The role of vascular endothelium in carbon dioxide-induced vascular relaxation

The effects of hypercapnia(Pco_2 91±2mmHg) on the contraction induced by norepinephrine in isolated rat aorta were studied. Isolated vascular rings obtained from rat aorta were mounted in the 20ml muscle shamber under the isometric sondition. The contractile concentration-effect curve for norepinephrine was altered to the right and maximal developed tension was significantly attenuated 28% by hypercapnia in rings with endothelium. By removal of endothelium, however, the curve shifted to the right without inhibition of maimal developed tension during hypercapnia compared with rings without endothelium during normocapnia. Hemoglobin abolished the depression of maximal developed tension caused by hypercapnia. The curve shifted to the right in hemoglobin pretreated rings with endothelium during hypercapnia compared with hemoglobin pretreated rings with endothelium during normocapnia. Indomethacin did not alter norepinephrine-induced contractions. These results suggest that endothlium-derived relaxing factor plays an important role in the attenuation of norepinephrine-induced contraction with hypercapnia.