ dependence of net lysine absorption. On the assumption th at b(0,+) in situ, like the transporter induced by retroperitoneal bro wn adipose tissue in Xenopus laevi oocytes, acts as an obligatory exch anger, this model can also explain the effects of lysine on short-circ uit current and net transport of sodium and the effect on transport ca pacity by preincubation at Na+-free conditions.