Effect of increasing circulating insulin on follicular development of Holstein cows

The aim of this study was to test the hypothesis that increased circulating insulin during the pre and post deviation period would increase the initial and final follicular development. Sixteen non-lactating and non pregnant Holstein cows were used. The estrous cycles were synchronized using an intravaginal P4 device of 1.9 g (CIDR®; Zoetis, SP, Brazil) and intramuscular (i.m.) 100 µg of GnRH (Gonadorelin diacetate; Cystorelin®, Merial, Canada) on day -12 of the protocol (random day of the estrous cycle). On day -5, 25 mg were administered i.m. of PFG2α (Dinoprost tromethamine; Lutalyse®, Zoetis, SP, Brazil). On day -3, the intravaginal P4 device was removed and cows received 25 mg of PGF2α im. On days 10 and 11 ± 1 of the estrous cycle all follicles > 5 mm were aspirated to synchronize the emergence of a new follicular wave. The second day of aspiration was considered D1 of the experiment, when treatments were initiated. For this, cows were divided into two treatments: water (control; C) or propylene glycol (P) provided orally in four daily doses of 300 mL every 6 hours for 3 consecutive days (D1 to D3, pre follicular deviation period), and another 3 consecutive days (D5 to D7; after follicular deviation period). The experimental design was a Latin square in a 2x2 factorial arrangement Thus, four groups were formed: 1) Group CC = water pre and post follicle deviation (n = 16); 2) Group CP = water pre and propylene glycol post follicle deviation, respectively (n = 16); 3) Group PC = propylene glycol pre and water post follicle deviation, respectively (n = 16); and 4) Group PP = propylene pre and post follicle deviation (n = 16). Blood samples were taken 0 (immediately before), 15, 30, 60 and 120 minutes after propylene glycol for circulating insulin. Ovarian ultrasonography examinations were performed daily for evaluation of follicular dynamics. Statistical analysis was performed by the MIXED procedures of SAS. Plasma insulin concentrations were higher for groups receiving propylene glycol compared to controls (0, 15, 30, 60 and 120 min: 17.5 ± 1.4; 26.3 ± 1.4; 31.2 ± 1.4; 21.8 ± 1.4, 16.9 ± 1.5 vs.12.1 ± 1.5; 11.6 ± 1.5; 11.2 ± 1.5; 10.8 ± 1 5; 11.1 ± 1.5; P ≤ 0.05). Despite this, there was no effect of circulating insulin increase (P > 0.05) on the rate of pre deviation (1.5 ± 0.14; 1.3 ± 0.15; 1.5 ± 0.14; 1.4 ± 0.15 mm/day) and post deviation (1.2 ± 0.13; 1.4 ± 0.14; 1.4 ± 0.13; 1.4 ± 0.13 mm/day) follicular growth and ovulatory follicle size (15.5 ± 0.56; 161 ± 0.55; 15.6 ± 0.52; 15.6 ± 0.54 mm) for CC, CP, PC and PP groups, respectively. We did not confirm the hypothesis that increased circulating insulin during pre and post deviation period interfere on follicle development.