On the interaction of histamine with the tryptaminergic smooth-muscle receptors

The effect of histamine (1 x 10(-8) to 1 x 10(-3) M) on the contractile effects of 5-hydroxytryptamine (1 x 10(-10) to 1 x 10(-3) M) has been studied in experiments on isolated smooth-muscle preparations (rabbit duodenum and guinea-pig trachea). Applied in lower concentrations in both smooth-muscle preparations, histamine does not influence essentially or slightly lowers the contractions induced by the low concentrations of 5-hydroxytryptamine (5-HT). At the same time the contractile effects of the high 5-HT concentrations are reduced by about 15-30 per cent. Increasing the histamine concentration in the organ bath reveals a potentiation of the contractile responses caused by the low 5-HT concentrations by about 10-25 per cent. The contractile effects of the high 5-HT concentrations are reduced by 10-30 per cent on the background of the high histamine concentrations in advance. The results obtained show that histamine influences the 5-HT-induced contractions most probably through allosteric interaction with the tryptaminergic receptors in the smooth-muscle organs studied.