5-Hydroxytryptamine receptor activity of the dopamine receptor agonist fenoldopam in canine tracheal smooth muscle.

Fenoldopam is a new vasodilator undergoing clinical trials for the treatment of hypertensive emergencies. Its pharmacologic effects result from activation of vascular dopamine-1 receptors. In canine tracheal smooth muscle strips, fenoldopam caused a concentration- and calcium-dependent increase in tension, which was not antagonized by atropine, indomethacin or the dopamine-1 receptor antagonist, SCH 23390. The EC50 (1.89 x 10(-6) M) exceeded that of serotonin or acetylcholine (8.38 x 10(-8) and 8.25 x 10(-8) M, respectively). Maximum tension was similar for fenoldopam and serotonin (11.6 +/- 1.5 g, n = 7 and 13.8 +/- 0.8 g, n = 24; P greater than .2) and considerably greater for acetylcholine (20.5 +/- 1.3 g, n = 14; P less than .005). The serotonin antagonists ketanserin and methysergide reversed completely the effect of fenoldopam (5 x 10(-7) M) with IC50 values of 2.5 x 10(-9) and 2.7 x 10(-9) M, respectively. Phentolamine, rauwolscine and chlorpheniramine were also effective, but they were less potent (IC50 values 6.6 x 10(-8), 1.0 x 10(-7) and 1.7 x 10(-7) M, respectively). By contrast, only very high concentrations (IC50, 5.3 x 10(-5) M) of terazosin produced an inhibition of fenoldopam-induced tension increases. The effect of antagonists could be overcome by increasing the fenoldopam concentration. Experiments performed on strips precontracted with serotonin (5 x 10(-8) M) revealed a very similar order of potency for the five antagonists. The addition of serotonin did not increase the tension produced by supramaximal concentrations of fenoldopam (and vice-versa), whereas acetylcholine increased tension further.(ABSTRACT TRUNCATED AT 250 WORDS)