The endothelium is the production site of several potent vasoactive factors that contribute to the modulation of the vascular tone. Because hemoglobin-based oxygen carriers (HBOC) have been demonstrated to cause vasoconstriction and thereby increase arterial pressure by interacting with endothelium-derived factors such as nitric oxide and endothelin-1, we hypothesized that hemoglobin could penetrate into the endothelial cells. Therefore, we investigated the presence of hemoglobin into guinea pig aortic endothelial cells by immunohistochemical staining after exchange transfusion with a hemoglobin-based oxygen carrier. Despite the large molecular size of HBOC due to chemical modifications designed to prevent hemoglobin subunit dissociation and extravascular leakage, hemoglobin was detectable by immunohistochemical staining into the endothelial cells. These findings suggest that the vascular endothelial cells could uptake hemoglobin by endocytosis mechanisms or could help hemoglobin to cross the endothelial barrier toward media by transcytosis mechanisms. These findings are very important to lead future investigations to the mechanisms by which HBOC cause vasoconstriction.

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The cardiovascular effects of human albumin (Alb) and three human hemoglobin (Hb) solutions, dextran-benzene-tetracarboxylate Hb, alphaalpha-crosslinked Hb, and o-raffinose-polymerized Hb were compared in anesthetized rabbits undergoing acute isovolemic hemodilution with Hct reduction from 41.4 +/- 2.7 to 28.8 +/- 1.6%. The impact of the vasoconstricting properties of Hb was examined by measuring heart rate (HR), mean arterial pressure (MAP), abdominal aortic, and femoral arterial blood flow, vascular resistance (VR), and aortic distension during the first 3 h after hemodilution. The impact of the hemorheological parameters was assessed by measurements of hemodiluted blood viscosity. In contrast to Alb, the Hb solutions elicited an immediate increase in MAP (20-38%). The effects of Alb and Hb solutions on HR, as well as on aortic and femoral arterial blood flow, were similar. VR decreased with Alb (20-28%) and increased with all three Hb solutions (30-90%), but the MAP and VR rising trends were different with each Hb solution. Aortic distension decreased in Hb groups compared with the Alb group for the first 60 min. The viscosity of hemodiluted blood was similar for all groups at high shear rates but was dependent on the viscosity of the solutions at low shear rates. We conclude that the vasoconstriction elicited by the Hb solutions overrides the vasodilation associated with viscosity changes due to hemodilution and would be the major factor responsible to the cardiovascular changes.

Infusion of hemoglobin-based oxygen-carrying solutions (HBOCs) produce an immediate rise in blood pressure with most solutions, both in animals and humans, as a result of systemic and pulmonary vasoconstriction. Autoregulation of the O2 supply by the microvasculature has been proposed as a phenomenon involved in the vasoconstriction elicited by HBOCs. Nevertheless, little is known about the ability of various HBOCs to induce constriction in the microcirculation according to their specific physicochemical properties (viscosity, molecular weight, P50, etc.). This study was therefore designed to assess the effects of three HBOCs, that is, bis(3.5-dibromosalicyl) fumarate-crosslinked hemoglobin (alphaalpha-Hb), dextran-benzene-tetracarboxylate-conjugated hemoglobin (Hb-Dex-BTC) and o-raffinose-oligomerized hemoglobin (o-raffinose-Hb), on the vascular tone of rat mesenteric arterioles (diameter, 15-25 microm) viewed microscopically in moderate hemodilution conditions. The effects of HBOCs were compared to those elicited by a reference solution of hydroxyethyl starch (HES-200) infused in the same conditions. In each experimental group, a fall in arteriolar diameter was observed 2 min and 5 min after infusion of the solution. The maximum changes were observed in Hb-Dex-BTC and o-raffinose-Hb groups, in which diameter decreased from 6.9 +/- 0.5% and 5.2 +/- 0.7%, respectively, 2 min after infusion. The changes in arteriolar diameter induced by Hb-Dex-BTC and o-raffinose-Hb were significantly higher than those elicited by HES-200 and alphaalpha-Hb. In conclusion, our data indicate that moderate hemodilution with HBOCs induces instantaneous constriction in rat mesenteric arterioles, with amplitudes depending on both pharmacological and physicochemical properties of the hemoglobin solution infused.