Stimulation of a pulse of LH and reduction in PRL concentration by a physiologic dose of GnRH before, during, and after luteolysis in heifers

A single physiologic dose (5.0 μg) of GnRH was given to 9 heifers each day (Hour 0) beginning on Day 15 postovulation until regression of the corpus luteum. Blood samples were taken each day for Hours −3, −2, −1, 0, 0.25, 0.5, 0.75, 1, 1.25, 1.50, 1.75, 2, 3, 4, and 5. Based on daily progesterone concentrations, data were grouped into phases of before (n = 4), during (n = 8), and after (n = 7) luteolysis. The number of LH pulses with a peak at pretreatment Hours −2 or −1 (0.35 ± 0.12 pulses/sampling session) was less (P < 0.0001) than for a pulse peak at posttreatment Hours 1 or 2 (1.0 ± 0.0 pulses/session). The characteristics and effects of LH pulses on progesterone and estradiol were similar between natural (pretreatment) and primarily induced (posttreatment) LH pulses. The same dose of GnRH stimulated an LH pulse with greater (P < 0.05) amplitude after luteolysis than during luteolysis. Concentrations of PRL and number and prominence of PRL pulses decreased (P < 0.05) between Hours 0 and 2 within each of the phases of before, during, and after luteolysis. The hypothesis that a physiologic dose of GnRH increases the concentration of PRL was not supported; instead, GnRH reduced the concentration of PRL. Results supported the hypotheses that an appropriate dose of GnRH stimulates an LH pulse during luteolysis that is similar to a natural pulse in characteristics and in the effects on progesterone and estradiol.