Overview

In this chapter we describe the role of purine nucleosides and nucleotides as chemical mediators subserving a wide range of functions. The mechanisms responsible for their synthesis and release are considered, as well as the various receptors on which they act, and drugs that affect purinergic signalling.

Introduction

Nucleosides (especially adenosine) and nucleotides (especially ADP and ATP) will already be familiar to you because of their crucial role in DNA/RNA synthesis and energy metabolism, but it may come as a surprise to learn that they also produce a wide range of unrelated pharmacological effects. Interest in this field probably began with the observation in 1929 that adenosine injected into anaesthetised animals caused bradycardia, hypotension, vasodilatation and inhibition of intestinal movements. Since then, it has become clear that purines participate in many physiological control mechanisms, including the regulation of coronary flow and myocardial function (Chs 21 and 22), platelet aggregation and immune responses (Chs 17 and 24), as well as neurotransmission in both the central and peripheral nervous system (Chs 12 and 38).

There is, therefore, increasing interest in purine pharmacology and the potential role of purinergic agents in the treatment of pain and a variety of disorders, particularly of thrombotic and respiratory origin. The full complexity of purinergic control systems, and their importance in many pathophysiological mechanisms, is only now emerging, and the therapeutic relevance of the various receptor subtypes is still being unravelled.¹ There is no doubt that drugs affecting these systems will assume growing significance but, recognising that the overall picture is far from complete, we will focus our discussion on a few prominent areas.

Figure 16.1 summarises the mechanisms by which purines are released and interconverted, and the main receptor types on which they act.

Fig. 16.1 Purines as mediators.

ATP (and, in platelets, ADP) is stored in vesicles and released by exocytosis. It is also present in the cytosol of all cells, from which large quantities may be released by cellular damage. Adenosine is present in the cytosol of all cells, and is taken up and released via a specific membrane transporter. When released, ATP and ADP are rapidly converted to adenosine by the action of tissue nucleotidases.

Purinergic Receptors

Purines exert their biological actions through three families of receptors. Table 16.1 lists these and summarises what is known about their signalling systems, their endogenous ligands and antagonists of pharmacological interest. It should be noted, however, that the family of purinergic receptors continues to grow and their pharmacology can be confusing. In part, this is because nucleotides are rapidly degraded by ecto-enzymes and there is also evidence of interconversion by phosphate exchange. Thus it is possible to envisage a situation where ATP may produce effects at all three receptor subclasses depending upon the extent of its enzymatic hydrolysis.

Table 16.1 Purinergic receptors

The three main types of purine receptor are:

The subtypes in each family may be distinguished on the basis of their molecular structure as well as their agonist and antagonist selectivity. The latter has usually been determined by the use of groups of experimental compounds with varying degrees of receptor selectivity and need not concern us here. The P2Y group is particularly problematic: several receptors have been cloned on the basis of homology with other family members, but their ligands have yet to be identified (in other words they are ‘orphan receptors’). In addition, some members of this family also recognise pyrimidines such as UTP and UDP as well as purines, and as such are sometimes classed as pyrimidinoceptors. Little is known about the role of pyrimidines in cell signalling.

With the exception of platelet P2Y₁₂ antagonists such as clopidogrel, there are so far few therapeutic agents that act on these receptors, and we will confine this account to some prominent and interesting aspects; the reading list provides further information.

Adenosine as a Mediator

The simplest of the purines, adenosine is found in biological fluids throughout the body. It exists free in the cytosol of all cells and is transported in and out mainly by a membrane transporter. Little is known about the way in which this is controlled but the extracellular concentrations are usually quite low compared with intracellular levels. Adenosine in tissues comes partly from this intracellular source and partly from extracellular hydrolysis of released ATP or ADP (Fig. 16.1).

Virtually all cells express one or more A-receptors and so adenosine produces many pharmacological effects, both in the periphery and in the CNS. Based on its ability to inhibit cell function and thus minimise the metabolic requirements of cells, one of its functions may be as an ‘acute’ protective agent that is released immediately when tissue integrity is threatened (e.g. by coronary or cerebral ischaemia; see Chs 21 and 39). Under less extreme conditions, variations in adenosine release may play a role in controlling blood flow and (through effects on the carotid bodies) respiration, matching them to the metabolic needs of the tissues.

ADP as a Mediator

ADP is usually stored in vesicles in cells. When released, it exerts its biological effects predominantly through the P2Y family of receptors.

ATP as a Mediator

ATP exerts its action primarily through the P2X receptors. The extracellular domain of these multimeric receptors can bind three molecules of ATP. When activated, the receptor gates the cation-selective ion channels that trigger ongoing intracellular signalling. The other actions of ATP in mammals are mediated through the P2Y receptors. Suramin (a drug originally developed to treat trypanasome infections) and an experimental compound PPADS antagonise ATP and have broad-spectrum inhibitory activity at most P2X and P2Y receptors. ATP is present in all cells in millimolar concentrations and is released, independently of exocytosis, if the cells are damaged (e.g. by ischaemia). ATP released from cells is rapidly dephosphorylated by a range of tissue-specific nucleotidases, producing ADP and adenosine (Fig. 16.1), both of which produce a wide variety of receptor-mediated effects. The role of intracellular ATP in controlling membrane potassium channels, which is important in the control of vascular smooth muscle (Ch. 22) and of insulin secretion (Ch. 30), is quite distinct from its transmitter function.

Future Prospects

While it is true that few currently available drugs act through purinergic receptors when compared, for example, with 5-HT receptors discussed in Chapter 15, the area as a whole holds promise for future therapeutic exploitation, particularly in the treatment of asthma (Brown et al., 2008), pain (Liu et al., 2005; Burnstock, 2006) and gastrointestinal disorders (Burnstock, 2008), provided compounds with sufficient receptor selectivity can be found.