Adamson R H, Lenz J F, Zhang X, Adamson G N, Weinbaum S, Curry F E
Department of Human Physiology, University of California, Davis, CA 95616, USA.
J Physiol. 2004 Jun 15;557(Pt 3):889-907. doi: 10.1113/jphysiol.2003.058255. Epub 2004 Apr 8.
We hypothesized that ultrafiltrate crossing the luminal endothelial glycocalyx through infrequent discontinuities (gaps) in the tight junction (TJ) strand of endothelial clefts reduces albumin diffusive flux from tissue into the 'protected region' of the cleft on the luminal side of the TJ. Thus, the effective oncotic pressure difference (sigma black triangle down pi) opposing filtration is greater than that measured between lumen and interstitial fluid. To test this we measured sigma black triangle down pi across rat mesenteric microvessels perfused with albumin (50 mg ml(-1)) with and without interstitial albumin at the same concentration within a few micrometres of the endothelium as demonstrated by confocal microscopy. We found sigma black triangle down pi was near 70% of luminal oncotic pressure when the tissue concentration equalled that in the lumen. We determined size and frequency of TJ strand gaps in endothelial clefts using serial section electron microscopy. We found nine gaps in the reconstructed clefts having mean spacing of 3.59 microm and mean length of 315 nm. The mean depth of the TJ strand near gaps was 67 nm and the mean cleft path length from lumen to interstitium was 411 nm. With these parameters our three-dimensional hydrodynamic model confirmed that fluid velocity was high at gaps in the TJ strand so that even at relatively low hydraulic pressures the albumin concentration on the tissue side of the glycocalyx was significantly lower than in the interstitium. The results conform to the hypothesis that colloid osmotic forces opposing filtration across non-fenestrated continuous capillaries are developed across the endothelial glycocalyx and that the oncotic pressure of interstitial fluid does not directly determine fluid balance across microvascular endothelium.
我们推测，通过内皮裂隙紧密连接（TJ）链中不常见的间断（间隙）穿过管腔内皮糖萼的超滤液会降低白蛋白从组织向TJ管腔侧裂隙“保护区”的扩散通量。因此，对抗滤过的有效胶体渗透压差值（σΔπ）大于管腔与组织间液之间测得的差值。为了验证这一点，我们通过共聚焦显微镜观察，测量了在大鼠肠系膜微血管中灌注白蛋白（50 mg ml⁻¹）时，在内皮几微米范围内有无相同浓度组织间白蛋白情况下的σΔπ。我们发现，当组织浓度与管腔浓度相等时，σΔπ接近管腔胶体渗透压的70%。我们使用连续切片电子显微镜确定了内皮裂隙中TJ链间隙的大小和频率。我们在重建的裂隙中发现了9个间隙，平均间距为3.59微米，平均长度为315纳米。间隙附近TJ链的平均深度为67纳米，从管腔到组织间的裂隙平均路径长度为411纳米。利用这些参数，我们的三维流体动力学模型证实，TJ链间隙处的流体速度很高，因此即使在相对较低的液压下，糖萼组织侧的白蛋白浓度也显著低于组织间液。这些结果符合以下假设：在无窗连续毛细血管上对抗滤过的胶体渗透力是在内皮糖萼上产生的，并且组织间液的胶体渗透压并不直接决定微血管内皮的液体平衡。
