The effects of meptazinol in comparison with pentazocine, morphine and naloxone in a rat model of anaphylactic shock.

The actions of meptazinol, pentazocine, morphine and naloxone on the cardiovascular changes accompanying anaphylactic shock were evaluated in ovalbumin-sensitized anaesthetized rats. Pretreatment with meptazinol and pentazocine prevented the fall in mean arterial pressure associated with antigen challenge, whereas morphine and naloxone attenuated but did not completely prevent, this change. None of the drugs significantly altered the antigen-induced decreases in heart rate. All the drugs partially reversed the fall in mean arterial pressure when given after antigen challenge although the activity of naloxone was less marked. Pretreatment with reserpine prevented the restoration of blood pressure by all drugs. Additional experiments with meptazinol showed that pretreatment with phentolamine prevented its pressor action. In pithed non-sensitized rats the frequency-pressor response curve to splanchnic stimulation was shifted to the left by meptazinol and shifted to the right by pentazocine, but the changes were small Morphine and naloxone had no significant effects. It was concluded that opioid mixed agonist-antagonists reverse the cardiovascular changes associated with anaphylactic shock. These effects appear to be mediated by facilitation of sympathetic neurotransmission.