Naloxone reversal of morphine-induced peripheral vasodilatation

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Abstract
Naloxone is used to reverse the CNS and respiratory depressant effects of morphine, but it is not known whether the narcotic antagonist reverses the peripheral vasodilatation. Since the cutaneous vasodilatation induced by morphine may be hemodynamically-important in cardiac patients, the effect of naloxone was studied on the morphine-induced vasodilatation in the cutaneous vascular bed of the hand using venous occlusion plethysmography. Morphine, 15 mg i.v., caused a 70% reduction in hand vascular resistance. Given 18 min after morphine, naloxone, 0.8 mg i.v., completely reversed the vasodilatation in 12-17 min. Saline placebo given 18 min after morphine induced no reversal; the vasodilatation lasted at least 35 min. Naloxone acts as a pure antagonist to these actions of morphine, since it has no actions on blood flow in the hand when given alone.