Pardridge W M
Fed Proc. 1984 Feb;43(2):201-4.
The transport of circulating nutrients (glucose, amino acids, ketone bodies, choline, and purines) through the brain endothelial wall, i.e., the blood-brain barrier (BBB), is an important regulatory step in several substrate-limited pathways of brain metabolism. The in vivo kinetics of nutrient transport has been well characterized in the rat, and the kinetic constants of saturable (Km, Vmax) and nonsaturable (KD) transport through the BBB are now known for more than 30 circulating nutrients. The kinetic constants can be used to gain insight into the important rate-limiting role played by BBB nutrient transport in the regulation of brain metabolism and function. Unlike most nutrients, steroid and thyroid hormones circulate tightly bound to plasma proteins. However, owing to favorable kinetic relationships among brain capillary transit times and rates of hormone dissociation from plasma proteins and hormone diffusion through the brain endothelia, the BBB is able to strip hormones off circulating plasma proteins. With regard to peptide hormone, no specific BBB transport systems for peptides have been identified thus far. However, peptides are able to rapidly distribute into brain interstitial space at the circumventricular organs. In addition, specific receptors for insulin are located on the BBB. The presence of BBB peptide receptors provides a mechanism by which circulating peptides may rapidly influence brain function without the peptide crossing the BBB.
循环营养物质（葡萄糖、氨基酸、酮体、胆碱和嘌呤）通过脑内皮壁，即血脑屏障（BBB）的转运，是脑代谢的几种底物限制途径中的一个重要调节步骤。营养物质转运的体内动力学在大鼠中已得到充分表征，目前已知超过30种循环营养物质通过血脑屏障的可饱和（Km、Vmax）和不饱和（KD）转运的动力学常数。这些动力学常数可用于深入了解血脑屏障营养物质转运在脑代谢和功能调节中所起的重要限速作用。与大多数营养物质不同，类固醇和甲状腺激素与血浆蛋白紧密结合循环。然而，由于脑毛细血管通过时间、激素从血浆蛋白解离的速率以及激素通过脑内皮扩散之间存在有利的动力学关系，血脑屏障能够从循环血浆蛋白中剥离激素。关于肽类激素，迄今为止尚未发现针对肽的特定血脑屏障转运系统。然而，肽能够在室周器官迅速分布到脑间质空间。此外，血脑屏障上存在胰岛素特异性受体。血脑屏障肽受体的存在提供了一种机制，通过该机制循环肽可以在不穿过血脑屏障的情况下迅速影响脑功能。
