Temporary suppression of pulsatile LH release following a single injection of a GnRH agonist (deslorelin) in ovariectomised Holstein dairy cows

The objective of the experiment was to investigate the potential for using a single injection of the GnRH agonist [D-Trp(6), Pro(9)-des-Gly(10)-NH(2)] GnRH-ethylamide (deslorelin) to suppress LH secretion in ovariectomised Holstein cows. Each dose of 10, 100 and 1000 microg deslorelin was injected intravenously into each of four ovariectomised cows on day 0. Blood samples were collected hourly on day 0 to profile the induced LH release. Frequent serial blood samples were collected at 10min intervals over 4h on days -3, -1, +2, +4 and +6. The injection of deslorelin induced a surge-like release of LH that begun within 1h in all cows. There was no difference between deslorelin doses in terms of maximum LH concentration, area under the LH curve (AUC) or log(10)(AUC). The average interval from injection to maximum LH concentration was longer for cows receiving 1000 microg than in those receiving 10 microg (3.5 versus 1.5h; P<0.01), though no different to 100 microg (2.8h; P>0.1). This relationship was described by a logarithmic function of deslorelin dose in micrograms (R(2)=73.3%, P<0.01). Pre-treatment smoothed mean LH concentration was significantly correlated with peak LH concentration of the induced surge: max_LH=5.37+9.57 x pre-amplitude (R(2)=33.2%, P=0.05). Similarly, LH pulse amplitude pre-deslorelin was also correlated with peak LH of the induced surge max_LH=0.07+12.9 x pre-amplitude (R(2)=53.7%, P=0.07). Pulsatile release of LH was suppressed only with the 1000 microg dose on day +2. Suppression was characterised by a reduction in mean LH, smoothed mean LH and LH pulse amplitude. By day +4, LH parameters were no different to pre-treatment ones. Pulse frequency was not affected by the treatment, although a small non-significant reduction at day +2 for 1000 microg dose was observed (3.9 versus 2.8, P=0.14). In conclusion, temporary suppression of LH output for at least 48h occurred following a single intravenous injection of 1000 microg of deslorelin, even though there were similar peak LH concentrations were for the three doses.