Circulating LH concentrations after intravaginal instillation of GnRH in lactating dairy cows

Our objective was to evaluate circulating LH concentrations after intravaginal (IVG) instillation of GnRH in dairy cows. Lactating primiparous (n = 6) and multiparous (n = 27) Holstein cows received two luteolytic doses of PGF2a 12 h apart 7 d after the Ovsynch protocol (GnRH-7d-PGF2a-56h-GnRH). Forty 8 h after the first PGF2a treatment cows were stratified by parity and randomly allocated to five different treatments: 2 mL of saline solution IVG (SAL-IVG, n = 6), 100 μg of GnRH i.m. (G100-im, n = 5), 100 (G100-IVG, n = 7), 500 (G500-IVG, n = 8), or 1000 μg of GnRH IVG (G1000-IVG, n = 7). For all GnRH treatments Gonadorelin diacetate tetrahydrate (Cystorelin) was used. Blood was collected using indwelling jugular catheters at -1 h, 0 h, every 15 min up to 4 h, and every 30 min from 4 to 6 h after treatment. Data for progesterone, estradiol, and LH concentrations were analyzed by ANOVA with (LH only) or without repeated measures using PROC MIXED of SAS. Concentrations of progesterone and estradiol did not differ (P > 0.10) among groups at time 0. Concentrations of LH were affected by treatment (P < 0.001), time (P < 0.001) and treatment by time interaction (P < 0.001). Cows in G100-im had greater (P < 0.05) mean LH than the IVG treatments from 15 to 195 min after treatment whereas the G1000-IVG group had greater (P < 0.05) mean LH than the SAL-IVG and the other IVG GnRH groups from 45 to 240 min (except at 60, 75, and 90 min) after treatment. Mean LH for SAL-IVG, G100-IVG, and G500-IVG did not differ (P > 0.05) at any time point. The greatest (P < 0.001) area under the curve (AUC) was observed for G100-im (1149 ± 51 ng) followed by G1000-IVG (546 ± 108 ng) which had greater AUC than the other IVG treatment groups (242 ± 40, 271 ± 34 and 247 ± 24 ng for SAL-IVG, G100-IVG, and G500-IVG, respectively). Mean LH peak was greater (P < 0.001) for G100-im (6.9 ± 0.4 ng/mL) than G1000-IVG (2.8 ± 0.6 ng/mL) whereas the SAL-IVG, G100-IVG, and G500-IVG groups did not have a discernible LH surge after treatment (maximum LH: 1.2 ± 0.2, 1.3 ± 0.2 and 1.2 ± 0.1 ng/mL for SAL-IVG, G100-IVG and G500-IVG respectively). We conclude that IVG instillation of 1000 μg of GnRH induced more LH release than IVG instillation of saline solution and a 100 or 500 ...g dose of GnRH. Also, the amount of LH released after IVG treatment with 1000 μg of GnRH was less than that released after i.m. treatment with 100 μg of GnRH. Supported by USDA NIFA Hatch project NYC-127434.