Naloxone reversal of morphine-induced peripheral vasodilatation

Naloxone is used to reverse the central nervous system amd respiratory depressant effects of morphine, but it is not known whether the narcotic antagonists also reverses the peripheral vasodilatation. Since the cutaneous vasodilatation induced by morphine may be hemodynamically important in cardiac patients, we studied the effect of naloxone on the morphine-induced vasodilatation in the cutaneous vascular bed of the hand using venous occlusion plethysmography. Morphine, 15 mg intravenously, caused a 70% reduction in hand vascular resistance. Given 18 min after morphine, naloxone, 0.8 mg intravenously, completely reversed the vasodilatation in 12 to 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.