Circulating concentrations of 17-estradiol influence pattern of LH in circulation of cows

The objective of the research was to determine the relationship between circulating 17β-estradiol (E2) and secretion of luteinizing hormone (LH) in cows. A second objective was to determine if response to E₂ was influenced by interval between ovariectomy and the start of E₂ treatment. Thirty-one nulliparous cows 3 yr of age were randomly assigned to a 2 × 4 factorial arrangement of treatments. Sixteen cows were ovariectomized at 18 mo of age (long term), and the other 15 cows were ovariectomized at 36 mo of age (short term). At the time of ovariectomy of cows in the short term group, 11 cows in the short term group and 12 cows in the long term group were implanted subcutaneously with 1, 2 or 4 polydimethylsiloxane capsules containing E₂. The other eight cows served as non-implanted controls (n=4-short term, n=4-long term). All cows were fitted with jugular vein catheters on day 29 of treatment, and on day 30 blood samples were collected at 12-min intervals for 6 hr. At the end of 6 hr, luteinizing hormone-releasing hormone (LHRH) was administered and blood sampling continued at 12-min intervals for an additional hour. Serum was analyzed for LH and E₂. Variables of LH secretion analyzed were mean concentration, frequency of pulses, amplitude of pulses and maximum concentration after LHRH. There were no significant interactions for any of the variables of LH among cows ovariectomized for the long and short term. There was a significant linear increase in mean concentration of LH with increased circulating concentration of E₂. Frequency of LH pulses was not affected by circulating concentration of E₂. As circulating concentration of E₂ increased, amplitude of LH pulses increased and response to LHRH increased - resulting in an increase in mean LH. Interval from time of ovariectomy to the start of E₂ treatment only had a minor influence on mean concentration of LH and profile of LH concentrations in circulation.