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Oxygen release from low and normal P₅₀ Hb vesicles in transiently occluded arterioles of the hamster window model

¹Advanced Research Institute for Science and Engineering, Waseda University, Tokyo, Japan; and ²Department of Bioengineering, University of California-San Diego, and ³La Jolla Bioengineering Institute, La Jolla, California

Submitted 27 November 2004 ; accepted in final form 24 January 2005

A phospholipid vesicle encapsulating Hb [Hb vesicle (HbV)] has been developed as a transfusion alternative. One characteristic of HbV is that the O₂ affinity [PO₂ at which Hb is 50% saturated (P₅₀)] of Hb can be easily regulated by the amount of the coencapsulated allosteric effector pyridoxal 5'-phosphate. In this study, we prepared two HbVs with different P₅₀s (8 and 29 mmHg, termed HbV₈ and HbV₂₉, respectively) and observed their O₂-releasing behavior from an occluded arteriole in a hamster skinfold window model. Conscious hamsters received HbV₈ or HbV₂₉ at a dose rate of 7 ml/kg. In the microscopic view, an arteriole (diameter: 53.0 ± 6.6 µm) was occluded transcutaneously by a glass pipette on a manipulator, and the reduction of the intra-arteriolar PO₂ 100 µm down from the occlusion was measured by the phosphorescence quenching of preinfused Pd-porphyrin. The baseline arteriolar PO₂ (50–52 mmHg) decreased to about 5 mmHg for all the groups. Occlusion after HbV₈ infusion showed a slightly slower rate of PO₂ reduction compared with that after HbV₂₉ infusion. The arteriolar O₂ content was calculated at each reducing PO₂ in combination with the O₂ equilibrium curves of HbVs, and it was clarified that HbV₈ showed a significantly slower rate of O₂ release compared with HbV₂₉ and was a primary source of O₂ (maximum fraction, 0.55) overwhelming red blood cells when the PO₂ was reduced (e.g., <10 mmHg) despite a small dosage of HbV. This result supports the possible utilization of Hb-based O₂ carriers with lower P₅₀ for oxygenation of ischemic tissues.

blood substitutes; artificial red blood cells; occlusion; microhemodynamics; liposome

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