INTERACTIONS BETWEEN 5-HYDROXYTRYPTAMINE, NORADRENALINE AND THE THROMBOXANE-A(2) MIMETIC U44069 IN THE MARMOSET ISOLATED AORTA

1. The effects of 5-hydroxytryptamine (5-HT) in the absence and presen ce of noradrenaline (NA) or the thromboxane-A(2) mimetic, U44069, were investigated in ring preparations of marmoset aorta. 2. 5-HT(0.001-10 mu mol/L) produced little or no contractile response in preparations at basal tone. When the tone was elevated to 50% of maximum with NA th e predominant response to 5-HT was relaxation. The 5-HT2 receptor anta gonist LY53857 (0.1 mu mol/L) unmasked a contractile response to low c oncentrations of 5-HT (0.01-1.0 mu mol/L) and reduced relaxation to hi gh concentrations of 5-HT (1.0-10 mu mol/L) in vessels precontracted w ith NA. 3. In U44069-contracted vessels, 5-HT was contractile in the r ange 0.01-1 mu mol/L and relaxant in concentrations of 6.0-10.0 mu mol /L. Ketanserin (1.0 mu mol/L) had no effect on the contraction or rela xation to 5-HT. 4. The relaxant response to 5-HT was not significantly diminished in endothelium-imp aired arteries. 5. In conclusion, 5-HT exerts complex inhibitory and excitatory actions on the marmoset aorta . The inhibitory actions are not endothelium-dependent and the excitat ory actions do not appear to involve the 5-HT2 receptor.