Dose–response curves for increases in perfusion pressure by tryptamine of rat isolated perfused mesenteric arterial bed. Doses of tryptamine were administered as individual boluses (nmoles/100 μl). Each response is the mean ± S.E.M. increase in perfusion pressure. A. Dose–response curves for vasoconstrictor responses to tryptamine in the absence (□) and repeated in the presence (■, n = 3) of ritanserin (100 pM). * Significantly different from the absence of ritanserin, P < 0.05. B. Dose–response curves for tryptamine with intact endothelium (□, n = 4) and in de-endothelialised (■, n = 4) mesenteric arterial bed. * Significantly different from intact endothelium, P < 0.05. C. Dose–response curves in the absence (□) and presence (■, n = 7) of l-NAME (100 μM). All points significantly different between tryptamine alone and with l-NAME, P < 0.05. D. Dose–response curves in the presence of ritanserin (◆, 100 pM) and in the additional presence (▲, n = 3) of l-NAME (100 μM). * Significantly greater than ritanserin alone, P < 0.05, ** P < 0.01.