ACINAR ORGANIZATION OF NITROGENMETABOLIZING PATHWAYS Different techniques (for recent reviews see [5, 9, 10]) have been employed to study metabolic zonation in hepatic nitrogen metabolism: histochemistry [12], immunohistochemistry [13-17], in situ hybridization [18, 19], retrograde/antegrade liver perfusion [20, 21], microdissection [22], autoradiography (B. Stoll, HP Buscher & D. Haiussinger, unpublished work), radiolabel-incorporation studies [23, 24], studies on zonal cell damage [25, 26], use ofmicro-lightguides and mini-oxygen electrodes [27], attempts to separate periportal from perivenous hepatocytes [28-30] andthe dual-digitonin-pulse perfusion [31]. These studies revealed a remarkable development of functional hepatocyte heterogeneity with respect to ammonia and glutamine meta-bolism. In the intactliver acinus, urea synthesis and glutaminase are present in periportal hepatocytes, whereas glutamine syn-thetase is found only in perivenous hepatocytes. This was shown in the structurally and metabolically intact perfused rat liver by comparison of metabolic fluxes during antegrade and retrograde perfusion [20], in experiments with zonal liver damage [25, 26], by immunohistochemistry of carbamoyl phosphate synthetase [15], argininosuccinate synthetase [13], arginase [13] and glutamine synthetase [14] and more recently with hepatocyte preparations enriched in periportal and perivenous cells [28, 29]. As shown by immunohistochemistry, the borderline between the periportal urea-synthesizing and the perivenous glutamine-synthesizing compartments is very strict [14, 15]: glutamine synthetase is exclusively found in a small hepatocyte population (about 7% of all hepatocytes of an acinus) surrounding the terminal hepatic venule. These cells are virtually free of carbamoyl phosphate synthetase [15] and exhibit several further features which distinguish them clearly from cells of the much larger periportal compartment capable of urea synthesis. Some 70-100% of total hepatic glutamate uptake must be ascribed to this small peri-venous cell population [21, 24, 30, 32, 33] and vascular oxoglu-tarate is taken up in a largely Na+-dependent manner almost exclusively by perivenous glutamine synthetase-containing hepatocytes, but not by the much larger periportal cell population [24]. Also, plasma membrane transport systems are distributed heterogeneously among subacinar cell populations and the data provide an example for a common subacinar localization of functionally linked processes: glutamate and oxoglutarate are taken up almost exclusively by perivenous hepatocytes as a substrate for glutamine synthesis [21, 23, 24]. Carbonic anhydrase isoenzymes II and III [34], as well as a phenobarbital-inducible cytochrome P-450 isoenzyme [35] show an immunohistochemical localization almost identical to that of glutamine synthetase. On the other hand, mitochondrial carbonic anhydrase V [36] is periportal, as suggested by its functional linkage to urea synthesis and gluconeogenesis [36-41]. There is some evidence that peri-venous glutamine synthetase-containing hepatocytes exhibit a higher proliferative activity in vitro than do periportal hepato-cytes [42]. Whereas glutamate is almost exclusively taken up by perivenous hepatocytes [21, 24], other amino acids such as proline, alanine, glutamine are taken up predominantly by periportal hepatocytes [20, 21, 30, 32], ie into a compartment with high gluconeogenic and ureogenic capacity. In addition, periportal hepatocytes exhibit higher activities of amino acid metabolizing enzymes, such as alanine aminotransferase, tyrosine aminotransferase and glutaminase [20, 43, 44]. On the …