Local regulation of microvascular blood flow is a complex process in which the needs of the tissue must be communicated to the vasculature, enabling the appropriate matching of O2 supply to demand. We hypothesize that the red blood cell is not only the major O2 carrier but also serves as an O2 sensor and affecter of changes in O2 delivery via its release of ATP, which subsequently binds to P2y receptors on the vascular endothelium, altering vessel caliber. Using the hamster as a model, we determined that the efflux of ATP from red blood cells after exposure to low-PO2 (PO2 = 17 +/- 6 mmHg) and low-pH (pH = 7.06 +/- 0.07) solutions was significantly (P < 0.01) greater than that after exposure to normoxic, normal pH (PO2 = 87 +/- 4; pH = 7.38 +/- 0.04) solutions, indicating that two factors that are associated with an impaired O2 supply relative to demand increase the release of ATP from the red blood cell. To ascertain whether ATP alters vascular caliber, we applied 10(-6) M ATP intraluminally to arterioles of the retractor muscle, using a micropressure system. Vessel diameter increased 8 and 10%, 140 +/- 60 microns upstream of the site of infusion after 50- and 500-ms pulses, respectively. Application of ATP to arteriolar and venular capillaries induced a 31 and 81% increase in red blood cell supply rate, respectively. These results support our hypothesis that the red blood cell is more than just an O2 carrier and has a direct role in the regulation of vascular tone.

The microcirculation is a complex and integrated system that supplies and distributes oxygen throughout the tissues. The red blood cell (RBC) facilitates convective oxygen transport via co-operative binding with hemoglobin. In the microcirculation oxygen diffuses from the RBC into neighboring tissues, where it is consumed by mitochondria. Evidence suggests that the RBC acts as deliverer of oxygen and 'sensor' of local oxygen gradients. Within vascular beds RBCs are distributed actively by arteriolar tone and passively by rheologic factors, including vessel geometry and RBC deformability. Microvascular oxygen transport is determined by microvascular geometry, hemodynamics, and RBC hemoglobin oxygen saturation. Sepsis causes abnormal microvascular oxygen transport as significant numbers of capillaries stop flowing and the microcirculation fails to compensate for decreased functional capillary density. The resulting maldistribution of RBC flow results in a mismatch of oxygen delivery with oxygen demand that affects both critical oxygen delivery and oxygen extraction ratio. Nitric oxide (NO) maintains microvascular homeostasis by regulating arteriolar tone, RBC deformability, leukocyte and platelet adhesion to endothelial cells, and blood volume. NO also regulates mitochondrial respiration. During sepsis, NO over-production mediates systemic hypotension and microvascular reactivity, and is seemingly protective of microvascular blood flow.

Through oxygen-dependent release of the vasodilator ATP, the mobile erythrocyte plays a fundamental role in matching microvascular oxygen supply with local tissue oxygen demand. Signal transduction within the erythrocyte and microvessels as well as feedback mechanisms controlling ATP release have been described. Our understanding of the impact of this novel control mechanism will rely on the integration of in vivo experiments and computational models.

Inherent in the remote organ injury caused by sepsis is a profound maldistribution of microvascular blood flow. Using a 24-h rat cecal ligation and perforation model of sepsis, we studied O₂ transport in individual capillaries of the extensor digitorum longus (EDL) skeletal muscle. We hypothesized that erythrocyte O₂ saturation (So ₂) levels within normally flowing capillaries would provide evidence of either a mitochondrial failure (increased So ₂) or an O₂ transport derangement (decreased So ₂). Using a spectrophotometric functional imaging system, we found that sepsis caused 1) an increase in stopped flow capillaries (from 10 to 38%, P < 0.05), 2) an increase in the proportion of fast-flow to normal-flow capillaries ( P< 0.05), and 3) a decrease in capillary venular-end So ₂ levels from 58.4 ± 20.0 to 38.5 ± 21.2%, whereas capillary arteriolar-end So ₂levels remained unchanged compared with the sham group. Capillary O₂ extraction increased threefold ( P < 0.05) and was directly related to the degree of stopped flow in the EDL. Thus impaired O₂ transport in early stage sepsis is likely the result of a microcirculatory dysfunction.

The release of ATP from red blood cells (RBC) in response to low O₂levels is linked to ATP production and the oxygenation state of hemoglobin. Because O₂is unloaded from the RBC, the concentration of deoxygenated hemoglobin increases, displacing phosphofructokinase from the cytoplasmic domain of band 3. We hypothesize that the ATP molecules produced through this glycolytic stimulation at the membrane surface result in the release of ATP from the RBC. Rat whole blood exposed to 5 min of low Po₂in vitro increased plasma [ATP] by 1.0 μM (+45%). This increase was reduced to 0.1 μM (+12%, P < 0.05) after citrate incubation and reversed after fluoride treatment (both glycolytic inhibitors) by −0.2 μM (−23%, P < 0.05). Plasma [ATP] of control RBC decreased −0.3 μM (−12%) when 8% CO ( P < 0.05) was added to the chamber. Because CO and O₂bind competitively to heme, these results support our hypothesis that the release of ATP from RBC is linked to ATP production through the oxygenation state of the hemoglobin molecule.