Methylxanthines, adenosine

Dulloo AG, Seydoux J, Girardier L. Potentiation of the thermogenic antiobesity effects of ephedrine by dietary methylxanthines Adenosine antagonism or phosphodiesterase inhibition Metabolism 1992 41 1233-1241. 

Adenosine is produced by many tissues, mainly as a byproduct of ATP breakdown. It is released from neurons, glia and other cells, possibly through the operation of the membrane transport system. Its rate of production varies with the functional state of the tissue and it may play a role as an autocrine or paracrine mediator (e.g. controlling blood flow). The uptake of adenosine is blocked by dipyridamole, which has vasodilatory effects. The effects of adenosine are mediated by a group of G protein-coupled receptors (the Gi/o-coupled Ai- and A3 receptors, and the Gs-coupled A2a-/A2B receptors). Ai receptors can mediate vasoconstriction, block of cardiac atrioventricular conduction and reduction of force of contraction, bronchoconstriction, and inhibition of neurotransmitter release. A2 receptors mediate vasodilatation and are involved in the stimulation of nociceptive afferent neurons. A3 receptors mediate the release of mediators from mast cells. Methylxanthines (e.g. caffeine) function as antagonists of Ai and A2 receptors. Adenosine itself is used to terminate supraventricular tachycardia by intravenous bolus injection. 

The often severe headaches, common in caffeine withdrawal, appear to be caused by vasodilation of cerebral blood vessels. This action is probably mediated by the action of the methylxanthines on adenosine receptors. 

Most likely the major action of methylxanthines as adenosine antagonist is the reason for these effects on the renal system. 

Rail, T. W., Evolution of the mechanism of action of methylxanthines From calcium mobilizers to antagonists of adenosine receptors, Pharmacologist, 24, 277, 1982. 

The generic name of the cacao tree (Theobroma) means food of the Gods and gives its name to a caffeine-like stimulant, theobromine (a methylxanthine). It has been claimed that the theobromine in chocolate is responsible for its addictive characteristics. This is based on the fact that methylxanthines bind to adenosine receptors in the central nervous system and act as antagonists to this neurotransmitter (Chapter 14). However, another group of substances, the amides formed between ethanolamine and unsaturated fatty acids, are also possible candidates for the title of the chocolate drug . 

Metabolism. The nucleotide cAMP (adenosine 3, 5 -cyclic monophosphate) is synthesized by membrane-bound adenylate cyclases [1] on the inside of the plasma membrane. The adenylate cyclases are a family of enzymes that cyclize ATP to cAMP by cleaving diphosphate (PPi). The degradation of cAMP to AMP is catalyzed by phosphodiesterases [2], which are inhibited by methylxanthines such as caffeine, for example. By contrast, insulin activates the esterase and thereby reduces the cAMP level (see p. 388). 

Action on the CNS depends directly on the dose of administered drug, and can be manifested as fatigue, anxiety, tremors, and even convulsions in relatively high doses. Theophylline acts on the cardiovascular system by displaying positive ionotropic and chronotropic effects on the heart, which, can likely be linked to the elevated influx of calcium ions by modulated cyclic adenosine monophosphate and its action on specific cardiac phosphodiesterases. In the gastrointestinal system, methylxanthines simultaneously stimulate secretion of both gastric juice and digestive enzymes. 

Metabolism of adenosine is slowed by dipyridamole, indicating that in patients stabilized on dipyridamole the therapeutically effective dose of adenosine may have to be increased. Methylxanthines antagonize the effects of adenosine via blockade of the adenosine receptors. 

B. Methylxanthines have been proposed to be inhibitors of phosphodiesterase, which would elevate intracellular levels of cAMP. However, the concentration of cAMP that is required for such action is above the threshold of CNS stimulation. Since the methylxanthines are relatively potent antagonists of adenosine and since adenosine has been shown to be a reasonably potent inhibitor of both central and peripheral neurons, the most likely mechanism by which CNS stimulation occurs is through antagonism of adenosine receptors. 

Mecfianism of Action A methylxanthine and competitive inhibitor of phosphodiesterase that blocks antagonism of adenosine receptors. Therapeutic Effect Stimulates respiratory center, increases minute ventilation, decreases threshold of or increases response to hypercapnia, increases skeletal muscle tone, decreases diaphragmatic fatigue, increases metabolic rate, and increases oxygen consumption. Pharmacokinetics Protein binding 36%. Widely distributed through the tissues and CSF. Metabolized in liver. Excreted in urine. Half-life 3-7 hr. 

The methylxanthines have positive chronotropic and inotropic effects. At low concentrations, these effects appear to result from inhibition of presynaptic adenosine receptors in sympathetic nerves increasing catecholamine release at nerve endings. The higher concentrations (more than 10 i mol/L, 2 mg/L) associated with inhibition of phosphodiesterase and increases in cAMP may result in increased influx of calcium. At much higher concentrations (more than 100 mol/L), sequestration of calcium by the sarcoplasmic reticulum is impaired. 

