Peripheral 5-HT2-like receptors. Can they be classified with the available antagonists?

Interactions between 5-hydroxytryptamine (5-HT) and the so-called 5-HT2 receptor antagonists ketanserin, spiperone, trazodone and methysergide were studied in isolated preparations of the rabbit aorta, rat jugular vein, and rat caudal artery. Trazodone and spiperone were apparently simple competitive antagonists since they produced antagonism that was surmountable over the concentration range studied and, in each tissue, their apparent affinity appeared to be independent of the antagonist concentration. Furthermore, concentration-ratios obtained with the two antagonists in combination suggested that antagonism was additive, implying mutual competition with a single population of 5-HT receptors. Ketanserin was a non-surmountable antagonist of 5-HT in the rat caudal artery and methysergide demonstrated surmountable, competitive antagonism only in the rabbit aorta. Antagonist dissociation constants estimated for apparently competitive interactions showed that ketanserin, spiperone and trazodone expressed affinities which differed according to the tissue used. In the case of trazodone, affinity estimates differed by as much as 12 fold. These discrepancies were independent of the 5-HT receptor agonist used and could not be attributed to an inadequate equilibration of the antagonist. These results can be interpreted in two ways: either the receptors in the different tissues are heterogeneous or the antagonists used here must be considered as unreliable probes for the classification of 5-HT2-like receptors.