Effects of oestradiol and progesterone on pulsatile secretion of luteinizing hormone in ovariectomized ewes

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Abstract
The effect of treatment with oestradiol, progesterone, a combination of the two steroids or no steroids on pulsatile release of luteinizing hormone (LH) was examined in ovariectomized ewes. Beginning 3 days after ovariectomy, 5 ewes were assigned to each of the following treatment groups: 0.7 mg oestradiol, 16 mg progesterone, 0.7 mg oestradiol plus 16 mg progesterone or no steroid. All treatments were administered twice daily for 3 weeks in a 0.5 ml injection of ethanol given sc. After 2 weeks of treatment and 1, 4, 8, 16 and 32 days after the treatment period ended, blood samples were obtained from all ewes at 10-min intervals for a 6-h period. At the end of the 6-h period, 100 μg gonadotrophin-releasing hormone (GnRH) was injected iv and blood samples were collected at 15 min intervals for an additional 5 h to estimate the relative pituitary content of LH. Ovariectomized ewes receiving no steroid presented regular pulses of LH at frequency of four to five pulses during a 6-h sampling period. Treatment with progesterone alone decreased the frequency of pulsatile release of LH to approximately 1 pulse/6 h, but did not affect the amplitudes of the pulses of LH. Recovery of pulsatile release of LH to a frequency of four or five pulses of LH in a 6-h period was complete between 16 and 32 days after treatment ended in progesterone-treated ewes. Oestradiol, administered alone or with progesterone, resulted in a decrease in both the frequency and the amplitude of pulses of LH compared to control ewes and a decrease in GnRH-induced release of LH. In these groups, the GnRH-induced release of LH had returned to normal by 32 days after cessation of treatment. Although frequency and amplitude of endogenous pulses had increased by 32 days post-treatment, neither parameter had returned to normal. These data suggest that oestradiol exerts an inhibitory effect on the pituitary by decreasing the content of LH to a level which precludes pulsatile release of LH. Progesterone, in contrast, may inhibit the frequency of release of pulses of LH by reducing the frequency of pulsatile release of GnRH from the hypothalamus.

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