Williams J C, Schafer J A
Department of Physiology, University of Alabama, Birmingham 35294.
Am J Physiol. 1988 Jun;254(6 Pt 2):F813-23. doi: 10.1152/ajprenal.1988.254.6.F813.
The possibility that significant concentration differences could exist between the interstitial fluid and capillary plasma was investigated by modeling the renal cortex as the following three compartments: tubular lumen, interstitium, and capillary lumen. A simple analysis of this system suggests that for the interstitium surrounding a proximal tubule, the concentration in the interstitium of a solute like glucose could be well over 1 mM greater than in the peritubular capillary if the solute permeability of the peritubular capillary were like that measured in other organs (i.e., less than 10 micron/s). The effect of varying capillary permeability on the interstitial concentrations of several solutes was examined using a modification of a model of the proximal tubule, and results were found to be similar to those obtained with the simpler analysis for glucose. This model was also used to see if placing an osmotic difference between the tubule lumen and capillary could cause significant solute polarization within the interstitium, as might occur in an experiment to measure the osmotic water permeability (Pf) of the proximal tubule in vivo. The results show that an apparent Pf calculated from the difference between the osmolalities of tubular perfusate and peritubular plasma is likely to underestimate the true Pf of the proximal tubule. Even for a high value of capillary permeability (10 micron/s, which allows relatively rapid diffusion between capillary and interstitium), the model predicts that the apparent Pf may underestimate the true value by half. Thus the analyses presented suggest that if the permeability of renal peritubular capillaries is similar to that measured in other organs the composition of the interstitium may be significantly different from capillary plasma, a situation that would have great impact on our view of the mechanism of volume absorption in the proximal tubule in vivo.
肾小管管腔、间质和毛细血管腔，研究了间质液和毛细血管血浆之间可能存在显著浓度差异的可能性。对该系统的简单分析表明，对于近端小管周围的间质，如果肾小管周围毛细血管的溶质渗透率与在其他器官中测得的相似（即小于10微米/秒），则间质中葡萄糖等溶质的浓度可能比肾小管周围毛细血管中的浓度高1 mM以上。使用近端小管模型的一种改进方法，研究了改变毛细血管渗透率对几种溶质间质浓度的影响，结果发现与对葡萄糖进行的更简单分析所得到的结果相似。该模型还用于研究在小管腔和毛细血管之间设置渗透差异是否会导致间质内显著的溶质极化，这可能发生在体内测量近端小管渗透水渗透率（Pf）的实验中。结果表明，根据肾小管灌注液和肾小管周围血浆的渗透压差异计算出的表观Pf可能会低估近端小管的真实Pf。即使对于较高的毛细血管渗透率值（10微米/秒，这允许毛细血管和间质之间相对快速的扩散），该模型预测表观Pf可能会低估真实值的一半。因此，所提出的分析表明，如果肾肾小管周围毛细血管的渗透率与在其他器官中测得的相似，间质的组成可能与毛细血管血浆有显著差异，这种情况将对我们对体内近端小管容积吸收机制的看法产生重大影响。
