Sympathoadrenal and other responses to hypoglycaemia in the young foal

The effects of insulin-induced hypoglycaemia on plasma catecholamines, cortisol and metabolites have been examined in newborn and 7-14-day-old foals. The fall in plasma glucose elicited by the highest dose of insulin (1.0 i.u./kg) given to the neonates was slower in onset and less severe in effect than 0.5 i.u./kg in the older foals. There was a significant inverse correlation between the concentrations of glucose and adrenaline (but not noradrenaline) in plasma once the glucose level had fallen below 2 mmol/l; the adrenergic response to hypoglycaemia was greater in the 7-14-day-old foals than in the neonates. No significant changes in glucose or catecholamines were seen after fasting alone. The adrenocortical response to hypoglycaemia was poor after birth, but significant changes occurred in the older foals with a 3-fold increase in plasma cortisol at 60 min after 0.5 i.u. insulin/kg. There were significant increases in plasma FFA after hypoglycaemia in both groups of animals, but the rise was less pronounced in the neonates. A significant positive correlation was found between plasma adrenaline and FFA values. Hypoglycaemia also resulted in a significant rise in plasma lactate and a slow fall in alpha-amino nitrogen. These findings show that hypoglycaemia in the foal is followed by stimulation of the adrenal medullary component of the sympathetic system and by activation of the adrenal cortex with a number of consequent metabolic changes. The hypoglycaemic effects of insulin were more intense and the response more rapid in the older foals than in the neonates, which exhibited some degree of insulin resistance.