Effects of morphine on noradrenaline release from cerebral and peripheral vascular smooth muscle.

Morphine reduced the [3H]noradrenaline uptake in cat cerebral and femoral arteries and induced a dose-dependent tritium release from these vessels prelabelled with this amine, which was diminished in Ca2+-free medium. The opioid decreased the radioactivity release induced by electrical field stimulation as well as its potentiation by tetraethylammonium. These effects were unaffected by naloxone. These results indicate: the reduction by morphine of tritium release from both kinds of vessels is through a mechanism unrelated to opiate receptors, and the noradrenaline release elicited by morphine could be due in part to inhibition of its reuptake.