Moeckel G W, Lai L W, Guder W G, Kwon H M, Lien Y H
Department of Medicine, University of Arizona Health Sciences Center, Tucson 85724, USA.
Am J Physiol. 1997 Jan;272(1 Pt 2):F100-6. doi: 10.1152/ajprenal.1997.272.1.F100.
Betaine is one of the major organic osmolytes that accumulate in the renal medulla in response to high extracellular tonicity. Recent studies in MDCK cells have shown that betaine is taken up by an Na(+)- and Cl(-)-dependent transporter located on the basolateral membrane. We demonstrate here the presence of Na(+)-Cl(-)-dependent betaine transporter(s) in tubule suspensions prepared from the rat outer and inner medulla. The betaine transport activity was two to three times higher in the inner medulla compared with the outer medulla. The removal of Na+ and Cl- reduced betaine uptake in the outer medullary tubules by 86% and 82%, respectively. The betaine uptake was decreased by 39% in hypotonic buffer (189 mosmol/ kgH2O) and increased by 82% in hypertonic buffer (545 mosmol/kgH2O), compared with isotonic buffer (308 mosmol/ kgH2O). Kinetic studies of Na(+)-dependent betaine uptake in the outer medullary tubules revealed both a low- and a high-affinity component as follows: low-affinity and high volume component with Michaelis constant (K(m)1) of 8.6 mM and maximal uptake rate (Vmax1) of 112 pmol.microgram protein-1.h-1; and a low-volume and high-affinity component with K(m)2 of 0.141 mM and Vmax2 of 10 pmol. microgram protein-1.h-1. To investigate whether the Na(+)-Cl(-)-dependent betaine transporter is regulated by tonicity in vivo, we quantitated its mRNA in rat renal cortex and outer and inner medulla using both canine and rat Na(+)-Cl(-)-dependent betaine transporter cDNA probes. A single band of 3.0 kb was seen in the Northern blots prepared from both outer and inner medulla, but not in the cortex. Water deprivation for 3 days increased the abundance of this mRNA in the outer and inner medulla by 140% and 170%, respectively, but did not affect its expression in the cortex. In conclusion, Na(+)-Cl(-)-dependent betaine transporter(s) is present in rat outer and inner medullary tubules, and betaine transporter mRNA abundance is regulated by the hydration state in vivo.
甜菜碱是主要的有机渗透溶质之一，在细胞外高渗状态下会在肾髓质中蓄积。最近对MDCK细胞的研究表明，甜菜碱通过位于基底外侧膜上的一种依赖Na⁺和Cl⁻的转运体被摄取。我们在此证明，在从大鼠外髓和内髓制备的肾小管悬液中存在依赖Na⁺-Cl⁻的甜菜碱转运体。内髓中的甜菜碱转运活性比外髓高两到三倍。去除Na⁺和Cl⁻分别使外髓肾小管中的甜菜碱摄取减少86%和82%。与等渗缓冲液（308 mosmol/kgH₂O）相比，在低渗缓冲液（189 mosmol/kgH₂O）中甜菜碱摄取减少39%，在高渗缓冲液（545 mosmol/kgH₂O）中增加82%。对外髓肾小管中依赖Na⁺的甜菜碱摄取的动力学研究揭示了一个低亲和力和一个高亲和力成分，如下：低亲和力和高容量成分，米氏常数（Kₘ1）为8.6 mM，最大摄取速率（Vₘₐₓ1）为112 pmol·μg蛋白⁻¹·h⁻¹；以及低容量和高亲和力成分，Kₘ2为0.141 mM，Vₘₐₓ2为10 pmol·μg蛋白⁻¹·h⁻¹。为了研究依赖Na⁺-Cl⁻的甜菜碱转运体在体内是否受渗透压调节，我们使用犬和大鼠依赖Na⁺-Cl⁻的甜菜碱转运体cDNA探针，对大鼠肾皮质、外髓和内髓中的其mRNA进行定量。在外髓和内髓制备的Northern印迹中可见一条3.0 kb的单带，但在皮质中未见到。3天禁水使外髓和内髓中该mRNA的丰度分别增加140%和170%，但不影响其在皮质中的表达。总之，依赖Na⁺-Cl⁻的甜菜碱转运体存在于大鼠外髓和内髓肾小管中，且甜菜碱转运体mRNA丰度在体内受水合状态调节。
