The role of nitric oxide synthase in reduced vasocontractile responsiveness induced by prolonged alpha1-adrenergic receptor stimulation in rat thoracic aorta.

Abstract

1 Prolonged exposure (6-12 h) of rat aorta to alpha1-adrenergic receptor (alpha(1)AR) agonist phenylephrine (Phe) leads to a decrease in alpha(1)AR-mediated vasoconstriction. This reduced responsiveness to alpha(1)AR stimulation was strongly dependent on the intactness of the endothelium. 2 We examined the effect of Phe on nitric oxide synthase (NOS) activity by measuring the conversion of [(3)H]L-arginine to [(3)H]L-citrulline in rat aorta or in endothelial cells isolated from rat aorta. Phe stimulation increased NOS activity in control aortas. This response was antagonized by prazosin. However, Phe increased neither the activity of NOS nor intracellular Ca(2+) in the isolated endothelial cells from the control aortas, whereas acetylcholine (Ach) was able to stimulate both responses in these cells. This result suggests that Phe stimulates alpha(1)AR on vascular smooth muscle cells and has an indirect influence on endothelial cells to increase NOS activity. 3 In Phe-exposed aortic rings, basal NOS activity was found to have increased compared to vehicle-exposed control rings. Stimulation with Phe or Ach caused a small increase over basal NOS activity in these preparations. Prolonged exposure to Phe also caused an enhancement of Ach-mediated vasorelaxation in rat aorta. 4 Immunoblot and reverse transcription-polymerase chain reaction experiments showed that prolonged exposure of rat aorta to Phe resulted in an increased expression of eNOS, but not iNOS. This increase was antagonized by nonselective antagonist prazosin. Immunohistochemical staining experiments also showed that expression of eNOS increased in endothelial cells after Phe exposure of the aortas. 5 These results, all together, showed that prolonged exposure of rat aorta to alpha(l)AR agonist Phe enhanced the expression of eNOS and basal NOS activity, which probably causes a decreased vasocontractile response to Phe or to other agonists such as 5HT (5-hydroxytryptamine) in rat aorta. 6 This phenomenon can be considered more as a functional antagonism of vasocontractile response to agonists mediated by endothelium than a specific desensitization of alpha(1)AR-mediated signalling in vascular smooth muscle cells.