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1 INSERM U 499, Laboratoire de Physiopathologie Metabolique et Renale, 69372 Lyon Cedex 08, France; CNRS URA 219, College de France, Laboratoire de Physiologie Cellulaire, France
2 CNRS URA 219, College de France, Laboratoire de Physiologie Cellulaire, France; INSERM U 36, College de France, Laboratoire de Pathologie Vasculaire et Endocrinologie Renale, 75231 Paris Cedex 05, France
3 CECIL, Centre Commun d'imagerie Laennec, France
4 Departement de Physiologie, Centre medical universitaire, CH-1211 Geneve 4, Switzerland
5 INSERM U 433, Laboratoire de Neurobiologie Experimentale et Physiopathologie, 69372 Lyon Cedex 08, France
6 CNRS URA 219, College de France, Laboratoire de Physiologie Cellulaire, France
* To whom correspondence should be addressed. E-mail: Olivier.Levillain{at}laennec.univ-lyon1.fr.
In the kidney, L-ornithine is reabsorbed along the proximal convoluted tubule (PCT), transported by basolateral carriers and produced by arginase II (AII). Hereby, the renal metabolic fate of Lornithine was analyzed in male and female rats. Kidneys and renal zones were dissected and used for western blot, immunofluorescence and electron microscopy studies. Ornithine aminotransferase (OAT) and AII were localized using specific antibodies. Ornithine oxidation was determined by incubating microdissected tubules with L-[1-14C] or L-U-14C] ornithine in the presence or not of energy-providing substrates. Ornithine decarboxylase (ODC) mRNAs were localized by in situ hybridization. The 48 kDa protein OAT was detected in male and female kidneys, but its level was fourfold higher in the latter. OAT relative distribution increased from the superficial cortex towards 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 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. L-[1-14C] ornithine decarboxylation occured in all tubules, but predominantly in proximal tubules. L-[1-14C] ornithine decarboxylation was enhanced when given as the sole substrate to tubules. The use of L-[U-14C] ornithine demonstrated the complete oxidation of ornithine. In conclusion, OAT gene was more expressed in female rat proximal tubules than in males. Because OAT and AII proteins overlapped in PST mitochondria, Larginine-derived ornithine may be preferentially converted into L-glutamate as proven by ornithine oxidation. However, the coexpression of ODC, glutamate decarboxylase and glutamine synthetase in PST suggests that L-ornithine can be also metabolized into putrescine, GABA, and L-glutamine. The fate of L-ornithine may depend on the cellular context.
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