Figure. Concentration-dependent activation of HRH1 by his­ta­mine. Reporter cells were transfected with either the expression plasmid for his­ta­mine receptor H1 (HRH1) or the mock plasmid and treated with various concentrations of his­ta­mine. Data points shown are the mean ± SEM of an experiment performed in triplicate, and the curve is a fit to Hill equation with an EC50 of 17 nM.