Studies on the Disappearance of LH and FSH in the Rat; A Quantitative Approach to Adenohypophysial Secretory Kinetics1

The disappearances of circulating endogenous and exogenous FSH and endogenous LH in the rat were compared, using HCG-augmentation and ovarian ascorbic acid depletion assays. The studies of endogenous LH and FSH were done in previously castrated male and female rats. Exogenous FSH was studied after iv injection of NIH-FSH (ovine) into several types of rats: 1) long-term castrates of both sexes (with and without prior hypophysectomy); 2) intact males and females, and hypophysectomized males; 3) rats orchidectomized, spayed or sham-castrated only 2–3 min prior to injection; 4) intact and previously castrated rats, treated chronically with estrogen or androgen and hypo-physectomized 2–3 min before they were injected. The following findings emerged. 1) Endogenous LH activity disappears from the circulation of the castrated rat at least 4 times as rapidly as endogenous FSH activity. Half-lives averaged 30 (19–38) and 149 (96–239) min, respectively. 2) The exogenous FSH preparations disappeared more rapidly than native FSH; the “half-lives” for exogenous FSH (in hypophysectomized rats) ranged from 48 to 93 min. 3) The rate of endogenous FSH release is sufficiently high, particularly in long-term castrates, to severely distort the apparent decay curves for exogenous FSH in non-hypophysectomized rats. This means that elevation of plasma FSH activity, to levels 4 or 5 times the presumed “set-point” in the castrate, did not abolish FSH release—at least during a 2–3 hr period. 4) Gonadal utilization of FSH does not contribute appreciably to the removal of exogenous FSH from the circulation. Disappearance curves in rats castrated 2–3 min before an FSH injection were indistinguishable from those in intact rats. 5) The decay constant for injected FSH (NIH-FSH-S4) was not appreciably affected either by wide variations in the levels of circulating sex steroids or by the full range of physiological variations of plasma LH and FSH activities; neither long-term castration nor chronic androgen or estrogen treatment altered it (in rats hypophysectomized 2–3 min prior to injection). These findings were used to derive and compare secretory rates and pituitary turnover times for LH and FSH in the castrated rat. Some limitations of the derived data, and the applicability of these data in the analysis of pituitary secretory kinetics, have been discussed. (Endocrinology84: 1118, 1969)