Insulin enhances endothelial alpha(2)-adrenergic vasorelaxation by a pertussis toxin mechanism

To investigate whether insulin effect on endothelium is related to a specific signal transduction pathway or reflects a more generalized action of the hormone, we studied in aortic rings of Wistar-Kyoto (WKY) rats the effects of the hormone on endothelium-dependent regulations generated by acetylcholine, adenosine diphosphate, the selective alpha(2)-adrenergic agonist UK 14,304, and the calcium ionophore ionomycin. The responses were evaluated both in control conditions and after 30 minutes of exposure to three different levels of insulin (30, 100, and 500 mu U/mL). Insulin failed to modify the phenylephrine aortic contractions and the relaxations induced by acetylcholine, adenosine diphosphate, and ionomycin. Ln contrast, both 100 and 500 mu U/mL insulin were able to potentiate the UK 14,304-induced vasorelaxation (+96+/-19% and +91+/-12%, respectively). Pertussis toxin, which causes alpha(2)-adrenergic receptor G(i) uncoupling, reduced the alpha(2)-adrenergic vasorelaxation and prevented the insulin potentiation of the response to UK 14,304. Furthermore, in primary cultured aortic endothelial cells from WKY, we evaluated the conversion of [H-3]arginine to [H-3]citrulline in response to acetylcholine, ionomycin, and UK 14,304, both in control conditions and during insulin exposure. Again, insulin did not affect basal citrulline production or the increase induced by acetylcholine and ionomycin, whereas it potentiated the response to UK 14,304. Finally, in aortic rings of spontaneously hypertensive rats, insulin treatment (100 and 500 mu U/mL) was unable to enhance the az-adrenergic vasodilator response; in vascular endothelial cells from spontaneously hypertensive rats, insulin did not potentiate the increase in citrulline production evoked by UK 14,304. In conclusion, insulin selectively enhances alpha(2)-adrenergic endothelial vasorelaxation through a pertussis toxin-sensitive mechanism, by potentiating endothelial nitric oxide production. This vasorelaxant mechanism is altered in spontaneously hypertensive rats.