Ornithine metabolism in male and female rat kidney: mitochondrial expression of ornithine aminotransferase and arginase II.

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-[1-14C] or L-[U-14C]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. L-[1-14C]ornithine decarboxylation occurred in all tubules, but predominantly in proximal tubules. L-[1-14C]ornithine decarboxylation was enhanced when L-[1-14C]ornithine was given to tubules as the sole substrate. The use of L-[U-14C]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-arginine-derived 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.