Fralix T A, Ceckler T L, Wolff S D, Simon S A, Balaban R S
Laboratory of Cardiac Energetics, National Heart Lung and Blood Institute, NIH, Bethesda, Maryland 20892.
Magn Reson Med. 1991 Mar;18(1):214-23. doi: 10.1002/mrm.1910180122.
Magnetization transfer between macromolecules and water can be a significant factor contributing to tissue water 1H relaxation. Using saturation transfer techniques, the degree of magnetization transfer between the macromolecular matrix and bulk water 1H can be directly measured and magnetization transfer contrast (MTC) can be generated in MR images. A significant degree of MTC has been observed in tissues with high plasma membrane content such as kidney and brain. The purpose of this study was to establish whether lipid bilayers, as models for cell membranes, could exchange magnetization with the water solvent and whether this effect could contribute to MTC observed in intact tissues. Magnetization transfer was measured in aqueous dispersions of egg phosphatidylcholine (EPC) in the presence and absence of cholesterol. It was found that neither EPC bilayers nor cholesterol by themselves significantly exchanged magnetization with bulk water 1H. However, as the concentration of cholesterol was increased, the pseudo-first-order magnetization exchange rate increased to a maximum value of approximately 1 s-1. The cholesterol-induced 1H magnetization exchange may be related either to longer correlation times of the lipid or to an increase in the number of water molecules associated with the bilayer. These results indicate that EPC-cholesterol bilayers exchange 1H magnetization with bulk water. These results are consistent with lipid bilayer contributions to bulk water relaxation and MTC in intact biological tissues.
大分子与水之间的磁化转移可能是导致组织水1H弛豫的一个重要因素。使用饱和转移技术，可以直接测量大分子基质与大量水1H之间的磁化转移程度，并在磁共振图像中生成磁化转移对比（MTC）。在具有高质膜含量的组织如肾脏和大脑中已观察到显著程度的MTC。本研究的目的是确定作为细胞膜模型的脂质双层是否可以与水溶剂交换磁化，以及这种效应是否有助于在完整组织中观察到的MTC。在有和没有胆固醇存在的情况下，测量了卵磷脂（EPC）水相分散体中的磁化转移。结果发现，单独的EPC双层或胆固醇本身都不会与大量水1H显著交换磁化。然而，随着胆固醇浓度的增加，伪一级磁化交换率增加到最大值约1 s-1。胆固醇诱导的1H磁化交换可能与脂质更长的相关时间或与双层相关的水分子数量增加有关。这些结果表明，EPC-胆固醇双层与大量水交换1H磁化。这些结果与脂质双层对完整生物组织中大量水弛豫和MTC的贡献一致。
