Oestradiol-17beta responsiveness, plasma LH profiles, pituitary LH and FSH concentrations in long-term ovariectomised Holstein cows at 24 h, 48 h and 21 days following treatment with an absorbable GnRH agonist implant.

Non-lactating OVX Holstein cows (N = 34) were used to investigate the effect of s.c. placement of an absorbable GnRH agonist implant (Ovuplant; deslorelin 2.1mg, Peptech Animal Health, Australia) on the relationship of plasma LH, oestradiol responsiveness and pituitary LH content. On the day of implant insertion (Day 0), one group (OVU-48h; N = 5) received Ovuplant and had blood samples collected at hourly intervals to characterize the LH response, while a second group (CON-48 h; N = 5) remained untreated and acted as controls. Blood samples were collected every 10 min over 6 h from CON-48 h and OVU-48 h, at 24 h post-implant insertion. These cows were then slaughtered at 48 h post-implant insertion and their pituitaries recovered. Another group received Ovuplant (OVU-21d+E2; N = 10) or were left untreated (CON-21d+E2) and 21 days later were injected i.m. with 0.5 mg 17beta-E2. Blood samples were collected every 10 min for 4 h on the day before E2 injection to characterize LH pulse frequency and amplitude. Beginning 14 h later, blood samples were collected hourly for 12 h to characterize the expected LH surge. These cows were slaughtered and their pituitary glands recovered and assayed for LH and FSH content. Peak plasma LH concentrations (59 +/- 19 ng/ml) were measured after 30 min of Ovuplant insertion. They had returned to pre-treatment levels by 7 h. By 24 h post-implant insertion, OVU-48 h plasma LH profiles were characterized by reduced LH pulse frequency (0.23 +/- 0.09 pulses/h versus 0.75 +/- 0.26 pulses/h; OVU-48 h versus CON-48 h; P < 0.05). The cows that received Ovuplant had lower LH pulse amplitude, LH pulse frequency and mean LH concentrations after 20 days. Injection of 0.5 mg 17beta-E2 induced an LH surge in every one of the control cows with their peak concentrations measured 18 h post injection. No increase in LH was detected in any Ovuplant treated cows. Pituitary FSH content was reduced in Ovuplant treated cows after 48 h, but not that of LH. In conclusion, absorbable deslorelin implants induced a substantial but temporary release of LH, but even 21 days later their LH profiles were characterized by marked suppression of pulsatile LH and an absence of response to E2. These results suggest the implant has prolonged biological activity.