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At birth, the pattern of nutrition changes from a continuous but limited supply of nutrients from the placenta to an intermittent yet more plentiful provision of metabolic substrates via the gut. The success of this transition from parenteral to enteral nutrition depends on adequate fuel reserves, functional mechanisms for regulating nutrient utilization and production, a gastrointestinal tract competent at digestion and sufficient motor development to allow suckling. Maturational changes in key tissues (eg liver, gut) and endocrine systems (eg pancreas, adrenal) therefore occur during late gestation in preparation for the nutritional transition at birth. In many species, these maturational changes are dependent on the prepartum increase in cortisol secretion by the fetal adrenal. In fetal sheep, cortisol has been shown to stimulate hepatic glycogen deposition, induce hepatic gluconeogenic enzymes and adrenoreceptors and accelerate the structural and functional development of the gut. In fetal horses, prepartum maturation of the glucogenic and glucoregulatory capacities occurs comparatively late in gestation coincidentally with the natural rise in fetal plasma cortisol which occurs much closer to term in this than other domestic species. The relatively narrow window for glucocorticoid dependent maturation in the foal has consequences for survival both in utero in response to adverse conditions and at birth. Fetal foals are unable to produce glucose endogenously during late gestation and develop severe hypoglycaemia if delivered prematurely. Foals born with low cortisol levels therefore adapt poorly to extrauterine life while, conversely, increasing fetal glucocorticoid levels by betamethasone administration or by stress induced fetal cortisol secretion can lead to early delivery of viable foals which are normoglycaemic and able to digest enteral feeds. Increase prenatal exposure to glucocorticoid therefore appears to be as much a prerequisite for the neonatal transition from parenteral to enteral nutrition in the horse as it is in other species. |
