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.