.-In the kidney, L-ornithine is reabsorbed along the proximal convoluted tubule (PCT), transported by basolateral carriers, and produced by arginase II (AII). Here, the renal metabolic fate of L-ornithine was analyzed in male and female rats. Kidneys and renal zones were dissected and used for Western blot analysis, immunofluorescence, and electron microscopic studies. Ornithine aminotransferase (OAT) and AII were localized using specific antibodies. Ornithine oxidation was determined by incubating microdissected tubules with L-[1-14 C] or L-[U-14 C]ornithine in the presence or absence of energy-providing substrates. Ornithine decarboxylase (ODC) mRNAs were localized by in situ hybridization. The 48-kDa OAT protein was detected in male and female kidneys, but its level was fourfold higher in the latter. OAT relative distribution increased from the superficial cortex toward the outer medulla to reach its highest level. Almost all OAT protein was localized in cortical and medullary proximal straight tubules (CPST and OSPST, respectively). In proximal straight tubule (PST), AII protein distribution overlapped that of OAT. No gender difference in AII protein level was found. OAT and AII were colocalized within PST mitochondria. 14 C]ornithine demonstrated the complete oxidation of ornithine. In conclusion, the OAT gene was expressed more in female rat proximal tubules than in male. Because OAT and AII proteins overlapped in PST mitochondria, L-argininederived ornithine may be preferentially converted to L-glutamate, as proven by ornithine oxidation. However, the coexpression of ODC, glutamate decarboxylase, and glutamine synthetase in PST suggests that L-ornithine can also be metabolized to putrescine, GABA, and L-glutamine. The fate of L-ornithine may depend on the cellular context. L-ornithine; L-arginine; proximal tubules; isolated nephron segments; Western blot analysis; immunofluorescence; electron microscopy; mitochondria; ornithine decarboxylase THE AMINO ACID L-ORNITHINE is absent from food and is not a constituent of proteins; therefore, vertebrate organisms are dependent on its formation. In kidneys, four sources of Lornithine can be considered: 1) L-ornithine is present in the blood and then transported across the basolateral membranes of a variety of renal cells (36,40); 2) L-ornithine is filtered in glomeruli and reabsorbed by specific carriers located in the apical membrane of the proximal convoluted tubule (PCT) (43); 3) in PCT, the enzyme L-arginine:glycine amidinotransferase (GAT; EC 2.1.4.1) metabolizes L-arginine in the presence of L-glycine and produces guanidinoacetic acid and Lornithine (42); and 4) in the whole proximal straight tubule (PST) of several mammals including rats, the extrahepatic arginase II isoenzyme (AII; EC 3.5.3.1) hydrolyzes L-arginine into urea and L-ornithine (23).L-Ornithine is known to play a pivotal role in several metabolic pathways as precursor for L-proline, L-glutamate, L-glutamine, L-citrulline, and L-arginine biosynthesis. Interestingly, renal tubular cells contain...