Functional sympatholysis and sympathetic escape in a theoretical model for blood flow regulation

Regional limb blood flow has been measured with dilution techniques (cardio-green or thermodilution) and ultrasound Doppler. When applied to the femoral artery and vein at rest and during dynamical exercise these methods give similar reproducible results. The blood flow in the femoral artery is approximately 0.3 L min(-1) at rest and increases linearly with dynamical knee-extensor exercise as a function of the power output to 6-10 L min[-1] (Q= 1.94 + 0.07 load). Considering the size of the knee-extensor muscles, perfusion during peak effort may amount to 2-3 L kg(-1) min(-1), i.e. approximately 100-fold elevation from rest. The onset of hyperaemia is very fast at the start of exercise with T 1/2 of 2-10 s related to the power output with the muscle pump bringing about the very first increase in blood flow. A steady level is reached within approximately 10-150 s of exercise. At all exercise intensities the blood flow fluctuates primarily due to the variation in intramuscular pressure, resulting in a phase shift with the pulse pressure as a superimposed minor influence. Among the many vasoactive compounds likely to contribute to the vasodilation after the first contraction adenosine is a primary candidate as it can be demonstrated to (1) cause a change in limb blood flow when infused i.a., that is similar in time and magnitude as observed in exercise, and (2) become elevated in the interstitial space (microdialysis technique) during exercise to levels inducing vasodilation. NO appears less likely since NOS blockade with L-NMMA causing a reduced blood flow at rest and during recovery, it has no effect during exercise. Muscle contraction causes with some delay (60 s) an elevation in muscle sympathetic nerve activity (MSNA), related to the exercise intensity. The compounds produced in the contracting muscle activating the group IIl-IV sensory nerves (the muscle reflex) are unknown. In small muscle group exercise an elevation in MSNA may not cause vasoconstriction (functional sympatholysis). The mechanism for functional sympatholysis is still unknown. However, when engaging a large fraction of the muscle mass more intensely during exercise, the MSNA has an important functional role in maintaining blood pressure by limiting blood flow also to exercising muscles.

Blood flow to contracting skeletal muscle is tightly coupled to the oxygenation state of hemoglobin. To investigate if ATP could be a signal by which the erythrocyte contributes to the regulation of skeletal muscle blood flow and oxygen (O2) delivery, we measured circulating ATP in 8 young subjects during incremental one-legged knee-extensor exercise under conditions of normoxia, hypoxia, hyperoxia, and CO+normoxia, which produced reciprocal alterations in arterial O2 content and thigh blood flow (TBF), but equal thigh O2 delivery and thigh O2 uptake. With increasing exercise intensity, TBF, thigh vascular conductance (TVC), and femoral venous plasma [ATP] augmented significantly (P<0.05) in all conditions. However, with hypoxia, TBF, TVC, and femoral venous plasma [ATP] were (P<0.05) or tended (P=0.14) to be elevated compared with normoxia, whereas with hyperoxia they tended to be reduced. In CO+normoxia, where femoral venous O2Hb and (O2+CO)Hb were augmented compared with hypoxia despite equal arterial deoxygenation, TBF and TVC were elevated, whereas venous [ATP] was markedly reduced. At peak exercise, venous [ATP] in exercising and nonexercising limbs was tightly correlated to alterations in venous (O2+CO)Hb (r2=0.93 to 0.96; P<0.01). Intrafemoral artery infusion of ATP at rest in normoxia (n=5) evoked similar increases in TBF and TVC than those observed during exercise. Our results in humans support the hypothesis that the erythrocyte functions as an O2 sensor, contributing to the regulation of skeletal muscle blood flow and O2 delivery, by releasing ATP depending on the number of unoccupied O2 binding sites in the hemoglobin molecule.

The aims were, first, to detect and quantify the release of ATP from human erythrocytes in response to a brief exposure to a hypoxic/hypercapnic environment, similar to that found in vigorously exercising skeletal or cardiac muscle; and second, to explore the mechanism of ATP release in response to hypoxia. Washed human erythrocytes suspended in Krebs-Henseleit solution were exposed for 50 s to an atmosphere of approximately 8.0 kPa PCO2 and 2.7 kPa PO2; ATP released into the suspension was assayed using the firefly luminescence technique. Samples of human blood were obtained by venepuncture of the median cubital vein from male and female volunteers ranging in age from 21 to 74 years. Anticoagulation was with EDTA. A background of 0.49 x 10(6) (SEM 0.037 x 10(6)) ATP molecules.cell-1 was attributed to spontaneous haemolysis of 1% of the cell population, as estimated by levels of haemoglobin measured in the suspension fluid. When the erythrocytes were exposed to the hypoxic/hypercapnic gas mixture at 37 degrees C the ATP concentration in the suspension fluid rose to 2.67 x 10(6) (0.27 x 10(6)) molecules.cell-1. An efflux rate of 276(37) molecules.mu-2.s-1 was calculated. The hypoxia induced ATP release was blocked in three different ways: first, by application of 50 microM of the specific band 3 anion channel blocking agents niflumic acid (a translocation inhibitor), DIDS (a transport site inhibitor), or dipyridamole (a channel blocker); secondly, by replacement of extracellular chloride and bicarbonate with the impermeant anion methanesulphonate; and thirdly, by application of 5 nM of the nucleoside transport blocker nitrobenzylthioinosine. None of these blocking techniques affected the background levels of ATP attributed to haemolysis. 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.