Interaction between 5-hydroxytryptamine and other vasoconstrictor substances in the isolated femoral artery of the rabbit; effect of ketanserin (R 41 468)

Experiments were designed to determine whether or not ketanserin (R 41 468) antagonizes the augmentation by 5-hydroxytryptamine of contractions evoked in isolated arteries by non-adregergic vasoconstrictor substances. Rings of rabbit femoral arteries were studied under isometric conditions in organ chambers filled with Krebs-Henseleit solution (37°C). Ketanserin, unlike methysergide and LSD, was devoid of agonistic properties. It competitively antagonized contractile responses to 5-hydroxytryptamine and, at higher concentrations, to histamine. 5-Hydroxytryptamine amplified the contractions evoked by threshold concentrations of histamine, angiotensin II and prostaglandin F_2α; in all three cases, the amplification was antagonized by comparable concentrations of ketanserin. These experiments indicate that the interaction of 5-hydroxytrptamine with S₂-receptors of the vascular smooth muscle cells is essential to allow the expression of the monoamine-induced amplification of the response to other vasoconstrictor substances. The inhibition by ketanserin of the amplifying effect of 5-hydroxytryptamine on vascular responses may help explain the antihypertensive properties of the compound.