Microdialysis Studies

微透析探针技术为可扩散组织成分的取样或生物活性物质的递送提供了进入组织微囊的途径。然而，对于任何感兴趣的分析物，探针灌注液和组织中的分析物浓度之间的关系是许多因素的复杂函数，例如分析物分子量、分析物物理化学性质、组织性质、探针膜性质、探针几何形状和灌注速率。需要更好地理解这种关系，以提高技术的量化效用。目前的研究特别强调两个因素。一种是探针插入组织造成的创伤效应。第二个是组织中清除过程的强烈影响，该清除过程从细胞外空间去除分析物，例如细胞摄取、化学转化和通过探针附近的微脉管系统损失到血液中。结合这些因素的数学建模用于描述探针内和周围组织中的扩散和对流溶质传输。模型的主要结果是探针提取效率的预测表达式。这些建模工作已经扩展了用于从灌注液浓度测量确定体内提取效率的校准技术的效用。除了允许估计组织细胞外液中的分析物浓度之外，提取效率的大小提供了关于组织的定量信息。这些定量分析需要了解探针的物理性质，例如探针膜的扩散和对流渗透性，这些物理性质可以在体外良好表征的条件下确定。这些定量方法在大脑微透析中的应用正在与酗酒和滥用药物的研究联系起来。其他应用涉及各种正常组织和肿瘤。感兴趣的内源性溶质包括神经递质，特别是多巴胺。使用的外源性物质的实例是齐多夫定（AZT）、顺铂和类似物、氟康唑、乙醇、可卡因和阿片类药物。动物（小鼠、大鼠和灵长类动物）的验证实验包括定量放射自显影、组织学和探针周围组织的化学分析，以及探针灌注液浓度的测量。 与模型预测一致，我们和其他人先前已经表明，多巴胺在大脑中的提取效率随着这种神经递质的细胞外清除率的降低而降低。然而，最近在大鼠纹状体中的一项研究表明，提取效率可能对多巴胺清除率的增加不敏感。根据我们今年完成的一项研究，在小鼠的丘脑核中情况并非如此。在用长效κ-阿片受体拮抗剂nor-binaltorphimine处理的小鼠中，提取效率高于对照动物。模型计算表明，治疗增加了多巴胺的释放和摄取的表观速率约6倍。 与扩散运动相比，灌注液和组织之间的溶质对流交换通常被认为是可以忽略的。然而，对流贡献可能是不可避免的。在某些情况下，对流增强对于增加溶质递送的速率和组织渗透的程度可能是期望的。我们以前的数学模型的微透析已扩大到包括对流无论是从灌注液的组织或在相反的方向。对于表现出浓度线性的分析物，修订后的模型预测，基于质量和浓度的提取效率的适当定义的测量是对称的，即，无论探针是取样还是输送分析物，它们都具有相同的值。如果没有这种对称性，大多数探头校准技术将是无效的。实验计划测试这一预测。

Microdialysis probe technology provides access to tissue interstitium for either sampling of diffusible tissue constituents or delivery of bioactive substances. For any analyte of interest, however, the relationship between the analyte concentration in the probe perfusate and in the tissue is a complex function of many factors, such as analyte molecular weight, analyte physicochemical properties, tissue properties, probe membrane properties, probe geometry, and perfusion rate. Better understanding of the relationship is needed in order to improve the quantitative usefulness of the technology. Current studies emphasize two factors in particular. One is the effect of trauma resulting from probe insertion into the tissue. The second is the strong influence of clearance processes in the tissue that remove analyte from the extracellular space, such as cellular uptake, chemical conversion and loss to blood through the microvasculature in the vicinity of the probe. Mathematical modeling incorporating these factors is used to describe diffusive and convective solute transport within the probe and in surrounding tissue. A principal outcome of the models is predictive expressions for the probe extraction efficiency. These modeling efforts have expanded the utility of calibration techniques for determining the extraction efficiency in vivo from perfusate concentration measurements. In addition to permitting estimation of analyte concentrations in tissue extracellular fluid, the magnitude of the extraction efficiency provides quantitative information about the tissue. These quantitative analyses require knowledge of physical properties of the probes, such as diffusive and convective permeabilities of probe membranes, that can be determined under well-characterized conditions in vitro. Applications of these quantitative approaches to microdialysis in the brain are being pursued in connection with alcoholism and studies of drugs of abuse. Other applications involve various normal tissues and tumors. Endogenous solutes of interest include neurotransmitters, particularly dopamine. Examples of exogenous substances employed are Zidovudine (AZT), cisplatin and analogs, fluconazole, ethanol, cocaine and opioids. Validation experiments in animals (mice, rats and primates) involve quantitative autoradiography, histology, and chemical assay of tissue surrounding the probe, as well as measurement of probe perfusate concentrations. In agreement with model predictions, we and others have previously shown that the extraction efficiency for dopamine in the brain decreases with reduction in the rate of extracellular clearance of this neurotransmitter. However, a recent study in the rat striatum suggested that the extraction efficiency may be insensitive to increases in dopamine clearance. This is not the case in mouse nucleus accumbens based on a study we completed this year. In mice treated with a long-acting kappa-opioid receptor antagonist, nor-binaltorphimine, the extraction efficiency was higher than in control animals. Model calculations indicated that the treatment increased the apparent rates of dopamine release and uptake approximately six-fold. Convective exchange of solutes between the perfusate and the tissue is usually assumed to be negligible in comparison to diffusive movement. However, a convective contribution may be unavoidable. In some cases convective enhancement may be desirable for augmenting the rate of solute delivery and the extent of tissue penetration. Our previous mathematical model of microdialysis has been expanded to incorporate convection either from the perfusate to the tissue or in the reverse direction. For analytes exhibiting concentration linearity, the revised model predicts that properly defined measures of mass- and concentration-based extraction efficiency are symmetric, i.e., they possess the same value whether the probe is sampling or delivering analyte. Without this symmetry, most probe calibration techniques would be invalid. Experiments are planned for testing this prediction.