Markovic Ivanka, Segal Malcolm, Djuricic Bogdan, Redzic Zoran
Department of Physiology, Faculty of Medicine, Kuwait University, Kuwait.
Exp Physiol. 2008 Mar;93(3):325-33. doi: 10.1113/expphysiol.2007.040980. Epub 2007 Nov 26.
Sheep choroid plexus epithelium expresses equilibrative nucleoside transporters (ENT) 1 and 2 and concentrative nucleoside transporter 2 at the transcript level. This study aimed to explore the kinetics and functional role of these transporters at the basolateral side of the sheep choroid plexus epithelium perfused in situ. The cellular uptake of [(3)H]adenosine and [(3)H]uridine was insensitive to 1 microm nitrobenzylthioinosine (NBTI), and the uptake of [(3)H]adenosine was reduced significantly when 10 microm NBTI was present in low-Na(+) Ringer solution. This might suggest that ENT2, a transporter sensitive to micromolar NBTI, is functionally active at the basolateral side of the choroid plexus epithelium while ENT1, a transporter sensitive to nanomolar NBTI, is not active. When low-Na(+) Ringer solution was used for the in situ perfusion, the Na(+) concentration in the venous effluent decreased to 14 mm; under these conditions the maximal uptake (U(max)) of [(3)H]adenosine and [(3)H]uridine did not change significantly when compared with the U(max) obtained when Ringer solution that contained 145 mm Na(+) was used. Kinetic analysis revealed apparent Michaelis-Menten constants (K(m,app)) for cellular uptake of [(3)H]adenosine, [(3)H]inosine and [(3)H]thymidine of 1.2 +/- 0.2, 15.7 +/- 2.6 and 3.8 +/- 0.9 microm, respectively. The HPLC and HPLC-fluorometric analysis of the sheep plasma and cerebrospinal fluid revealed nanomolar concentrations of adenosine and thymidine and micromolar levels of inosine and nucleobases. Considering the estimated K(m,app) values, it appears that under normal conditions inosine is the more important nucleoside substrate for uptake by the basolateral membrane of the choroid plexus epithelium than other nucleosides.
绵羊脉络丛上皮在转录水平表达平衡核苷转运体（ENT）1和2以及浓缩核苷转运体2。本研究旨在探讨这些转运体在原位灌注的绵羊脉络丛上皮基底外侧的动力学及功能作用。[(3)H]腺苷和[(3)H]尿苷的细胞摄取对1 μM的硝基苄硫肌苷（NBTI）不敏感，而当低钠林格液中存在10 μM NBTI时，[(3)H]腺苷的摄取显著降低。这可能表明，对微摩尔NBTI敏感的转运体ENT2在脉络丛上皮基底外侧具有功能活性，而对纳摩尔NBTI敏感的转运体ENT1无活性。当使用低钠林格液进行原位灌注时，静脉流出液中的Na(+)浓度降至14 mM；在这些条件下，[(3)H]腺苷和[(3)H]尿苷的最大摄取量（U(max)）与使用含145 mM Na(+)的林格液时获得的U(max)相比，没有显著变化。动力学分析显示，[(3)H]腺苷、[(3)H]肌苷和[(3)H]胸苷细胞摄取的表观米氏常数（K(m,app)）分别为1.2±0.2、15.7±2.6和%3.8±0.9 μM。对绵羊血浆和脑脊液的高效液相色谱（HPLC）及HPLC荧光分析显示，腺苷和胸苷的浓度为纳摩尔级，肌苷和核苷碱的水平为微摩尔级。考虑到估计的K(m,app)值，似乎在正常条件下，肌苷是脉络丛上皮基底外侧膜摄取的比其他核苷更重要的核苷底物。
