Lewis S E, Goldspink D F
Biochem J. 1984 Jul 1;221(1):71-9. doi: 10.1042/bj2210071.
alpha-Aminoisobutyric acid (AIB) transport into the intracellular compartment of extensor digitorum longus and soleus muscles was measured (in vitro) after allowance for the equilibration of the amino acid in the extracellular space. The latter was determined with three markers, [14C]inulin, 60Co-EDTA and [3H]mannitol. Net transport of AIB was subsequently divided into its two components, i.e. influx and efflux. Rates of influx were measured as the intracellular accumulation of [14C]AIB after a short incubation (5 min), and efflux was measured as the release of AIB with time (up to maximum of 50 min) from muscles that had previously been preloaded with AIB. This intracellular efflux was resolved into two phases, which probably represent two separate components of exit. The influence of extracellular Na+ on the transport of this neutral amino acid (representing the A system) was investigated. Na+ depletion resulted in lower accumulations of AIB, the effects becoming more pronounced with progressive depletions of external Na+. These changes arose from an inhibition of AIB influx, concomitant with an enhancement of its efflux. In contrast, all components of tyrosine transport (representing the L system) were unaffected by lowering external Na+ concentrations. The net accumulation of AIB was also suppressed by cortisol. This inhibitory effect was, however, Na+-dependent and resulted solely from the steroid's enhancement of AIB efflux, the hormone being without effect on AIB influx.
在考虑氨基酸在细胞外空间达到平衡后，（在体外）测定了α-氨基异丁酸（AIB）进入趾长伸肌和比目鱼肌细胞内区室的转运情况。后者通过三种标志物[¹⁴C]菊粉、⁶⁰Co-乙二胺四乙酸和[³H]甘露醇来测定。随后，AIB的净转运被分为其两个组成部分，即流入和流出。流入速率通过短时间孵育（5分钟）后[¹⁴C]AIB在细胞内的积累来测定，流出则通过预先用AIB加载的肌肉随时间（最长50分钟）释放AIB来测定。这种细胞内流出可分为两个阶段，这可能代表了两种不同的流出成分。研究了细胞外Na⁺对这种中性氨基酸转运（代表A系统）的影响。Na⁺耗竭导致AIB的积累减少，随着细胞外Na⁺的逐渐耗竭，这种影响变得更加明显。这些变化是由于AIB流入受到抑制，同时其流出增强所致。相比之下，酪氨酸转运的所有成分（代表L系统）不受细胞外Na⁺浓度降低的影响。皮质醇也抑制了AIB的净积累。然而，这种抑制作用依赖于Na⁺，并且完全是由于该类固醇增强了AIB的流出，该激素对AIB的流入没有影响。
