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

A phospholipid vesicle encapsulating Hb [Hb vesicle (HbV)] has been developed as a transfusion alternative. One characteristic of HbV is that the O(2) affinity [Po(2) at which Hb is 50% saturated (P(50))] 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 P(50)s (8 and 29 mmHg, termed HbV(8) and HbV(29), respectively) and observed their O(2)-releasing behavior from an occluded arteriole in a hamster skinfold window model. Conscious hamsters received HbV(8) or HbV(29) at a dose rate of 7 ml/kg. In the microscopic view, an arteriole (diameter: 53.0 +/- 6.6 mum) was occluded transcutaneously by a glass pipette on a manipulator, and the reduction of the intra-arteriolar Po(2) 100 mum down from the occlusion was measured by the phosphorescence quenching of preinfused Pd-porphyrin. The baseline arteriolar Po(2) (50-52 mmHg) decreased to about 5 mmHg for all the groups. Occlusion after HbV(8) infusion showed a slightly slower rate of Po(2) reduction compared with that after HbV(29) infusion. The arteriolar O(2) content was calculated at each reducing Po(2) in combination with the O(2) equilibrium curves of HbVs, and it was clarified that HbV(8) showed a significantly slower rate of O(2) release compared with HbV(29) and was a primary source of O(2) (maximum fraction, 0.55) overwhelming red blood cells when the Po(2) was reduced (e.g., <10 mmHg) despite a small dosage of HbV. This result supports the possible utilization of Hb-based O(2) carriers with lower P(50) for oxygenation of ischemic tissues.