Caulfield J P, Farquhar M G
J Cell Biol. 1974 Dec;63(3):883-903. doi: 10.1083/jcb.63.3.883.
Graded dextrans have been used as tracers to identify the primary permeability barrier(s) to macromolecules among the structural elements (endothelium, mesangium, basement membrane, epithelium) of the glomerular capillary wall. Three narrow-range fractions of specified molecular weights and Einstein-Stokes radii (ESR) were prepared by gel filtration: (a) 32,000 mol wt, ESR = 38 A; (b) 62,000 mol wt, ESR = 55 A; and (c) 125,000 mol wt, ESR = 78 A. These fractions are known to be extensively filtered, filtered in only small amounts, and largely retained, respectively, by the glomerular capillaries. Tracer solutions were infused i.v. into Wistar-Furth rats, and the left kidney was fixed after 5 min to 4 h. The preparations behaved as predicted: initially, all three fractions appeared in the urinary spaces, with 32,000 > 62,000 >> 125,000. The smallest fraction was totally cleared from the blood and urinary spaces by 2.5 h, whereas the intermediate and largest fractions were retained in the circulation at high concentrations up to 4 h. With all fractions, when particles occurred in high concentration in the capillary lumina, they were present in similarly high concentrations in the endothelial fenestrae and inner (subendothelial) portions of the basement membrane, but there was a sharp drop in their concentration at this level-i.e., between the inner, looser portions of the basement membrane and its outer, more compact portions. With the two largest fractions, accumulation of particles occurred against the basement membrane in the mesangial regions with time. No accumulation was seen with any of the fractions in the epithelial slits or against the slit membranes. Dextran was also seen in phagosomes in mesangial cells, and in absorption droplets in the glomerular and proximal tubule epithelium. It is concluded that the basement membrane is the main glomerular permeability barrier to dextrans, and (since their behavior is known to be similar) to proteins of comparable dimensions (40,000-200,000 mol wt). The findings are discussed in relation to previous work using electron-opaque tracers to localize the glomerular permeability barrier and in relation to models proposed for the functions of the various glomerular structural elements.
分级葡聚糖已被用作示踪剂，以确定肾小球毛细血管壁结构成分（内皮细胞、系膜、基底膜、上皮细胞）中对大分子物质的主要通透屏障。通过凝胶过滤制备了三种特定分子量和爱因斯坦-斯托克斯半径（ESR）的窄范围级分：（a）分子量32,000，ESR = 38 Å；（b）分子量62,000，ESR = 55 Å；（c）分子量125,000，ESR = 78 Å。已知这些级分分别被肾小球毛细血管大量滤过、少量滤过和大量滞留。将示踪剂溶液静脉注射到Wistar-Furth大鼠体内，5分钟至4小时后固定左肾。制备物的表现符合预期：最初，所有三种级分都出现在尿腔中，32,000的级分> 62,000的级分>> 125,000的级分。最小的级分在2.5小时内从血液和尿腔中完全清除，而中间和最大的级分在长达4小时内以高浓度保留在循环中。对于所有级分，当颗粒在毛细血管腔中高浓度出现时，它们在内皮窗孔和基底膜的内侧（内皮下）部分也以类似的高浓度存在，但在这个水平——即在基底膜内侧较疏松部分与其外侧较致密部分之间，它们的浓度急剧下降。对于两种最大的级分，随着时间的推移，颗粒在系膜区域的基底膜处积聚。在任何级分中，在上皮裂隙或裂隙膜处均未观察到积聚。在系膜细胞的吞噬体以及肾小球和近端小管上皮细胞的吸收液滴中也可见到葡聚糖。得出的结论是，基底膜是肾小球对葡聚糖的主要通透屏障，并且（由于已知它们的行为相似）也是对尺寸相当的蛋白质（分子量40,
