Higher Plasma Disappearance Rate in the Mouse for Pituitary Follicle-Stimulating Hormone of Young Women Compared to that of Men and Elderly Women*

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Abstract
Female mice were injected iv with extracts of different human pituitaries and the recovery of hFSH in plasma and the disappearance rates of the hormone from the circulation were measured. In the first samples taken 5 min after injection, the recovery of pituitary FSH of young women (52%) was significantly lower (P < 0.001) than that of men (64%) and elderly women (63%). The plasma concentration of human FSH was measured in the mice up to 242 min after injection. During this period the disappearance rate of the FSH of a young woman continued to be higher than that of the man and the elderly woman. The plasma disappearance curves were multiexponential and gave a poor fit in a two-component exponential model, probably due both to distribution into more than one compartment and to heterogeneity of the material investigated. The MCR and the t½ of irreversible loss of human FSH from the circulation were therefore also calculated from graphic integration of the areas under the disappearance curves extrapolated to 480 min. The values obtained in this way for MCR of FSH of a young and an elderly woman and a man were 2.4, 1.4, and 1.7 ml/h, respectively, and the values for t½ were 19, 33, and 28 min, respectively. The differences between these values for FSH of the three individuals were highly significant (P < 0.001). The results indicate that the relatively low in vivo biological activity reported for pituitary FSH of young women compared to that of men and elderly women is due to more rapid clearance of the hormone from the circulation of the test animal.