Dantzler W H, Silbernagl S
Department of Physiology, College of Medicine, University of Arizona, Tucson 85724.
Am J Physiol. 1991 Sep;261(3 Pt 2):F495-504. doi: 10.1152/ajprenal.1991.261.3.F495.
Amino acids can be reabsorbed distal to tips of Henle's loops and may be recycled between loops and vasa recta in rat papilla. Transport specificity was examined during continuous microinfusions of ascending Henle's loops and vasa recta with radiolabeled amino acids. Percent of recovered radiolabel as intact amino acid was also determined. Previous data indicated that, relative to simultaneously microinfused inulin, 30-40% of radiolabeled L- and D-Ala, L-Glu, L-Glu(NH2), and Gly, but no taurine (Tau) or mannitol, microinfused into Henle's loops was reabsorbed. In the present study, reabsorption was shown to involve intact L- and D-Ala, D-Glu, and L-Ser. L-Phe (50 mM) in infusate had no effect on reabsorption of L-Ala (2.5 mM) or L-Glu(NH2) (42.6 microM), and D-Asp (50 mM) had no effect on reabsorption of L-Glu (1.5 mM). Thus reabsorption from Henle's loops is not stereospecific, not different for neutral and acidic amino acids, and not inhibited by competitive inhibitors of proximal tubule amino acid transport, but it was not completely nonspecific and not a simple leak. Previous vasa recta microinfusions suggested that Ala could move directly from vasa recta to tubules. These studies were extended with simultaneous collections from ipsilateral and contralateral kidneys. Relative to simultaneously microinfused inulin, 40-50% of radiolabeled L- and D-Ala, L-Glu, and L-Glu(NH2) and 30% of L-Ser microinfused into ascending vasa recta appeared intact in urine from ipsilateral kidney, whereas only 1-3% appeared in urine from contralateral kidney. Fifty percent of infused D-Glu was excreted intact by each kidney; 70% of infused Tau was excreted intact by ipsilateral kidney, and 22% was excreted by contralateral kidney. L-Phe (50 mM) in infusate inhibited appearance of L-Ala (2.5 mM) and D-Ala (10 mM) but not L-Glu(NH2) (42.6 microM) in ipsilateral urine. D-Asp (50 mM) inhibited appearance of L-Glu (1.5 mM), and beta-Ala (50 mM) inhibited appearance of Tau (78 microM) in ipsilateral urine. Thus some amino acids can move directly from vasa recta into tubules (probably descending thin limbs of Henle's loops) by a process showing significant specificity.
氨基酸可在髓袢顶端远侧段被重吸收，并且可能在大鼠乳头的髓袢和直小血管之间循环利用。在用放射性标记氨基酸持续微量灌注升支髓袢和直小血管的过程中，对转运特异性进行了检测。还测定了回收的放射性标记物中完整氨基酸的百分比。先前的数据表明，相对于同时微量灌注的菊粉，微量灌注到髓袢中的放射性标记的L-和D-丙氨酸、L-谷氨酸、L-谷氨酰胺和甘氨酸中有30%-40%被重吸收，但微量灌注的牛磺酸或甘露醇没有被重吸收。在本研究中，显示重吸收涉及完整的L-和D-丙氨酸、D-谷氨酸和L-丝氨酸。灌注液中50 mM的L-苯丙氨酸对2.5 mM的L-丙氨酸或42.6 μM的L-谷氨酰胺的重吸收没有影响，50 mM的D-天冬氨酸对1.5 mM的L-谷氨酸的重吸收没有影响。因此，从髓袢的重吸收不是立体特异性的，对中性和酸性氨基酸没有差异，也不受近端小管氨基酸转运竞争性抑制剂的抑制，但它并非完全非特异性，也不是简单的渗漏。先前对直小血管的微量灌注提示丙氨酸可直接从直小血管进入肾小管。这些研究通过同时收集同侧和对侧肾脏的样本得以扩展。相对于同时微量灌注的菊粉，微量灌注到升支直小血管中的放射性标记的L-和D-丙氨酸、L-谷氨酸和L-谷氨酰胺中有40%-50%以及L-丝氨酸中有30%完整地出现在同侧肾脏的尿液中，而只有1%-3%出现在对侧肾脏的尿液中。灌注的D-谷氨酸中有50%由每个肾脏完整排泄；灌注的牛磺酸中有70%由同侧肾脏完整排泄，22%由对侧肾脏排泄。灌注液中50 mM的L-苯丙氨酸抑制了同侧尿液中2.5 mM的L-丙氨酸和10 mM的D-丙氨酸的出现，但不抑制42.6 μM的L-谷氨酰胺的出现。50 mM的D-天冬氨酸抑制了同侧尿液中1.5 mM的L-谷氨酸的出现，50 mM的β-丙氨酸抑制了同侧尿液中78 μM的牛磺酸的出现。因此，一些氨基酸可通过一个显示出显著特异性的过程直接从直小血管进入肾小管（可能是髓袢降支细段）。
