Tripartite function of ATP in vascular signalling.

Adenosine triphosphate is not only the main energy source for most cellular functions, it is also an important signalling molecule in blood vessels via its action on purinergic P2x or P2y receptors. These receptors are components of the control system that regulates skeletal muscle blood flow – matching the competing functions of constriction to maintain systemic blood pressure and dilatation to increase oxygen delivery. When ATP activates ionotropic P2x receptors (found primarily on vascular smooth muscle cells) constriction ensues, whereas activation of metabotropic P2y receptors (found primarily on endothelial cells) produces dilatation. To these two functions of ATP we must now add a third – sympatholysis. The article by Rosenmeier et al. (2008) in this issue of The Journal of Physiology along with a recent article by Kirby et al. (2008) demonstrate convincingly that ATP attenuates sympathetic vasoconstriction. Before examining their findings, let us briefly review the evidence that implicates ATP in the control of skeletal muscle blood flow during exercise. The evidence for the involvement of ATP in sympathetic vasoconstriction in exercising skeletal muscle is straightforward. ATP is found in varicosities of sympathetic nerve terminals and is released along with noradrenaline in proportions which may vary according to the pattern of action potentials. The presence of functional P2x receptors during dynamic exercise is inferred from vasoconstriction elicited by intra-arterial infusion of a P2x agonist. The observation that blockade of P2x receptors elevates skeletal muscle blood flow (Buckwalter et al. 2004) demonstrates that tonic release of ATP from sympathetic nerve terminals during exercise contributes to sympathetic restraint of blood flow. The role of ATP in skeletal muscle vasodilatation during exercise is more complicated. Plasma [ATP] increases during exercise and dilatation can be evoked by intra-arterial infusion of ATP, presumably from activation of endothelial P2y receptors. The source of intravascular ATP during exercise is unknown. Although present in high concentrations in the interstitial space of contracting muscle, ATP does not readily cross the endothelium. The demonstration that erythrocytes release ATP in response to mechanical deformation or reduced oxygen tension has led to the intriguing idea that ATP release by erythrocytes could match oxygen delivery with metabolic requirements during exercise. However, release of ATP in this fashion is under the control of the cystic fibrosis transmembrane conductance regulator (Sprague et al. 1998) and identical blood flow responses to rhythmic forearm exercise in cystic fibrosis patients and healthy controls (Schrage et al. 2005) argue against ATP as the essential mediator of exercise vasodilatation. Research on the role of ATP in exercise hyperaemia has been hampered by the absence of a selective P2y antagonist. The magnitude of sympathetic vasoconstriction in exercising muscle is modulated in proportion to exercise intensity, a phenomenon called functional sympatholysis. Theoretically, this may be due to prejunctional inhibition of neurotransmitter release, increased degradation of neurotransmitter, interference with postjunctional receptors, or altered transduction pathways. Diminished vasoconstriction to tyramine-evoked endogenous release of noradrenaline could be ascribed partially to inhibition of noradrenaline release or increased breakdown, but attenuated constrictor responses to exogenous administration of α-adrenergic and P2x receptor agonists attest to an effect at postjunctional receptors or downstream pathways. Although the mechanism for functional sympatholysis has generally been attributed to metabolic factors produced by the contracting skeletal muscle, there is an evolving story implicating ATP in the process. The study by Rosenmeier and colleagues adds another chapter to this story. In human subjects, tyramine was infused into the femoral artery while leg blood flow was monitored by constant infusion thermal dilution. Vasoconstriction occurred during concomitant infusion of ADP, AMP, or adenosine, but not during infusion of ATP or UTP. On the basis of the known interactions with the family of P2y receptors, the authors reasoned that the similarity of responses between ATP and UTP suggests involvement of the P2y2 receptor. Kirby et al. (2008) took a complimentary approach by examining postjunctional α-adrenergic responsiveness during elevations in blood flow induced by contractions, adenosine or ATP. ATP blunted the constriction to the same extent as contractions, whereas adenosine was without effect. Importantly, there was a dose–response relationship whereby graded doses of ATP produced progressive attenuation of α-adrenergic vasoconstriction. Together, these studies show that ATP is distinctive in its ability to attenuate sympathetic vasoconstriction. As convincing as the above data are, several important issues remain. First, infusion of ATP had no effect on femoral artery or vein [ATP], leading Rosenmeier et al. to argue that it was immediately broken down by circulating nucleotidases. In a previous study by the same group (Gonzalez-Alonso et al. 2006) a wide range of experimentally induced oxygen levels failed to change the measured values of arterial or venous [ATP]. It is possible that the unchanged [ATP] is a result of sampling and measurement techniques which need to be refined to allow detection of transient changes. Second, it is widely assumed that the vascular effects of ATP are mediated by second messengers NO, prostacyclin, and EDHF. If ATP is the crucial mediator, it is perplexing why NO and prostaglandin blockade have shown little effect on sympatholysis in humans. Third, the cellular mechanism for ATP-mediated sympatholysis needs to be elucidated. Since most vasodilators cause smooth muscle hyperpolarization, this is unlikely to be the sole explanation. Until recently, little consideration has been given to a substance originating outside of contracting skeletal muscle as a mechanism to explain functional sympatholysis. The results of these two studies provide impetus for further investigation of ATP, the singular molecule with tripartite function in vascular signalling.