Ornithine metabolism along the female mouse nephron: localization of ornithine decarboxylase and ornithine aminotransferase

The fide of ornithine in the nephron of the female OF-1 Swiss mouse remains unknown. The aim of the present study was to identify the nephron segments containing the key enzymes involved in ornithine metabolism: ornithine dec arboxylase (ODC) and ornithine aminotransferase (OAT). Viable tubules isola ted by microdissection were incubated with [1-C-14]ornithine to study the o xidative pathway. Other tubules were permeabilized to measure the ODC activ ity. Ornithine was de carboxylated in all intact tubules, Gabaculine, a sui cide inhibitor of OAT, and rotenone sharply decreased the production of (CO 2)-C-14 from [1-C-14]ornithine. No ODC activity was round in permeabilized tubules isolated from untreated mice. Testosterone increased ODC activity i n the proximal tubule substantially and to a minor extent in other nephron segments. In situ hybridization showed ODC messenger ribonucleic acid (mRNA ) to be absent in kidneys of untreated females but abundant in the cortex a nd the outer stripe of the outer medulla of testosterone-treated female mic e. The whole proximal tubule contained a great density of silver grains cor responding to ODC mRNA. In conclusion, no basal ODC activity was found in t he nephron of female mice. The testosterone-inducible ODC is localized main ly in the proximal tubule, but is also present in distal tubules and collec ting ducts. OAT is distributed along the whole nephron, but its activity is higher in proximal tubules than in distal tubules.