Melatonin potentiates NE-induced vasoconstriction without augmenting cytosolic calcium concentration

Because little is known of the intracellular mechanisms involved in the vasoconstrictor effect of melatonin (Mel), we examined the in vitro effects of Mel by using perfused cylindrical segments of the rat tail artery loaded with the intracellular Ca2+ concentration ([Ca2+]i)-sensitive fluorescent dye, fura 2. Mel (10-14 to 10-4 M) had no effect on baseline perfusion pressure or [Ca2+]i but increased, at submicromolar concentrations, the vasoconstrictor effect of norepinephrine (NE) (P = 0.0029).