Microvascular responses to hemodilution with Hb vesicles as red blood cell substitutes: Influence of O₂ affinity

Phospholipid vesicles encapsulating purified hemoglobin (HbV) were developed to provide O₂-carrying capacity to plasma expanders. Microvascular perfusion was determined for HbV with different O₂ affinity (P₅₀ = 9, 16, and 30 mmHg) prepared by coencapsulating pyridoxal 5'-phosphate (PLP) at the molar ratios of [PLP]/[Hb] = 0, 0.5, and 3, respectively (cf. hamster blood, P₅₀: 28 mmHg), and suspended in 8 g/dl human serum albumin (HSA). Eighty percent of the red blood cell (RBC) mass of conscious Syrian golden hamsters fitted with dorsal skinfold windows was substituted with either of the HbV- HSA suspensions, washed hamster RBC suspended in HSA(RBC-HSA), and HSA alone. All three HbV-HSA groups and RBC-HSA groups showed stable blood pressure and heart rate, which could not be sustained with HSA alone. Only the HbV (P₅₀ = 9)-HSA group showed an increase in arterial O₂ tension (89.8 ± 14.7 mmHg, baseline 58.4 ± 4.0 mmHg) because of hyperventilation, and microvascular perfusion was decreased, indicating that facilitated O₂ unloading of HbV by decreasing the O₂ affinity (increasing P₅₀) with PLP as an allosteric effector is important. Microvascular perfusion and microvascular and interstitial O₂ tensions in the HbV (P₅₀ = 16 and 30)-HSA groups were significantly higher than those in the HSAgroup. The O₂ release rate from the HbV was 18-32 s^-1 vs. 4.4 s^-1 for RBC. Functional capillary density was improved from 17 to 41% on average by decreasing P₅₀ from 30 to 16 mmHg, which appears to be an optimal value for the P₅₀ in this system.