Modulation of alpha-adrenergic-induced contractions by endothelium-derived relaxing factor in rat aorta.

1. In rat thoracic aorta, endothelium removal produced a significant increase of the maximal contraction (Emax) and of the pD2 value (-log ED50) induced by norepinephrine, phenylephrine and clonidine, and did not affect the maximal contractile response to 70 mM KCl. 2. Clonidine did not induce a contraction in aorta with intact endothelium, but after endothelium removal, the contractile response was 94.8% of the Emax produced by norepinephrine in aorta with endothelium. 3. Pre-incubation with methylene blue (10(-5) M) and hemoglobin (0.02%), which inhibit EDRF effects, produced the same effects as the mechanical removal of endothelium on the contractile responses to alpha-adrenergic agonists. 4. These results suggest that EDRF formation and release is an important factor in the modulation of alpha-adrenergic-induced vasoconstriction.