ONTOGENY OF OVARIAN INHIBITION OF PULSATILE LUTEINIZING-HORMONE SECRETION IN POSTNATAL HOLSTEIN HEIFERS
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abstract
The hypotheses that pulsatile secretion of luteinizing hormone (LH) increases in heifers between 3 and 9 weeks of age and that ovarian inhibition of pulsatile LH release is first established between 3 and 9 weeks of age were tested in Holstein heifers. Heifers were bilaterally ovariectomized at 3 (n=5), 6 (n=4) or 9 (n=4) weeks of age. Blood samples were collected via indwelling jugular cannulae at 10-min intervals for 8 hr 1 week before and 1 and 4 weeks after ovariectomy. During the preovariectomy sampling period, no LH pulses were detected in the 3- or 6-week-old heifers, while one LH pulse occurred in three of four heifers in the 9 week age group. Both mean plasma LH concentration and number of LH pulses increased (P<0.01) by 1 week after ovariectomy only in the 6 and 9 week age groups. Between 1 and 4 weeks following ovariectomy, mean LH concentration increased (P<0.01) in all age groups, while number of LH pulses increased (P<0.01) only in the 3 week group. When the data from the three age groups were combined, amplitude of LH pulses increased (P<0.01) between 1 and 4 weeks following ovariectomy. Mean plasma levels of corticoids did not differ among age groups within sampling periods. We conclude that ovarian inhibition of pulsatile LH secretion is established by 6 weeks of age in Holstein heifers and that the initial increase in mean LH concentration following ovariectomy is mainly due to an increased frequency of LH pulses and the subsequent increase is due to a greater amplitude of LH pulses. Furthermore, the delayed development of pulsatile LH secretion following ovariectomy in the 3-week-old heifers provides a basis for further studies to determine the integrity of the hypothalamic-pituitary axis in postnatal heifers. 1986.
