PITUITARY-ADRENAL ACTIVITY AND OPIOID RELEASE IN PONIES DURING THIOPENTONE HALOTHANE ANESTHESIA

The effect of thiopentone/halothane anaesthesia on the release of endo genous opioid, adrenocorticotrophin, arginine vasopressin, cortisol an d catecholamine was investigated in ponies. The contribution made by h alothane itself was studied by maintaining six ponies with a constant 1.2 per cent end tidal halothane concentration and five with a concent ration ranging between 0.8 and 1.2 per cent. Cardiorespiratory depress ion was more prolonged in the ponies receiving a constant 1.2 per cent end tidal halothane concentration than in those which received less h alothane. Plasma lactate concentration increased and haematocrit decre ased during halothane anaesthesia. The concentrations of met-enkephali n, dynorphin and catecholamines did not change and those of beta-endor phin, adrenocorticotrophin, arginine vasopressin and cortisol increase d during halothane anaesthesia. Halothane appeared to be a major stimu lus to pituitary adrenocortical activation because the adrenocortical secretion was proportional to the amount of halothane inhaled. beta-en dorphin increased proportionally more than adrenocorticotrophin and th eir plasma concentrations were not correlated, suggesting that they ha ve independent secretion mechanisms.