Intravaginal instillation of gonadotropin-releasing hormone analogues with an absorption enhancer induced a surge of luteinizing hormone in lactating dairy cows.

Our objectives were to evaluate circulating LH concentrations after intravaginal (IVG) instillation of GnRH analogs in lactating dairy cows. In 2 experiments, lactating Holstein cows (experiment 1: n = 32; experiment 2: n = 47) received the experimental treatments 48 h after the first of 2 PGF2α treatments given 12 h apart and 7 d after a modified Ovsynch protocol (GnRH at -7 d, PGF2α at -24 h, PGF2α at -56 h, GnRH at 0 h). In experiment 1, cows were stratified by parity and randomly allocated to receive the following treatments: 2 mL of saline IVG (SAL, n = 6), 100 µg of gonadorelin (Gon) i.m. (G100-IM, n = 5), and 100 (G100, n = 7), 500 (G500, n = 8), or 1,000 µg of Gon IVG (G1000, n = 7). In experiment 2, treatments were SAL (n = 8), G100-IM (n = 8), G1000 (n = 7), 1,000 µg of Gon plus 10% citric acid (CA) IVG (G1000CA, n = 8), 80 µg of buserelin IVG (B80, n = 8), and 80 µg of buserelin plus 10% CA IVG (B80CA, n = 8). In both experiments, blood was collected every 15 min from -15 min to 4 h, and every 30 min from 4 to 6 h after treatment. Data for area under the curve (AUC), mean LH concentrations, and time to maximum LH concentration were analyzed by ANOVA with (mean LH only) or without repeated measures using PROC MIXED of SAS (version 9.4, SAS Institute Inc., Cary, NC). The proportion of cows with a surge of LH was evaluated with Fisher's exact test using PROC FREQ of SAS. In both experiments, LH concentrations were affected by treatment, time, and the treatment by time interaction. In experiment 1, the AUC for LH and maximum LH concentration were greatest for the G100-IM treatment and were greater for the G1000 than for the SAL and G500 treatments. The proportion of cows with an observed surge of LH was 100 and 0% for cows that received Gon i.m. and IVG, respectively. In experiment 2, the AUC and maximum LH concentrations were greater for the G100-IM, G1000CA, and B80CA treatments than for the other IVG treatments. The proportion of cows with a surge of LH differed by treatment (SAL = 0%, G100-IM = 100%, G1000 = 14%, G1000CA = 88%, B80 = 13%, and B80CA = 100%). For the treatments with a surge of LH, time to maximum concentration of LH was the shortest for the G100-IM treatment, intermediate for the G1000CA treatment, and the longest for cows in the B80CA treatment. In conclusion, Gon (up to 1,000 µg) absorption through intact vaginal epithelium after a single IVG instillation was insufficient to elicit a surge of LH of normal magnitude. Conversely, IVG instillation of 1,000 µg of Gon and 80 µg of buserelin with the addition of citric acid as absorption enhancer resulted in a surge of LH of similar characteristics than that induced after i.m. injection of 100 µg of Gon.