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AJ Forhead, JC Ousey, WR Allen and AL Fowden

This study examined the effects of intrauterine growth on insulin secretion and resistance in newborn foals. Embryo transfer between small pony and large Thoroughbred mares was used to produce four groups of foals with different birth weights (pony in pony n=7; pony in Thoroughbred n=7; Thoroughbred in Thoroughbred n=8; Thoroughbred in pony n=8). On day 2 after birth, glucose (0.5 g/kg) was administered intravenously to the foal and blood samples were taken for 2 h to determine plasma glucose and insulin concentrations. On day 3, insulin sensitivity was assessed by giving insulin (0.75 U/kg i.v.) and measuring the decrement in plasma glucose in the foals. There were no significant differences in insulin secretion, insulin sensitivity or glucose tolerance between the control and growth-retarded Thoroughbred foals. Overgrown pony foals delivered by Thoroughbred mares had higher basal insulin levels and greater beta cell responses to glucose than the other groups of foals. The relationship between plasma glucose and insulin was also significantly steeper in overgrown pony foals than in the other groups. Variations in intrauterine growth rate, therefore, affect postnatal insulin secretion in the horse. More specifically, it is overgrowth, not growth retardation in utero that alters equine beta cell function in the immediate neonatal period.

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WR Allen, S Wilsher, F Stewart, F Stewart, J Ousey, J Ousey and A Fowden

Within-breed artificial insemination and between-breed embryo transfer were carried out in small pony (P) and large Thoroughbred (Tb) mares to create 4 types of horse pregnancy in which the fetus experienced spatial and nutritional deprivation (Tb-in-P; n=8), luxury (P-in-Tb; n=7) or normality (Tb-in-Tb; n=7 and P-in-P; n=7) in utero. Measurement of equine chorionic gonadotrophin (eCG), total conjugated oestrogens and progestagen concentrations in serial peripheral serum samples recovered from all the mares throughout gestation showed that the amount of eCG produced during the first half of gestation was dependent upon the breed of the mare rather than the breed of the fetus being carried. In contrast, the mean total amounts of oestrogens produced, as measured by area under the curve, were significantly greater (P=0.003) in the two types of pregnancy in which a Thoroughbred fetus was being carried (Tb-in-Tb and Tb-in-P) than those in which a pony fetus was gestated (P-in-P and P-in-Tb); the evidence suggests that the Tb fetus may have larger gonads than the P fetus and thereby secrete more C-19 precursor steroids for aromatisation to oestrogens by the placenta. In the final weeks of pregnancy mean plasma progestagen concentrations rose much earlier, and to significantly higher levels (P<0.001), in the Tb-in-P than in the P-in-Tb pregnancies, thereby reflecting the increased fetal stress in the former causing premature maturation of the fetal adrenal gland. This, in turn, resulted in increased secretion of pregnenolone by the adrenal cortex for conversion to progestagens by the placenta.