AJP - Regulatory, Integrative and Comparative Physiology, Vol 272, Issue 4 1060-R1068, Copyright © 1997 by American Physiological Society
Na+-independent transport of bipolar and cationic amino acids across the luminal membrane of the small intestine
B. G. Munck and L. K. Munck
Department of Medical Physiology, The Panum Institute, University of Copenhagen, Denmark.
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 beta-alanine inhibitable B(0,+)
present in rabbit ileum was blocked by combining inhibition by beta-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.
