Are the Vascular Effects of Naloxone Attributable to the Preservatives Methyl- and Propylparaben?

In vitro, the Nalonee® preparation of naloxone caused a concentration-dependent relaxation of human pial cortical arteries contracted by potassium, noradrenaline, serotonin, prostaglandin F_2α (PGF_2α), and haemorrhagic cerebrospinal fluid, or inhibited contractions elicited by these agents. However, the preservatives in the Nalonee preparation, methyl- and propylparaben, had similar effects. Pure naloxone alone had no effect on potassium or PGF_2α-induced contractions. It is suggested that the relaxant effects on vascular smooth muscle of Nalonee can be attributed to the alkylparabens rather than to naloxone. The pronounced relaxations induced by the alkylparabens had a rapid onset, and they were stable and could easily be cleared after rinsing.