Developmental Changes in the Ability of Follicle Stimulating Hormone to Stimulate Estrogen Synthesis in vivo by the Testis of the Rat

Abstract

FSH [follicle-stimulating hormone, follitropin] injection into immature rats is known to stimulate estradiol [E2] synthesis by the testis. Experiments were designed to determine whether age influenced the gonadal response to such gonadotropin stimulation. At 2, 6, 12, 18 or 24 days of age Sprague-Dawley-derived rats were injected i.p. with 200 ng NIH-FSH-S13/g BW or an equivalent volume of vehicle. Six hours later the animals were killed, and the concentration of testosterone [T] and E2 was determined in methanolic extracts purified by Sephadex LH-20 column chromatography prior to radioimmunoassay. In control animals the T concentration in the testes decreased steadily from 611 .+-. 90 ng/g at 2 days of age to 9 .+-. 4 ng/g at 24 days of age. The E2 concentration showed a parallel decline from 298 .+-. 28 pg/g to 24 .+-. 5 pg/g. FSH injection had no effect on the T concentration in the gonads; the E2 concentration was increased by FSH injection at all ages tested except 24 days. The largest incremental response was noted at 12 days when the FSH treatment increased the E2 concentration to 580 .+-. 64 pg/g. The possibility that the estrogen response was due to the LH [luteinizing hormone, lutropin] which contaminated the FSH preparation was tested. LH alone or combined with FSH was injected i.p. at 5-50 ng NIH-LH-S21/g BW. At all doses tested LH had no measurable effect on the E2 concentration in the testes. When combined with 200 ng/g of FSH the same doses of LH had no more stimulatory effect on estrogen synthesis than FSH injection alone. Evidently in vivo treatment of infant rats with FSH increases the E2 amount in the testes. The effect is not due to LH contamination of the FSH preparation. The responsiveness of the rat testes to FSH changes with age. While the 12 day old rat seemed most responsive, significant stimulation of estrogen concentration was also noted at 2, 6 and 18 days of age. The smallest increase of concentration occurred at 18 days, and by 24 days of age FSH was unable to alter measurably the E2 concentration in the testes.