On the mechanism by which theophylline inhibits histamine release from rat mast cells

Theophylline (2.5 mM) did not influence the spontaneous release of histamine but inhibited histamine release induced by antigen, compound 48/80 or phosphatidylserine. The effect on 48/80-induced histamine release could not be reversed by increasing extracellular Ca2+. Exogenous adenosine (10(-8) to 10(-4) M) did not influence spontaneous histamine release or 48/80-induced release but potentiated antigen-induced release. The adenosine potentiation was competitively inhibited by theophylline in concentrations (10(-5) to 10(-4) M) lower than those required to inhibit antigen-induced histamine release in the absence of adenosine. In order to see if endogenous adenosine levels are high enough to potentiate an anaphylactic histamine release in vivo, adenosine was determined in mast cell incubates and in plasma from 4 different strains of rat. The levels were 0.18 to 0.99 microM in plasma, which is sufficient to cause significant potentiation of histamine release, but only 3 x 10(-8) M in mast cell incubates. Theophylline (2.5 mM) increased cAMP levels about 100%, whereas adenosine (10(-5) M) had little effect on cAMP and cGMP levels. However, when incubated together, adenosine could inhibit the theophylline-induced increase in cAMP levels but not the inhibition of histamine release. It is concluded that the effect of low concentrations of theophylline could be due partly to antagonism of adenosine effects. In addition, in higher doses, theophylline appears to exert an inhibitory action that is unrelated to cyclic nucleotides, extracellular calcium and adenosine.     

Effect of theophylline and other drugs on rabbit renal cyclic nucleotide phosphodiesterase, 5'-nucleotidase and adenosine deaminase.

The effect of theophylline and several other drugs on cyclic AMP and cyclic GMP phosphodiesterase, 5′-nucleotidase and adenosine deaminase was tested in homogenates of rabbit renal cortex and medulla.Theophylline was a competitive inhibitor of cyclic nucleotide phosphodiesterase, a non-competitive inhibitor of 5′-nucleotidase and alkaline phosphatase but did not influence adenosine deaminase. Cyclic AMP and cyclic GMP hydrolysis were inhibited to an equal extent by theophylline, furosemide, caffeine, 3-isobutyl-methylxanthine, and SQ 20006. Dipyridamol, ZK 62.711, Ro 20-1724 and ICI 63.197 inhibited cyclic AMP hydrolysis in at least ten times lower concentrations than cyclic GMP hydrolysis. Conversely, M &; B 22948 and dilazep were more potent as inhibitors of cyclic GMP than of cyclic AMP hydrolysis.All the diuretic agents tested (furosemide, ethacrynic acid, chlorthalidone, cyclopentiazide and theophylline) inhibited 5′-nucleotidase at 1 mM concentration or lower.Of the drugs tested at 1 mM concentration only dipyridamol and chlorthalidone produced significant (30 per cent) inhibition of adenosine deaminase activity.     

Effect of propentofylline (HWA 285) on extracellular purines and excitatory amino acids in CA1 of rat hippocampus during transient ischaemia.

1. The adenosine uptake blocker propentofylline (HWA 285) has previously been shown to protect hippocampal CA1 pyramidal cells from ischaemia-induced delayed neuronal death. The influence of propentofylline, on the extracellular concentrations of purines, aspartate and glutamate in the CA1 of the rat hippocampus during transient forebrain ischaemia was investigated. 2. Twenty min of ischaemia was induced by four-vessel occlusion in Wistar rats, extracellular compounds were sampled by use of microdialysis and EEG was recorded by a tungsten electrode attached to the dialysis probe. 3. Propentofylline (10 mg kg-1 i.p.) did not influence the basal levels of any of the compounds in the hippocampal dialysates. 4. The EEG became isoelectric within 20 s after induction of ischaemia. 5. Extracellular adenosine, inosine, hypoxanthine, aspartate and glutamate increased several fold during ischaemia and remained elevated during early reflow. Within 2 h of reperfusion the concentration of all compounds was normalized. Xanthine increased upon reperfusion and remained elevated after 2 h. 6. Propentofylline (10 mg kg-1 i.p.) administered 15 min before ischaemia significantly enhanced the ischaemia-evoked increase of adenosine but attenuated the increases of the other purine catabolites and of glutamate. 7. In separate in vitro experiments, propentofylline did not inhibit adenosine deaminase activity. 8. The present data show that propentofylline enhances extracellular adenosine and lowers extracellular glutamate in vivo during ischaemia. These findings may be important in relation to the neuroprotective properties of propentofylline.     

