The effects of alpha-adrenoceptor antagonists on contractile responses to 5-hydroxytryptamine in dog saphenous vein.

1 5-Hydroxytryptamine (5-HT) contracted isolated saphenous vein strips of the dog, producing a biphasic concentration-effect curve. The first phase occurred with low concentrations of 5-HT (1.0 X 10(-8) TO 5.0 X 10(-6) mol/l) with a plateau between 1.0 x 10(-6) mol/l and 1.0 x 10(-5) mol/l. The second phase occurred with high concentrations of 5-HT (greater than 1.0 x 10(-5) mol/l). 2 The alpha-adrenoceptor antabonists, phentolamine (5.0 x 10(-8) to 5.0 x 10(-7) mol/l), labetalol (1.0 x 10(-6) to 1.0 x 10(-5) mol/l) and thymoxamine (1.0 x 10(-6) to 1.0 x 10(-5) mol/l), antagonized responses to high concentrations of 5-HT but responses to low concentrations of 5-HT were not antagonized. 3 The effects of high concentrations of 5-HT were antagonized by cocaine (1.0 x 10(-6) to 1.0 x 10(-5) mol/l) and were not evident in veins removed from dogs pretreated with syrosingopine. 5-HT receptors and that high concentrations of 5-HT also act indirectly on alpha-adrenoceptors by displacing noradrenaline from neuronal stores.