Adenosine is an autacoid that plays a critical role in regulating cardiac function, including heart rate, contractility, and coronary flow. In this chapter, current knowledge of the functions and mechanisms of action of coronary flow regulation and electrophysiology will be discussed. Currently, there are four known adenosine receptor (AR) subtypes, namely A1, A2A, A2B, and A3. All four subtypes are known to regulate coronary flow. In general, A2AAR is the predominant receptor subtype responsible for coronary blood flow regulation, which dilates coronary arteries in both an endothelial-dependent and -independent manner. The roles of other ARs and their mechanisms of action will also be discussed. The increasing popularity of gene-modified models with targeted deletion or overexpression of a single AR subtype has helped to elucidate the roles of each receptor subtype. Combining pharmacologic tools with targeted gene deletion of individual AR subtypes has proven invaluable for discriminating the vascular effects unique to the activation of each AR subtype. Adenosine exerts its cardiac electrophysiologic effects mainly through the activation of A1AR. This receptor mediates direct as well as indirect effects of adenosine (i.e., anti-β-adrenergic effects). In supraventricular tissues (atrial myocytes, sinua-trial node and atriovetricular node), adenosine exerts both direct and indirect effects, while it exerts only indirect effects in the ventricle. Adenosine exerts a negative chronotropic effect by suppressing the automaticity of cardiac pacemakers, and a negative dromotropic effect through inhibition of AV-nodal conduction. These effects of adenosine constitute the rationale for its use as a diagnostic and therapeutic agent. In recent years, efforts have been made to develop A1R-selective agonists as drug candidates that do not induce vasodilation, which is considered an undesirable effect in the clinical setting.

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Adenosine 5'-triphosphate (ATP) is released from the cytoplasm under physiologic and pathophysiologic conditions and enters the extracellular space, where it acts on a group of recently cloned cell-surface receptors termed P2-purinoceptors (subtypes P2X and P2Y). We examined the effects of extracellular ATP, uridine triphosphate (UTP), the stable ATP analogues alpha,betamethylene-ATP (alpha,betamATP), beta,gammamethylene-ATP (beta,gammamATP), and 2-methylthio-ATP (2mSATP), and adenosine (10(-6)-10(-3) M) on histamine release from human lung mast cells (HLMC) induced by anti-IgE and the calcium ionophore A23187. None of the nucleotides or adenosine directly induced histamine release. Adenosine exhibited a bimodal effect, enhancing histamine release at 10(-6) to 10(-4) M (P > 0.05, NS) and inhibiting it at 10(-3) M (P < 0.05). ATP (10(-4) M) enhanced anti-IgE-induced histamine release (10.9 +/- 2.7% to 19. 2 +/- 2.9%, n = 20, P < 0.01), but not ionophore A23187-induced histamine release (n = 10). The adenine nucleotides consistently enhanced anti-IgE-induced histamine release; the rank order for this action was: ATP > 2mSATP > alpha,betamATP > beta,gammamATP, suggesting mediation by a P2Y-purinoceptor subtype. The selective P2X purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid failed to influence the effect of ATP, further supporting P2Y-purinoceptor mediation of anti-IgE-induced histamine release. UTP, an agonist at P2Y-purinoceptors, also significantly enhanced anti-IgE-induced histamine release. Application of the reverse transcription-polymerase chain reaction indicated that HLMC constitutively express the messenger RNAs encoding the P2Y1- and P2Y2-purinoceptor subtypes, and not that encoding the P2X7-purinoceptor (i.e., P2Z), a subtype implicated in ATP-induced histamine release in rodent peritoneal mast cells. The data produced in the study suggest that ATP plays an important modulatory role in histamine release from HLMC, and that it may therefore be mechanistically involved in human allergic/asthmatic reactions.

1. The effects of extracellular adenosine 5'-triphosphate (ATP) on pulmonary vagal afferent fibres (n = 46) was studied in a canine model in vivo (n = 38).2. ATP (3-6 ,umol kg-'), administered as a rapid bolus into the right atrium, elicited a transient burst of action potentials in cervical vagal fibres, which was not affected by either blockade of ganglionic transmission (hexamethonium) or a drop in arterial blood pressure (nitroglycerine). 3. The fibres with ATP-sensitive terminals were otherwise quiescent with no activity related to either cardiac or respiratory cycles and their conduction velocity was 0-85 + 0'13 m 8-(n = 7).4.' Inflation of the lungs to 2-3 times the tidal volume triggered brief bursts of action potentials in these fibres. 5. Capsaicin (10 jug kg-), given as a rapid bolus into the right atrium, elicited a burst of action potentials in these ATP-sensitive fibres.6. Smaller amounts of ATP and capsaicin (0 5-3 /smol kg-1 and 1-5 jug kg', respectively) had similar effects when the two compounds were given into the right pulmonary artery. 7. Adenosine, adenosine 5'-monophosphate, or adenosine 5'-diphosphate did not excite these fibres (n = 30). 8. The non-degradable analogue of ATP a,,-methylene ATP (a,/J-mATP) was tenfold more potent than ATP while /,y-methylene ATP (,hy-mATP) was inactive. 9. The selective P2x-purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid markedly attenuated the effect of ATP but not of capsaicin. The P2y-purinoceptor antagonist Reactive Blue 2 was without effect. 10. 11. Pretreatment with pertussis toxin (PTX) did not affect this action of ATP. In the canine lungs ATP activates vagal C fibre nerve terminals. This action is mediated by P2x-purinoceptors and is independent of a PTX-sensitive guanine nucleotide binding protein (G protein).