Meloxicam, a Specific COX-2 Inhibitor, Does Not Enhance the Isoflurane Minimum Alveolar Concentration Reduction Produced by Morphine in the Rat

Unlabelled: A synergistic effect of nonselective cyclooxygenase (COX) inhibitors on morphine-induced decrease of isoflurane minimum alveolar concentration (MAC(ISO)) has been observed in the rat. We studied the influence of specific COX-2 inhibitors on this decrease of MAC. Sixty-four female rats were anesthetized with isoflurane in oxygen. The animals were grouped into saline solution, aspirin (30 mg/kg), morphine (1 mg/kg), morphine (1 mg/kg) + aspirin (30 mg/kg), meloxicam (1 and 3 mg/kg), and morphine (1 mg/kg) + meloxicam (1 and 3 mg/kg). Then the MAC(ISO) was determined from alveolar gas samples at the time of tail clamp. The groups treated with saline solution, aspirin, and 1 and 3 mg/kg meloxicam did not express any statistically relevant changes among them. The administration of morphine + meloxicam 1 or 3 mg/kg significantly reduced the MAC(ISO) just as in the group where only morphine was administered (morphine 1.35% +/- 0.07%, morphine + 1 mg/kg meloxicam 1.36% +/- 0.04%, and morphine + 3 mg/kg meloxicam 1.37% +/- 0.08%). The greatest reduction of MAC(ISO) was after administration of morphine + aspirin (1.19% +/- 0.05%). The administration of meloxicam does not potentiate the morphine-induced decrease of MAC(ISO) in the rat.

Implications: A synergistic effect between morphine and aspirin on isoflurane minimum alveolar concentration has been observed in the rat--an effect that does not occur between morphine and meloxicam.