Normal adult male hamsters have low levels (10-20 micrograms) of female protein (FP) in serum which increase approximately fivefold during an acute phase response. In contrast, normal females have 50- to 100-fold higher serum levels and the acute phase reaction consists of a transient decrease in FP (approximately 50%), followed by a return to normal levels even under adverse conditions such as cortisone treatment (which by itself has a depressing effect on FP levels in normal females). The acute phase response was not inherently associated with gender, as the pattern of response could be changed to that of the opposite sex by appropriate hormonal manipulation. That is, castrated or diethylstilbestrol-treated males with high FP levels showed a female-type response whereas testosterone-treated females with low FP levels showed a male-type response. Studies on catabolism of 125I-FP showed a similar rapid half-life (T1/2, 9-16 h) in normal males and females and indicated that the sex difference in serum concentration was due to greater synthesis of FP in females. The divergent acute phase reaction of serum FP was related directly to changes in the FP synthetic rate (increased in males, decreased in females). As an indicator of serious pathology, a decrease of FP to very low levels in females was associated frequently with impending death.

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