1. Intraperitoneal injections of zymosan were given to rats, according to a modified procedure, in order to create a pattern of illness with an acute critical phase for 36 h followed by a prolonged recovery phase lasting for at least 10 days. Changes in amino acid and protein metabolism were studied in both phases.

2. Differences between this modified and the original zymosan model are a lower mortality (16%), which is limited to the first 36 critical hours, and the absence of signs of severe illness during the prolonged recovery phase.

3. Wasting of muscle protein and decreased protein synthesis rates in muscle were observed during the acute phase of illness. Liver size and liver protein synthesis rates were increased during the same period. The decrease in the total amount of muscle protein and the increase in liver weight were still present 12 days after zymosan treatment, despite a normalization of protein synthesis rates. Large decreases were observed in the concentrations of the conditionally essential amino acids glutamine and arginine in muscle over 6 days. Decreases in plasma glutamine and arginine on day 12 after zymosan indicated that the rats were still not fully recovered on this day.

4. We conclude that injection of a single dose of zymosan in rats leads to metabolic derangements both during the acute phase of critical illness and during the prolonged recovery phase. The model seems suited for investigating the biochemical mechanisms behind these metabolic derangements and for studying therapeutic and nutritional interventions during recovery from critical illness.

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