Amann R P, Walker O A
J Anim Sci. 1983 Aug;57(2):433-42. doi: 10.2527/jas1983.572433x.
The temporal pattern of the endocrine changes associated with puberty were studied using 52 bulls born in October or April. Blood samples were taken weekly and at 30-min intervals for 5 h every 4-wk. Bulls were castrated at one of six 4-wk intervals between 12 and 32 wk and blood samples were taken. Season of birth affected concentrations of testosterone (greater for spring-born) in intact bulls, but not luteinizing hormone (LH) or follicle stimulating hormone (FSH). The concentration of FSH increased about 30% between 4 and 32 wk, without evidence of pulsatile discharge. Basal concentration of LH was low and pulsatile discharges were infrequent at 4 or 8 wk. At 12, 16 and 20 wk, however, basal LH concentration was elevated and LH discharges were at less than 2-h intervals. Testosterone concentration did not rise until 18 to 20 wk, but then continued to rise; LH discharge was suppressed concomitantly. Bulls castrated at 16 or 20 wk had higher concentrations of LH in their blood both before and shortly after castration values for bulls, but by 21 d after castration values for bulls of all ages were similar. It was concluded that elimination of an unidentified suppressive factor allows frequent discharges of LH between 12 an 16 wk, but the testes do not respond by secreting more testosterone until 18 to 20 wk. By 24 wk, the testes are secreting more testosterone and pituitary production of LH is restored to a lower level; LH discharges decline in frequency and basal LH level declines. The high frequency discharges of LH between 12 and 20 wk are postulated to induce responsiveness of Leydig cells to LH and, thus, enable elevation of intratesticular testosterone to levels necessary for Sertoli cell differentiation and initiation of spermatogenesis.
利用52头10月或4月出生的公牛，研究了与青春期相关的内分泌变化的时间模式。每4周采集一次血样，每周一次，每次5小时，每隔30分钟采集一次。在12至32周之间的六个4周间隔中的一个时间点对公牛进行阉割，并采集血样。出生季节影响未阉割公牛体内睾酮的浓度（春季出生的公牛浓度更高），但不影响促黄体生成素（LH）或促卵泡激素（FSH）。FSH的浓度在4至32周之间增加了约30%，没有脉冲式释放的迹象。4周或8周时，LH的基础浓度较低，脉冲式释放不频繁。然而，在12周、16周和20周时，基础LH浓度升高，LH释放间隔时间少于2小时。睾酮浓度直到18至20周才升高，但随后持续升高；LH释放同时受到抑制。在16周或20周阉割的公牛，在阉割前和阉割后不久，其血液中的LH浓度都高于其他公牛，但阉割后第21天，所有年龄公牛的值相似。得出的结论是，消除一个未知的抑制因子可使LH在12至16周之间频繁释放，但睾丸直到18至20周才通过分泌更多睾酮做出反应。到24周时，睾丸分泌更多睾酮，垂体产生的LH恢复到较低水平；LH释放频率下降，基础LH水平下降。推测12至20周之间LH的高频释放可诱导睾丸间质细胞对LH产生反应，从而使睾丸内睾酮升高到支持生精细胞分化和启动精子发生所需的水平。
