The amplifying effect of serotonin on responses of isolated arteries of rabbits to adrenergic stimuli

Experiments were designed to determine the possible involvement of calcium mobilization on the amplifying effect of 5-hydroxytryptamine (5-HT) on cont ractions induced in isolated arteries by transmural nerve stimulation and e xogenous noradrenaline. Ring segments of the pulmonary and mesenteric arter ies of rabbits were studied under isometric conditions in organ baths fille d with a Krebs solution. 5-HT, at threshold concentration (10(-8) mol/L), a mplified the contractile responses evoked by nerve stimulation and noradren aline in both types of arteries. The magnitude of amplification was markedl y higher in mesenteric than in pulmonary artery segment. The calcium antago nist diltiazem reduced noradrenaline-induced contractions. 5-HT did not aff ect either the diltiazem-resistant component of noradrenaline-contraction o r the residual noradrenaline-contraction in Ca2+-free solution. These resul ts support an assumption that the amplifying effect of 5-HT is exerted on t he component of the noradrenaline-contraction that is dependent on the entr y of extracellular calcium. The process of amplification of adrenergic cont ractions appears to be due to an activation of 5-HT2 subtype receptors that are coupled to an increase in Ca2+ influx into the Vascular smooth muscle cells.