Progesterone, 20beta-Hydroxypregn-4-En-3-One, And Luteinizing-Hormone Levels In Superovulated Prepuberal And Postpuberal Cattle

Pituitary and plasma luteinizing hormone (LH) and luteal and plasma progesterone were measured in 47 prepuberal Holstein calves and in eight postpuberal heifers for 30 days after induced ovulation or superovulation. Mean pituitary LH in six of the eight heifers sacrificed 30–31 days after superovulation averaged 1.1 mg LH/g of pituitary tissue. Plasma LH rose to 7.2 ng/ml the day after PMSG injection and gradually declined to normal levels of about 2 ng/ml. Plasma progesterone peaked at 63 ng/ml 10 days after LH injection to induce superovulation and declined to nearly undetectable levels in three of the heifers by Day 24. These three heifers ovulated spontaneously, plasma progesterone rose to 6.3 ng/ml by Day 28, and each had a new active corpus luteum at slaughter. The others had not ovulated when slaughtered on Day 30, and still had some old active luteal tissue. Mean pituitary LH in control calves was 5.0 mg/g of wet pituitary tissue and ranged from 0.7 to 1.6 mg/g during the 30 days after superovulation. Plasma LH in calves rose from 1.5 ng/ml before injection of pregnant mare serum gonadotropin (PMSC) to 5.5 ng/ml the day after PMSG and returned to the original level by 10 days after PMSG. Calf corpora lutea had a maximum average progesterone concentration of 43 μg/g of luteal tissue 10–11 days after ovulatory LH. This declined to a minimum on Day 28, but some calves had as much as 36 μg progesterone/g of luteal tissue on Day 30. Plasma progesterone rose to nearly 100 ng/ml 8–10 days after superovulation, and peak progesterone was correlated with the number of corpora lutea (r = 0.76, P < 0.01). Plasma progesterone declined after Day 15, but detectable levels were present throughout the experiment. Plasma progesterone and 20β-hydroxypregn-4-en-3-one rose and fell simultaneously in eight superovulated calves. The major difference among the prepuberal and postpuberal animals was the fact that plasma progesterone in the prepuberal animals never declined to essentially undetectable levels and none ovulated spontaneously. Whether this reflects a difference between uteri of immature and mature cattle in regulating corpus luteum function has not been investigated.