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.
WR Allen, S Wilsher, F Stewart, F Stewart, J Ousey, J Ousey and A Fowden
A L Fowden, D S Gardner, J C Ousey, D A Giussani and A J Forhead
At birth, the endocrine pancreas becomes more directly involved in the control of glycaemia than in utero. However, compared with other tissues, relatively little is known about the maturational changes that occur in the fetal endocrine pancreas in preparation for extrauterine life. This study examined the pancreatic β-cell response to exogenous administration of glucose and arginine in fetal horses with respect to their gestational age and concentration of cortisol, the hormone responsible for prepartum maturation of other fetal tissues. Glucose administration had no effect on fetal insulin secretion between 175 and 230 days of gestation but evoked a rapid insulin response in fetuses closer to term (290–327 days). In late gestation, the β-cell response was more rapid and greater in magnitude in fetuses with basal cortisol levels higher than 15 ng/ml than in those with lower cortisol values at the time of glucose administration. The fetal β-cell response to arginine was unaffected by the rise in fetal plasma cortisol towards term. These findings show that there are maturational changes in pancreatic β-cell function in fetal horses as cortisol levels rise close to term. Primarily, these prepartum maturational changes were in the mechanisms of glucose-stimulated insulin secretion, which would enable the β cells to regulate glycaemia at the higher glucose levels observed postnatally.