ABSTRACT
The effects of prolonged gonadotrophin-releasing hormone (GnRH) infusions on LH, FSH, oestrogens and progesterone secretion were studied in female rhesus monkeys at various times of the menstrual cycle and after castration. GnRH was infused at the rate of 15 μg/h for 48 h. This resulted in mean peripheral GnRH levels of 398 ± 31.5 pg/ml (± se) as measured by radioimmunoassay.
As expected the pattern of gonadotrophin responses to GnRH varied considerably with the phase of the menstrual cycle. The largest LH increase was seen during the late follicular phase (6-fold over baseline), with a 3-fold increase during the luteal phase and a 2-fold one during the early follicular phase and in the period following the LH surge. Significant FSH increases (4-fold) were seen only during the follicular phase. Oestrogens increased about 2-fold within 4 h of the start of the infusion during the early follicular phase. In the late follicular phase and during the LH surge, they declined within 24 and 1 h, respectively. Large progesterone increases were seen only during the luteal phase.
Of special interest is the fact that the increase in gonadotrophin secretion could not be maintained for the entire duration of the experiment even though GnRH continued to be infused at rates sufficient to elicit initial increases of several fold over baseline. Gonadotrophin release declined 4–28 h after the initial stimulation. A further decrease below pre-infusion control levels was particularly evident during the midcycle surge and, for FSH, after ovariectomy. These results indicate that a continuous mode of administration may rapidly induce a desensitization phenomenon at the level of the gonadotroph.
Clinical Applications of Ultrasound in Assessment of Follicle Development and Growth
