Suppression of Serum Gonadotropin Levels by Testosterone and Porcine Follicular Fluid in Castrate Male Rats*

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Abstract
The identification of the testicular negative feedback hormone(s) regulating the secretion of FSH has been a controversial area, with adherents of an inhibin or testosterone. We castrated and cannulated male rats and gave immediate testosterone replacement by Silastic implants using two doses: the lower for its known marginal suppression of serum LH, and the higher to match serum testosterone in intact males. On day 3, each rat was sampled at 0800 h for testosterone, LH, and FSH and then received charcoal-extracted porcine serum or porcine follicular fluid; blood was again removed at 1800 h. On days 6 and 12, the same procedures were carried out, but on day 7, the steroid implants were removed. On day 3, the testosterone threshold for the suppression of FSH was identical to that for the suppression of LH (≥1.1 ng/ml). The higher dose of testosterone suppressed both LH and FSH to near-intact levels. By day 6, testosterone was less successful in suppressing either LH or FSH. Follicular fluid suppressed FSH significantly by about 60% in every rat regardless of the FSH level at the time of injection. Serum LH was not altered by follicular fluid. Thus, serum testosterone is an excellent negative feedback signal for both LH and FSH, and there is no basis, from the present study, in believing that testosterone is an inadequate feedback hormone. However, under in vivo circumstances where there is a marked elevation of serum FSH over LH, a testicular inhibin might be involved as a missing specific negative feedback signal for FSH.

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