Echanisms for Vasoconstriction and Decreased Blood Flow Following Intravenous Administration of Cell-Free Native Hemoglobin Solutions

Acellular free hemoglobin-based oxygen carriers (HBOC) are being developed as red cell substitutes. However, following intravenous administration of some HBOC, decreased systemic blood flow and decreased functional capillary density have been observed. In isolated blood vessels, hemoglobin (Hb) in solution free of erythrocyte membranes has been shown to elicit vascular contraction. Therefore, the decreased blood flow and functional capillary density may be due to inherent vasoactive property of native Hb. There are two plausible mechanisms for the Hb-mediated vasoconstriction: nitrosylation of heme-irons and S-nitrosation of reactive β-chain cysteines (Cys93β). In this study, we investigated whether Hb Cys93β thiols play a role in Hb-mediated vascular contraction using functional bioassays with isolated rat thoracic aorta. To better define the roles of globin thiols and heme-iron, Hbs modified at the heme-iron and/or Cys93β sites were prepared and their vasoactivities tested. In addition, vasoactivities of natural heme proteins with heme and/or cysteine sites unavailable for NO reaction were also examined.