Prepubertal and peripubertal changes in secretory patterns of LH and FSH in beef heifers

Abstract

Hormonal changes associated with puberty in 14 Norwegian Red×Hereford heifers, born during March to May, were studied in March to June of the following year. Concentrations of LH were determined in plasma samples collected at 20-min intervals for 24 h on four separate days (0, 30, 60 and 75 days after the first collection). These were referred to as prepubertal periods, 1, 2, 3 and 4 in seven heifers which remained prepubertal. In seven heifers which reached puberty during the study, periods were designated as peripubertal (i.e., within 60 days prior to the first rise in progesterone) and luteal or follicular phase. Episodic releases of LH (pulses/24 h) were more frequent during the fourth prepubertal period (9.0 ± 0.8, P < 0.05) than during the first, second or third prepubertal periods or the luteal phase (6.8 ± 0.8, 6.4 ± 0.8, 5.6 ± 0.8 and 5.1 ± 2.1, respectively), but were similar to the peripubertal period (10.7±1.6) and follicular phase (13.8±2.4). Amplitudes of episodic increases in LH were greater during the third prepubertal (5.0 ± 0.4 ng/ml) than during the first, second, or fourth prepubertal period, the peripubertal period or the luteal or follicular phases (3.3 ± 0.4, 3.8 ± 0.4, 3.1 ± 0.4, 2.7 ± 0.4, 1.8 ± 0.5 and 2.3 ± 0.5 ng/ml, respectively). Neither frequency nor amplitude of episodic increases in LH varied between daylight and darkness. There was no evidence of episodic release or of diurnal variation in FSH. Concentrations of estrone were elevated during frequent sampling from catheters compared to single samples collected by venipuncture thrice weekly (9.2 vs. 3.5 pg/ml, P<0.001), whereas concentrations of estradiol averaged 2.0 pg/ml in both types of samples. It is concluded that diurnal variations in LH and FSH are not associated with the maturation process, but increases in either frequency or amplitude of pulses of LH can be detected as heifers mature, depending upon the time before puberty when frequent samples are collected.