Unveiling the Role of IGF-I in Fertility: Effect of Long-Acting Bovine Somatotropin (bST) on Terminal Follicular Development and Fertility during an Annual Reproductive Cycle in Sheep

Simple Summary An understanding of the mechanisms involved in the nutritional regulation of the ovulation rate would increase sheep productivity and would improve animal feeding strategies and environmental footprints. The interaction of the metabolic axis controlling energy balance and the reproductive axis occurs at central and peripheric levels, ultimately signaling their influence directly in the reproductive tract. Growth Hormone (GH), insulin-like growth factors (IGF), and insulin are major peripheric players in this linkage. In addition to controlling energy homeostasis, their specific roles in modulating reproduction efficiency are unclear because they are functionally associated. Based on a single administration of long-acting bST, this study used a model that increased blood concentrations of IGF and kept basal that of insulin to assess, during an annual reproductive cycle, the effect of bST on terminal follicular development and oocyte competence for fertility. Results show that bST increased the number of lambs born from each ovulatory-sized follicle only during the breeding season, without affecting any other clinical marker. This model and the information gathered by this study allow a precise search for the molecular mechanisms involved in this effect.Abstract The study aimed to assess the effect of long-acting bST treatment, in a dose that only increases IGF-I plasma concentrations, on ovarian and fertility markers of estrous synchronized ewes that were fed to keep their bodyweight. Three experiments were designed to evaluate this effect: in Experiment 1, 18 ewes were distributed in groups (bST 0, 30, 50 mg) to measure plasma IGF-I and insulin for 15 days; in Experiment 2, 92 ewes (5 replicates) in two groups (0 and 30 mg bST) were synchronized using a 6-day progesterone protocol during the breeding season to assess the effect of bST on follicular and luteal performances, estrous and ovulation, and fertility after mating. In Experiment 3, 50 ewes (3 replicates) were used to repeat the study before but during anestrus. Results indicate that 50 mg bST increased IGF-I and insulin plasma concentrations, but 30 mg bST only increased IGF-I concentrations; and that only during the breeding season did 30 mg bST increase the number of lambs born and the reproductive success of ovulatory-sized follicles compared to controls. This occurred without it affecting any other reproductive marker. In conclusion, 30 mg bST treatment may improve oocyte competence for fertility during the breeding season.