Radioimmunoassay of Serum and Pituitary LH and FSH Levels in Intact Male Rats and of Serum and Pituitary LH in Castrated Rats of Both Sexes—Apparent Absence of Diurnal Rhythms1

In 3 separate studies, each spanning 32–50 hr, serum and pituitary LH levels of male rats killed at 1½ to 6½ hr intervals were studied by radioimmunoassay. In 2 of these studies orchidectomized rats also were examined and in one of them (spanning 50 hr) ovariectomized rats were studied as well. In no case was evidence obtained for any rhythmic diurnal variation of either serum or pituitary LH levels. In the third study, on male rats only, serum and pituitary FSH were examined by 3 different radioimmunoassay systems. Serum corticosterone and pineal serotonin levels also were measured. The animals displayed pineal serotonin and serum corticosterone rhythms very similar to those other male rats have been reported to exhibit, but they showed no sign of any rhythmic variation in pituitary or serum FSH levels. These findings lead us to conclude that: 1) the rates of LH and FSH secretion remain fairly constant throughout the day and night in intact rats, even though such rats do display striking diurnal rhythms of pineal and adrenal activity; 2) LH secretion is also fairly stable in castrated rats of both sexes; 3) any rhythmic variations which may exist in the serum and pituitary hormone levels we studied are of such small amplitude that they were not detectable by the methods we used. (Endocrinology87: 798, 1970)