Crone C
J Physiol. 1984 Aug;353:317-37. doi: 10.1113/jphysiol.1984.sp015338.
Selectivity to passive permeation of small ions through capillary walls was studied by measurements of diffusion potentials in response to ionic gradients established across capillary walls in frog brain and muscle in superfusion and perfusion experiments. Average dilution potentials in response to 2:1 or 10:1 gradients of NaCl across brain capillaries were 4.2 and 10.2 mV, respectively. The 'diluted' side was negative with respect to the 'undiluted' side, reflecting higher mobility of Cl- than of Na+ ions. Bi-ionic potentials in response to isosmotic KCl:NaCl gradients averaged 5.0 mV in brain capillaries, negative on the KCl side, reflecting higher mobility of K+ than of Na+ ions. The potential variations were symmetrical across the capillary wall. From Planck-Henderson formalism, the relative permeabilities in brain capillaries of Na+, K+ and Cl- were PCl/PNa:1.54 and PK/PNa:1.56, rather close to mobility ratios in free solution. Experiments on muscle capillaries showed similar results to those in brain. Streaming potentials created with excess (200 mosmol) mannitol or sucrose were congruent to 1 mV in brain capillaries and zero in muscle capillaries. It is concluded that the transcapillary permeation pathway in muscle and brain is neutral or weakly charged. The dominant ion permeation is paracellular in all 'continuous' capillaries. The large range of ion permeability of 'continuous' capillaries may be explained by variation in the length of the effective open portion of interendothelial junctions. The very low passive permeability of the blood-brain barrier may be due to an almost closed endothelial junction, leaving only about 0.1% of the length open. The open fraction is functionally similar to that in muscle and mesentery. This interpretation is in accordance with a finite, but very low, permeability to hydrophilic non-electrolytes. Such a brain capillary would still display strong preference for lipid-soluble solutes, but its behaviour cannot be satisfactorily understood from a simple analogy to a cellular plasma membrane.
通过在灌流和灌注实验中测量蛙脑和肌肉毛细血管壁两侧离子梯度所产生的扩散电位，研究了小离子通过毛细血管壁的被动渗透选择性。在脑毛细血管中，针对2:1或10:1的NaCl梯度产生的平均稀释电位分别为4.2 mV和10.2 mV。“稀释”侧相对于“未稀释”侧呈负电位，这反映了Cl-比Na+离子具有更高的迁移率。在脑毛细血管中，针对等渗KCl:NaCl梯度产生的双离子电位平均为5.0 mV，KCl侧为负电位，这反映了K+比Na+离子具有更高的迁移率。电位变化在毛细血管壁两侧是对称的。根据普朗克 - 亨德森公式，脑毛细血管中Na+、K+和Cl-的相对渗透率为PCl/PNa:1.54和PK/PNa:1.56，与自由溶液中的迁移率比值相当接近。对肌肉毛细血管的实验结果与脑毛细血管的相似。用过量（200 mosmol）甘露醇或蔗糖产生的流动电位在脑毛细血管中相当于1 mV，在肌肉毛细血管中为零。结论是，肌肉和脑内的跨毛细血管渗透途径呈中性或带弱电荷。在所有“连续”毛细血管中，主要的离子渗透是通过细胞旁途径。“连续”毛细血管的离子渗透率范围较大，这可能是由于内皮细胞间连接有效开放部分的长度变化所致。血脑屏障极低的被动渗透率可能是由于内皮连接几乎完全封闭，仅约0.1%的长度开放。开放部分在功能上与肌肉和肠系膜中的相似。这种解释与对亲水性非电解质有限但极低的渗透率一致。这样的脑毛细血管对脂溶性溶质仍会表现出强烈偏好，但其行为无法通过简单类比细胞膜来令人满意地理解。
