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.

A situation of hypoxia/hypercapnia, such as would be found in exercising muscle, induces release of ATP from the erythrocyte via the plasma membrane protein moiety known as band 4.5 (a nucleoside transporter) and electrical balance across the erythrocyte membrane is maintained by the simultaneous influx of extracellular chloride and/or bicarbonate via the plasma membrane protein known as band 3 (anion channel). The circulation of erythrocytes into a region of hypoxia in vivo could promote an increase in local blood flow through release of endothelium dependent relaxing factor in response to released ATP.

SUMMARY1. Adult rat heart cells were isolated enzymically and ATP was identified in the cell suspension using the firefly luminescence technique. Adenosine 5'-triphosphate (ATP) was not detected from cell suspensions obtained from hearts which had been left asystolic for 10 min. 4am/min at 370 C. Q10 was found to be 4 between 25 and 370 C. Enzyme activity remained unaffected by either hypoxic conditions or ouabain.5. If these amounts of ATP are released from myocardial cells rendered hypoxic in vivo, then it must be concluded that ATP plays a principal role in the local control of myocardial blood flow.6. It is proposed that release of ATP occurs through the sarcolemma from an intracellular pool, and that alteration of the configuration of structural membrane protein controls the amounts of ATP extruded.

SUMMARY1. Human subjects performed a sustained contraction of the forearm muscles for 4 min in the presence of arterial and venous occlusion.2. The contraction was maintained at 5 % of the maximum voluntary contraction, a tension during which the muscle blood flow might be expected to increase by about three times (Lind & McNicol, 1967).3. Adenosine triphosphate (ATP) was identified in the venous effluent from occluded exercising forearm, but not in the venous effluent from occluded forearm without exercise.4. The rate of degradation of ATP was assessed in plasma at 370 C, with an estimate of the percentage loss occurring between sampling and testing. This enabled the rate of appearance of ATP in the blood at the time of exercise to be calculated as approximately 7-5-105 ,ug/min (14-20 n-