Interconversion into a low active state protects vascular 5-HT2-receptors against irreversible antagonism by phenoxybenzamine.

Recently, Kaumann and Frenken (1985) proposed an allosteric model of vascular 5-HT2-receptors. We now present experiments in both bovine coronary and pulmonary artery, using the method of irreversible receptor occlusion, that support and extend the model. 1) Phenoxybenzamine was found to cause irreversible antagonism of the effects of 5-hydroxytryptamine (5-HT). Maximal contractile effects induced by 5-HT were depressed and, with further receptor occlusion, concentration-effect curves for 5-HT became biphasic. The high-sensitivity and the low-sensitivity component of the curve for 5-HT consisted of quickly and slowly developing contractions, respectively. 2) Biphasic concentration-effect curves for 5-HT after receptor occlusion were shifted to the right in non-parallel manner by ketanserin and became monophasic with an unexpected partial restoration of maximal responses to 5-HT. The magnitude of the shift of the partially restored concentration-effect curve for 5-HT by ketanserin after receptor occlusion by phenoxybenzamine is consistent with an interaction of ketanserin with 5-HT2-receptors. 3) Preincubation with methysergide before phenoxybenzamine-treatment followed by washout of both drugs, and subsequent incubation with ketanserin completely prevented a depression of 5-HT-induced effects by phenoxybenzamine. 4) Estimates for the equilibrium dissociation constant of 5-HT for the 5-HT2-receptor derived from fast developing contractions range from 0.1 mumol/l to 0.4 mumol/l. 5) The results are consistent with a model of two interconvertible states of the 5-HT2-receptor. Phenoxybenzamine occludes the 5-HT2-receptor in the R-state but not in the R'-state. The low active R'-state of the 5-HT2-receptor appears to pre-exist in the absence of drugs and is not affected by phenoxybenzamine. By converting R' into R ketanserin restores partially the response to 5-HT after occlusion of the R-state by phenoxybenzamine. Methysergide prevents the 5-HT2-receptor occlusion induced by phenoxybenzamine indirectly by favouring isomerisation into the R'-state.