Melatonin- and temperature-related changes in adrenoceptor stimulation of canine saphenous veins

Experiments were designed to determine whether or not melatonin causes temperature-dependent changes in adrenoceptor sensitivity in canine saphenous veins. Dogs were housed at 22°C with a 12/12 light/dark cycle (5000 Kelvin) for two weeks. Saphenous veins were removed, cut into rings, and suspended for the study of isometric force in organ chambers. The endothelium was removed deliberately from some rings. After an equilibration period of 40 minutes, a maximal response to norepinephrine was obtained. Concentration-response curves were obtained to an alpha₁-and alpha₂-adrenergjc agonists in the absence and presence of melatonin (10^-9 through 10^-7 M) 37°C and 24°C. Melatonin differentially affected alpha adrenoceptors: inhibiting at 10^-6 M contractions to an alpha₂-agonist and augmenting at 10^-9 M an alpha₁-agonist in rings with but not without endothelium. These effects were not temperature-dependent. These results suggest that melatonin may dose-dependently and differentially modify sensitivity of alpha- adrenoceptors through endothelium-dependent mechanisms in canine saphenous veins.