English Pointer puppies were used in experiments designed to assess
urea cycle function. A preliminary assay revealed a dietary
arginine (Arg) requirement of 0.40% for maximal
weight gain of puppies fed a chemically defined L-
amino acid diet. Subsequent studies showed that consumption of an Arg-free diet resulted in growth failure,
emetic episodes, mild
hyperammonemia, decreased plasma Arg and
urea, and orotic aciduria.
Oral administration of
ornithine (Orn), equimolar to 0.40% Arg, failed to correct these Arg deficiency symptoms. Consumption of an equimolar concentration of
citrulline (
Cit), on the other hand, resulted in near normal
weight gains, but blood and urine metabolite levels still did not parallel those of dogs fed Arg. Plasma
glucose concentration was unaffected by dietary treatment. Older dogs (20 weeks of age) performed no better than younger dogs (7 weeks of age) when fed an Arg-free diet. In a serial
bleeding study, plasma
ammonia increased after ingestion of an Arg-free diet, reached a peak during
emesis and declined immediately thereafter. In contrast, plasma
ammonia of dogs fed an Arg-adequate diet remained relatively constant. It was apparent that in terms of meaningful Arg replacement value, Orn had minimal activity.
Cit could replace Arg for growth, but was not as efficient as Arg for
urea cycle function.