Opioids preserve the adrenal medullary response evoked by severe hemorrhage: studies on adrenal catecholamine and met-enkephalin secretion in halothane anesthetized cats.

Possible modulatory effects of mu-, delta-, and kappa-receptor agonists on the concurrent adrenal secretion of catecholamines and met-enkephalin evoked by staged hemorrhage were examined in four groups of cats (n = 5 in each group) anesthetized with halothane (1 MAC). Group I received saline, group II received the mu-agonist sufentanil (25 micrograms/kg i.v., followed by a maintenance infusion), group III received the delta/mu agonist metkephamid (3 mg/kg i.v.), and group IV the kappa agonist U50488H (3.5 mg/kg i.v.). Samples for norepinephrine, epinephrine, dopamine, and met-enkephalin were taken simultaneously from the adrenal vein, femoral vein, and femoral artery at baseline, after drug administration, and after induction of 25% and 50% hemorrhage. In cats receiving saline, 25% hemorrhage resulted in a significant decline in mean arterial blood pressure (MABP) and no change in adrenal secretion. Fifty percent hemorrhage evoked no significant further fall in MABP, but led to prominent increases in adrenal vein hormone levels (norepinephrine, 30-fold; dopamine, 14-fold; and epinephrine, ten-fold) as compared to post-saline values. During the pre-hemorrhage baseline state, administration of sufentanil evoked a significant six- to 20-fold rise in adrenal vein catecholamine and met-enkephalin levels, whereas the administration of metkephamid and U50488H produced no change in adrenal secretion and a decrease in MABP. After 25% and 50% hemorrhage, there was no difference in adrenal vein hormone levels in cats receiving the mu-, delta-, or kappa-agonists compared to those receiving saline. No differences were observed in the different treatment groups with regard to the proportional levels of catecholamines and met-enkephalin in the adrenal vein during the course of the experiment. The authors conclude that opioids are not involved in the regulation of the secretory adrenal medullary response evoked by hemorrhage, and that the systems involved in mediating these cardiovascular reflexes differ pharmacologically from those systems mediating the autonomic response evoked by pain.