Endocrine and metabolic effects of hypotension or halothane inhalation in sheep anaesthetized with pentobarbital

We have studied the mechanism whereby halothane induces adrenocortical activity in eight sheep anaesthetized twice for 2 h with pentobarbital. During the second hour they received infusion of nitroprusside to lower mean arterial pressure by 40 mm Hg (group NP) or on another occasion they inhaled 0.5% halothane (group HAL). Plasma concentrations of cortisol, adrenocorticotrophic hormone (ACTH), arginine vasopressin (AVP), glucose and lactate were measured. Infusion of nitroprusside increased cortisol concentrations from mean 44 (SD 33) to 256 (131) nmol litre-1 (P < 0.05), AVP from 19 (14) to 174 (102) pmol litre-1 (P < 0.05) and ACTH from 30 (17) to 54 (32) pmol litre-1. Glucose and lactate concentrations did not change significantly. After 60 min of halothane administration, there were fewer effects: plasma cortisol increased from 47 (39) to 63 (44) nmol litre-1 (P > 0.05) and AVP from 14 (11) to 32 (29) pmol litre-1 (P > 0.05) while ACTH decreased from 32 (13) to 25 (10) pmol litre-1. The AUC60-120 values for all three hormones were significantly lower in group HAL than in group NP (P < 0.01). Glucose was unchanged but lactate concentration decreased. These results support, but do not prove, the hypothesis that hypotension is the main stimulus to pituitary-adrenocortical activity during halothane anaesthesia in sheep. A direct effect of nitroprusside cannot be ruled out.