Reeves JJ, Jones CA, Sheehan MJ, Vardey CJ, Whelan CJ (1997) Adenosine A3 receptors promote degranulation of rat mast cells both in vitro and in vivo. Inflamm Res 46(5) 180-184 Ribeiro JA, Walker J (1975) The effects of adenosine triphosphate and adenosine diphosphate on transmission at the rat and frog neuromuscular junctions. Br J Pharmacol 54(2) 213-218 Ribeiro JA, Sebastiao AM (1984) Enhancement of tetrodotoxin-induced axonal blockade by adenosine, adenosine analogues, dibutyryl cyclic AMP and methylxanthines in the frog sciatic nerve. Br J Pharmacol 83(2) 485—492... 

The methylxanthine molecule is built on a foundation common to many biologic compounds, the xanthine double ring of carbons. The three methylxanthines, caffeine, theophylline, and theobromine, all block the action of the body s adenosine molecule, sending a signal that helps slow the chemical buildup inside cells. Because the methylxanthines closely resemble adenosine at the molecular level, they can occupy the molecular sites on cells that normally recognize, and react to, adenosine. Caffeine prevents the normal slowing action of adenosine at the cellular level, in both nerves and muscle. 

Caffeine, by blocking the action of the body s adenosine, affects a wide variety of organs, as well as the brain, the gut, and basic metabolism. Theophylline works more actively on respiration and the heart. Caffeine is more active in the gut and in the central nervous system. Theobromine has very weak, if any, effect on the brain, but it retains the methylxanthine effect on the kidneys, increasing urination. 

Mechanism of action The methylxanthines may act by several mechanisms, including translocation of extracellular calcium, increase in cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) caused by inhibition of phosphodiesterase, and blockade of adenosine receptors. 

Adenosine binds to adenosine receptors (AD-Rs) (subtypes Ah A2A, A2b and A3). Ap and Ap R activation gives Gai-mediated inhibition of adenylyl cyclase (resulting in decreased cAMP) and Gai/Gao-mediated activation of a K+ channel (with a de-excitatory hyperpolarizing effect). A2A and A2B activation gives Gas-mediated stimulation of adenylate cyclase (resulting in elevated cAMP). Adenosine acting via particular receptors variously has cardioprotective, neuroprotective, sedative, anticonvulsant, soporific, vasodilatory and bronchoconstrictive effects. The plant-derived methylxanthines theophylline and caffeine are adenosine A1 and A2 receptor antagonists (Table 5.1). 

The methylxanthines, theophylline and caffeine, act primarily as antagonists for both adenosine A, and Aj receptors (65-68). Caffeine is a relatively weak adenosine antagonist as compared to foeophylline (69). Studies to examine these methylxanthines as adenosine antagonists on platelet ftinction have been limited. Both theophylline and caffeine are shown to stimulate platelet reactivity in vitro and in vivo studies (49,50,67,69,70). Platelets from subjects after chronic consumption of caffeine show decreased antiplatelet activity of the adenosine analog NECA on thrombin-induced... 

Agarwal, KC, Clarke, E, Rounds, S, Parks, RE, Jr, Huzoor- Akbar, Platelet-activating factor (PAF> induced platelet aggregation modulation by plasma adenosine and methylxanthines, Btochem. Pharmacol 1994,48 1909-1916. 

Fredholm, BB, Are methylxanthine effects due to antagonism of endogenous adenosine Trends Pharmacol Sci., 1982,1 129-132. 

Agarmi KC, Oarice E, Rounds S, Parks RE Jr, Huzoor-Akbar. Platdet-activating factor (PAF)>induced platelet aggregatkn. Moduladon by plasma adenosine and methylxanthines. Biocfaem Pharmacol 1994 48 1909-1916... 

Methylxanthines inhibit phosphodiesterase in many tissues, and theophylline is about six times as potent as caffeine [6]. The potentiation of drug effects by methylxanthines has often been interpreted as accumulation of cyclic AMP, but methylxanthines also promote the accumulation of cyclic GMP [37]. Methylxanthines also inhibit adenylate cyclase activity in some tissues, as, for example, the noradrenaline-stimulated adenylate cyclase activity in rat erythrocyte ghosts [38], the vasopressin-stimulated adenylate cyclase activity in toad bladder epithelium [39], the increase in cyclic AMP produced in brain slices by depolarising stimuli and by adenosine [40], and the adenylate cyclase activity in guinea-pig lung particles [41]. Both basal and glucagon-stimulated phosphorylase activity in rat liver slices are inhibited by theophylline [42]. However, methylxanthines also have pharmacological effects which are not related to inhibition of phosphodiesterase [43]. 

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