Threshold phenomena and interactions between receptors.

Threshold or subthreshold concentrations of serotonin can augment contractile responses of isolated blood vessels to other vasoconstrictor agents. This report presents a theoretical analysis of threshold facilitation and concludes that the amplifying effect of serotonin may result from the removal of a response threshold. Physiological or pathophysiological alterations in the magnitude of this response threshold between different effector systems or in the efficacy of the vasoconstrictor agonist may account for the variability in the amplifying effect of serotonin observed in vivo and in vitro.