An opioid basis for early-phase isoflurane-induced hypotension in rats.

This study was conducted to more clearly delineate the possible role of endogenous opioid receptors and opioid peptides in general anesthesia-associated hypotension in rats. Exposure to 2% isoflurane in oxygen produced a triphasic change in mean arterial pressure (MAP), including an early phase in which MAP fell by -28.4 +/- 2.2%. The magnitude of this early-phase hypotension was attenuated in rats pretreated with intravenous (i.v.) mu-subtype-selective doses of either naloxone or methylnaloxone but not central doses of the selective mu-opioid antagonist beta-funaltrexamine. This early hypotensive phase was also reduced following i.v. pretreatment with antiserum against methionine-enkephalin but not beta-endorphin. These findings suggest that early-phase isoflurane-induced hypotension may be due to activation of peripheral mu-opioid receptors by an endogenous opioid peptide, possibly related to methionine-enkephalin.