ALPHA ADRENERGIC ACTIVATION AND HEMOGLOBIN MEDIATED CONTRACTION IN THE ISOLATED RAT THORACIC AORTA

A primary mechanism for Hb mediated vascular contraction appears to be Hb scavenging of endothelium derive nitric oxide (NO), a potent vasodilator. In isolated rat thoracic aorta, however, Hb elicits contraction only after precontraction. The present study investigated a possible role of the alpha adrenergic activation in the Hb mediated contraction. Thoracic aortic rings harvested from normal male SD rats were prepared in a tissue bath and isometric tension changes were evaluated. In vessel rings precontracted with 50nM norepinephrine (NE), 1 μM Hb produced an additional 21.8±13.2% increase in tension. Pretreatment with 70nM phentolamine, an alpha adrenergic antagonist, prevented the 50nM NE induced contraction. In these vessels, subsequent treatment with 2–4μM Hb did not elicit contraction. In vessel rings precontracted with 37mM KCl, 2μM Hb produced an additional 21.8±20.1% tension increase (P<0.05). Pretreatment with phentolamine did neither prevent KCl induced contraction nor affect subsequent Hb med...