Hypothermia enhances the effects of morphine on hormonal and histamine release.

1. The effects of intravenous (i.v.) morphine on adrenocorticotrophic hormone (ACTH), beta-endorphin (beta-END), total catecholamines (CA) and histamine (HIS) plasma concentrations, were determined in anaesthetized dogs at 30 degrees C and 37 degrees C. 2. Hypothermia initially increased CA levels by 29%, but the values returned to baseline after 2 h. Morphine (1 mg/kg, i.v.) produced a significant decrease in CA both at 37 degrees C and 30 degrees C (34% and 54%, respectively). Subsequent administration of naloxone (1 mg/kg, i.v.) significantly increased CA levels in both groups. 3. Hypothermia per se had no effect on ACTH, beta-END, and HIS concentrations. Morphine produced a significant increase in pituitary hormones and HIS, in hypothermic but not in normothermic animals. Morphine concentrations were significantly higher at 30 degrees C during the first 45 min. 4. The results suggest that the effects of morphine on hormonal and histamine release observed at 30 degrees C are concentration-dependent and related to changes in morphine pharmacokinetics.