159 EFFECTS OF EMBRYO TRANSFER IN A LARGER BREED ON POSTNATAL GROWTH AND GLUCOSE METABOLISM IN HORSES

In equids, the size of the uterus determines fetal intrauterine development, which in turn affects postnatal insulin sensitivity and growth rate. We induced intrauterine growth enhancement through embryo transfer using Pony (P), Saddlebred (S), and Draft (D) horses and studied growth and insulin sensitivity in foals from birth to one year of age. Control pregnancies of S-in-S (n = 14) and P-in-P (n = 10) were obtained by AI. Enhanced fetal growth was obtained by transferring S (S-in-D, n = 7) and P embryos (P-in-D, n = 5) into D mares. From birth to weaning (180 days), each foal was kept with its surrogate P, S, or D dam. At 3, 140, and 380 days, glucose clearance and pancreatic beta cell response to exogenous glucose were assessed with an intravenous glucose tolerance test (IVGTT). At 200 days, the euglycemic-hyperinsulinemic clamp method was used to determine the sensitivity and responsiveness of tissues to exogenous insulin. Plasma T3, T4, and IGF1 were assayed at 3 and 180 days. Data were analysed using one-way ANOVA and Tukey post hoc tests. S-in-S were heavier and taller than P-in-P from birth to 380 days (P < 0.001). Before weaning, plasma concentrations of several hormones involved in growth were lower in S-in-S than P-in-P (at 3 and 180 days, respectively, T3: P = 0.08 and P = 0.02, T4: P < 0.001 and P = 0.06, IGF1: P = 0.04 and P < 0.001). No difference was found in glucose regulation between these groups. In contrast, post-weaning insulin resistance was observed in P-in-P at 200 days (P < 0.001) and confirmed at 380 days where they exhibited slower glucose clearance (P = 0.03) associated with higher fasting glucose (P < 0.001) than S-in-S. Fetal growth was not enhanced in S-in-D with no difference in height and weight at birth. Although S-in-D grew faster from 30 to 140 days, growth rates were not different from S-in-S after weaning, weaning coinciding with lower T3 (P < 0.001) in S-in-D than in S-in-S. Glucose regulation was not different between the two groups, but insulin remains to be assayed at 140 and 380 days. Fetal growth was enhanced in P-in-D: at birth, they were heavier (P = 0.01) and taller (P < 0.001) than P-in-P. Growth of P-in-D was faster until weaning. No more difference, however, was observed between P-in-D and P-in-P at 380 days. Plasma concentrations of T3 (P = 0.03) and those of T4 (P < 0.001) were lower at 3 days and T3 was still lower at 180 days (P < 0.001) in P-in-D compared with P-in-P. Moreover, P-in-D developed early insulin resistance: insulin secretion was higher in P-in-D compared with P-in-P (P = 0.002) after IVGTT at 3 days. At 200 days, however, P-in-D and P-in-P had the same sensitivity to insulin. There was no difference in glucose clearance rates at 380 days, but P-in-D had lower fasting glucose (P < 0.001) than P-in-P. Insulin assays at 140 and 380 days are pending. In conclusion, these data indicate that transfer of a small breed embryo into a large breed mare and subsequent suckling by the recipient mare can enhance fetal and postnatal growth and affect the foal’s glycaemia and sensitivity to insulin at birth and in subsequent months. Ongoing work includes analyses of milk samples and effects on general health.