ENDOTHELIN-1 IS NOT INVOLVED IN HEMOGLOBIN ASSOCIATED VASOACTIVITIES

This study tested the hypothesis: is hemoglobin (Hb) associated vasoco nstriction mediated by endothelins (ET)? Two millimeter segments of th e thoracic aorta with intact endothelium were obtained from Sprague-Da wley rats (200-350g) and suspended in oxygenated Krebs buffer (37C, pH 7.4). The vessels were equilibrated at 2g of imposed tension for 1 ho ur. After submaximal tone was induced with norepinephrine (50nM), the presence of functional endothelium was verified by an acetylcholine (3 3uM) dilation test. The vessel rings were then treated with either ET- 1 (3nM) or human stroma-free Hb (SFH; 2.2uM) and vascular response cha racteristics observed. Subsequently, the vessel rings were treated wit h BQ-123 (15uM), an ET-1 receptor antagonist, to test whether the ET-I pathway is involved in the Hb associated vasoconstriction. Both ET-I and SFH elicited contractory responses in aortic rings; the maximal te nsion increases at the doses tested were 54.8+/-8.5% and 39.2+/-22.5%, respectively, over the pretreatment values. The contractory response characteristics were, however, distinct; ET-I caused a slow (time to m aximal response; TMR=28.8+/-6.4 min, N=6) but prolonged contraction (> 30 min) while SFH caused a faster contraction (TMR=7.2+/-1.7 min, N=6) with a shorter duration (<30 min). The TMR of ET-1 treated rings were significantly longer than that of SFH (P<0.0005, t-test) Treatment wi th BQ-123 caused an immediate reversal of the SFH induced contraction but had no significant effect on the SFH induced contraction. If the H b associated vasoconstriction were mediated by ET-1, BQ-123 should hav e also reversed the contraction. These results suggest that ET-I pathw ay is not involved in the Hb mediated vasoconstriction.