New micro-optical methods to study metabolism in periportal and pericentral regions of the liver lobule

New techniques employing microlight guides allow metabolic events to be studied noninvasively in tiny volumes of liver tissue located in defined regions within the liver lobule. These events include oxygen utilization, carbohydrate metabolism, ethanol oxidation, monooxygenation, glucaronidation, and sulfation. Two-fiber microlight guides are constructed from 80-microns optical fibers and are placed on periportal or pencentral regions of the liver lobule based on the pattern of liver pigmentation. Light of selected wavelengths is transmitted to the liver surface by one fiber, and the resultant fluorescence or reflectance is delivered to a photomultiplier by the second fiber. The experimental strategy is to monitor native fluors which are metabolically linked (e.g., NADH) or to infuse nonfluorescent substrates which are converted to fluorescent products by specific enzyme systems (e.g., 7-ethoxycoumarin for monooxygenation). Alternatively, disappearance of fluorescence can also be employed (e.g., 7-hydroxycoumarin for sulfation or glucuronidation). With these methods, rate-limiting steps in situ (e.g., substrate uptake, enzyme activity, cofactor supply) have been studied for several metabolic systems in periportal and pericentral regions of the liver lobule.