Pardridge W M, Mietus L J
J Clin Invest. 1979 Jul;64(1):145-54. doi: 10.1172/JCI109433.
These studies were undertaken to investigate (a) the permeability properties of the blood-brain barrier (BBB) to the major gonadal and adrenal steroid hormones, and (b) the role of the binding proteins of plasma (albumin and specific globulins) in the regulation of BBB steroid hormone transport. The permeability of the BBB to [(3)H]-labeled progesterone, testosterone, estradiol, corticosterone, aldosterone, and cortisol, was measured relative to [(14)C]butanol, a freely diffusable reference, in the barbiturate anesthetized rat using a tissue sampling-single injection technique. The isotopes were rapidly injected in a 200-mul bolus of Ringer's solution (0.1 g/dl albumin) via the common carotid artery and the percent extraction of unidirectional influx of hormone was determined after a single pass through brain: progesterone, 83+/-4%; testosterone, 85+/-1%; estradiol, 83+/-3%; corticosterone, 39+/-2%; aldosterone, 3.5+/-0.8%; and cortisol, 1.4+/-0.3%. The selective permeability of the BBB was inversely related to the number of hydrogen bonds each steroid formed in aqueous solution and directly related to the respective 1-octanol/Ringer's partition coefficient. When the bolus injection was 67% human serum, >95% of the labeled steroid was bound as determined by equilibrium dialysis. However, the influx of the steroids through the BBB was inhibited by human serum to a much less extent than would be expected if only the free (dialyzable) hormone was transported; progesterone, estradiol, testosterone, and corticosterone transport was inhibited 18, 47, 70, and 85% respectively, or in proportion to the steroid binding to plasma globulins. Rat serum (67%) only inhibited the transport of these four hormones, 0, 13, 12, and 69%, respectively, reflecting the absence of a sex hormone-binding globulin in rat plasma. However, neonatal rat serum (67%) inhibited progesterone, testosterone, and estradiol transport 0, 0, and 91%, respectively, consistent with the presence of an estradiol-binding protein in neonatal rat serum. The binding of steroid hormone to bovine albumin in vitro (as determined by equilibrium dialysis) was compared to albumin binding in vivo (as determined by the single injection technique). The ratio of apparent dissociation constant in vivo, K(D)(app), to the in vitro K(D) was: >>200 for progesterone, >200 for testosterone, 120 for estradiol, and 7.7 for corticosterone. Assuming the steady-state condition, the K(D)(app)/K(D) was found to be proportional to the BBB permeability for each steroid. These data demonstrate (a) the selective permeability properties of the BBB to the major steroid hormones is proportional to the tendency of the steroid to partition in a polar lipid phase and is inversely related to the number of hydrogen bond-forming functional groups on the steroid nucleus; (b) the presence of albumin in serum may bind considerable quantities of steroid hormone, but exerts little inhibitory effects on the transport of steroids into brain, whereas globulin-bound hormone does not appear to be transported into brain to a significant extent. Therefore, the hormone fraction in plasma that is available for transport into brain is not restricted to the free (dialyzable) fraction, but includes the larger albumin-bound moiety.
（a）血脑屏障（BBB）对主要性腺和肾上腺类固醇激素的通透性；（b）血浆结合蛋白（白蛋白和特异性球蛋白）在调节血脑屏障类固醇激素转运中的作用。在巴比妥麻醉的大鼠中，采用组织取样 - 单次注射技术，相对于[(14)C]丁醇（一种可自由扩散的参比物质），测量血脑屏障对[(3)H]标记的孕酮、睾酮、雌二醇、皮质酮、醛固酮和皮质醇的通透性。通过颈总动脉将同位素快速注入200微升林格氏溶液（0.1克/分升白蛋白）的大剂量注射液中，在单次通过脑循环后，测定激素单向流入的提取率：孕酮为83±4%；睾酮为85±1%；雌二醇为83±3%；皮质酮为39±2%；醛固酮为3.5±0.8%；皮质醇为1.4±0.3%。血脑屏障的选择性通透性与每种类固醇在水溶液中形成氢键的数量呈负相关，与各自的1 - 辛醇/林格氏分配系数呈正相关。当大剂量注射为67%的人血清时，通过平衡透析测定，>95%的标记类固醇被结合。然而，类固醇通过血脑屏障的流入受到人血清的抑制程度远低于仅运输游离（可透析）激素时预期的程度；孕酮、雌二醇、睾酮和皮质酮的运输分别被抑制18%、47%、70%和85%，或与类固醇与血浆球蛋白的结合成比例。大鼠血清（67%）仅分别抑制这四种激素的运输0%、13%、12%和69%，这反映了大鼠血浆中不存在性激素结合球蛋白。然而，新生大鼠血清（67%）分别抑制孕酮、睾酮和雌二醇的运输0%、0%和91%，这与新生大鼠血清中存在雌二醇结合蛋白一致。通过平衡透析测定体外类固醇激素与牛白蛋白的结合，并与通过单次注射技术测定的体内白蛋白结合进行比较。体内表观解离常数K(D)(app)与体外K(D)的比值为：孕酮>>200，睾酮>200，雌二醇为120，皮质酮为7.7。假设处于稳态条件下，发现K(D)(app)/K(D)与每种类固醇的血脑屏障通透性成正比。这些数据表明：（a）血脑屏障对主要类固醇激素的选择性通透性与类固醇在极性脂质相中的分配倾向成正比，与类固醇核上形成氢键的官能团数量呈负相关；（b）血清中白蛋白的存在可能结合大量类固醇激素，但对类固醇进入脑的运输几乎没有抑制作用，而球蛋白结合的激素似乎在很大程度上不会被运输到脑内。因此，可用于运输到脑内的血浆中激素部分不限于游离（可透析）部分，还包括较大的白蛋白结合部分。
