Sex Difference in de novo Synthesis of FSH in Developing Rats

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Abstract
A study of the FSH synthesis pattern in the maturing (10–40 days of age) male and female rats has been conducted. The plasma and pituitary FSH levels were measured by double antibody radioimmunoassay (RIA). De novo synthesis of FSH was studied by incubation of pituitaries with 3H-leucine and immunoprecipitation of the total FSH (including the newly synthesized hormone) with an FSH antibody. A marked difference in the pattern of changes in the pituitary and plasma levels and the de novo synthesis of FSH was noted between the two sexes. In the females the highest circulatory FSH levels were noted on the 10th day of life (2,009 ng NIHFSHS-1/ml). The level dropped progressively until the 30th day of age and remained approximately at the level of 500 ng/ml until the 40th day of age. Incorporation of 3H-leucine in the FSH molecules or the de novo synthesis of this hormone was also highest between the 10th and 15th days of age and gradually diminished until the 30th day when it leveled off. In the males the incorporation of 3H leucine increased gradually from 10 days of age and peaked on the 35th day at the same time that the highest FSH levels were observed in both serum and the pituitary. At the age of 40 days, the plasma and the pituitary FSH levels decreased, paralled to a decrease in synthetic rate. These data are interpreted to suggest that (1) the changes in the rates of FSH synthesis are primarily responsible for the dramatic changes in circulatory FSH levels, and (2) the factors which inhibit the release of FSH in these animals ultimately inhibit its synthesis as well.

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