Munck B G, Munck L K
Department of Medical Physiology, The Panum Institute, University of Copenhagen, Denmark.
Am J Physiol. 1997 Apr;272(4 Pt 2):R1060-8. doi: 10.1152/ajpregu.1997.272.4.R1060.
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.
通过测量完整大鼠空肠和兔回肠上皮刷状缘膜的单向流入，在体外研究了钠在双极性和阳离子氨基酸转运中的作用及其相互作用。兔回肠中存在的依赖氯离子且可被β-丙氨酸抑制的B(0,+)，通过将β-丙氨酸抑制与无钠或无氯条件相结合而被阻断。在这些条件下，赖氨酸穿过刷状缘膜的流入不依赖于钠。阳离子和双极性氨基酸的所有不依赖钠的流入都是通过两种动物刷状缘膜中相当于系统b(0,+)的机制进行的，其中不存在系统y+。系统b(0,+)被证明是细胞内亮氨酸与细胞外赖氨酸以及细胞内赖氨酸与细胞外亮氨酸的有效交换体。用于解释亮氨酸刺激黏膜到浆膜赖氨酸转运的模型可以解释赖氨酸净吸收对钠的依赖性。假设原位的b(0,+)，就像非洲爪蟾卵母细胞中腹膜后棕色脂肪组织诱导的转运体一样，作为一种强制性交换体，该模型还可以解释赖氨酸对短路电流以及钠的净转运的影响，以及在无钠条件下预孵育对转运能力的影响。