Adenosine A1 receptors are crucial in keeping an epileptic focus localized

Adenosine is an endogenous neuromodulator with anticonvulsant and neuroprotective properties presumably mediated by activation of adenosine A1 receptors (A1Rs). To study the involvement of A1Rs in neuroprotection during epileptogenesis, we induced status epilepticus by a unilateral intrahippocampal kainic acid (KA) injection (1 nmol) in wild-type C57BL/6 and homozygous adenosine A1R knock out (A1R-KO) mice of the same genetic background. Whereas the KA injection caused non-convulsive status epilepticus in wild-type mice, in A1R-KO mice KA induced status epilepticus with severe convulsions and subsequent death of the animals within 5 days. 24 h after KA injection, brains from wild-type C57BL/6 mice were characterized by slight neuronal cell loss confined to the immediate location of the KA injection. In contrast, KA-injected A1R-KO mice displayed massive neuronal cell loss in the ipsilateral hippocampus, and, importantly, the contralateral hippocampus was also affected with significant cell loss in the hilus and in the CA1 region of the pyramidal cell layer. We conclude that activation of A1 receptors by ambient adenosine is crucial in keeping epileptic foci localized. These results open up a new dimension of the A1 receptor's role in controlling excitotoxic cell death and further demonstrate its importance in preventing the progression of status epilepticus to lethal consequences.     

Propentofylline and a hydroxy metabolite augment A2-receptor mediated cyclic AMP accumulation and attenuate A1-receptor mediated inhibition of acetylcholine release in the rat hippocampus

In contrast to the adenosine receptor antagonist theophylline, the xanthine derivative propentofylline (HWA 285) is reported to protect against ischaemic cell damage. We examined the effect of propentofylline on two adenosine actions in the rat hippocampus; the A2-mediated stimulation of 3H-cAMP accumulation and the A1-mediated inhibition of 3H-ACh release. Propentofylline (0.5-1 mM) increased the adenosine (3 and 30 microM) -induced 3H-cAMP accumulation. This effect was shared by its metabolite A 72 0287. The mechanism may be a decreased inactivation of adenosine, since the effect of the stable adenosine derivative NECA was not altered. In contrast, the inhibitory effect of adenosine on evoked 3H-ACh release was inhibited by propentofylline and by its metabolite A 72 0287, but enhanced by the uptake inhibitor dipyridamole. The effect of the stable, A1-receptor selective analogue R-PIA (1 microM) was also blocked by propentofylline and by A 72 0287. In addition, propentofylline (0.5 mM) blocked the presynaptic inhibitory effect of carbachol (1 and 50 microM). Thus, propentofylline and one of its metabolites inhibits adenosine A1-receptor mediated presynaptic inhibition while it enhances adenosine A2-mediated cAMP accumulation in the rat hippocampus differently. This selectivity in action can only partly be explained by receptor subtype selectivity, and effects at sites other than the adenosine receptor are of major importance.     

Adenosine in the tuberomammillary nucleus inhibits the histaminergic system via A1 receptors and promotes non-rapid eye movement sleep

Adenosine has been proposed to promote sleep through A₁ receptors (A₁R's) and/or A_2A receptors in the brain. We previously reported that A_2A receptors mediate the sleep-promoting effect of prostaglandin D₂, an endogenous sleep-inducing substance, and that activation of these receptors induces sleep and blockade of them by caffeine results in wakefulness. On the other hand, A₁R has been suggested to increase sleep by inhibition of the cholinergic region of the basal forebrain. However, the role and target sites of A₁R in sleep–wake regulation remained controversial. In this study, immunohistochemistry revealed that A₁R was expressed in histaminergic neurons of the rat tuberomammillary nucleus (TMN). In vivo microdialysis showed that the histamine release in the frontal cortex was decreased by microinjection into the TMN of N⁶-cyclopentyladenosine (CPA), an A₁R agonist, adenosine or coformycin, an inhibitor of adenosine deaminase, which catabolizes adenosine to inosine. Bilateral injection of CPA into the rat TMN significantly increased the amount and the delta power density of non-rapid eye movement (non-REM; NREM) sleep but did not affect REM sleep. CPA-promoted sleep was observed in WT mice but not in KO mice for A₁R or histamine H₁ receptor, indicating that the NREM sleep promoted by A₁R-specific agonist depended on the histaminergic system. Furthermore, the bilateral injection of adenosine or coformycin into the rat TMN increased NREM sleep, which was completely abolished by coadministration of 1,3-dimethyl-8-cyclopenthylxanthine, a selective A₁R antagonist. These results indicate that endogenous adenosine in the TMN suppresses the histaminergic system via A₁R to promote NREM sleep.     

The xanthine derivative 1-(5'-oxohexyl)-3-methyl-7-propyl xanthine (HWA 285) enhances the actions of adenosine

The effect of two closely related xanthine derivatives, pentoxifylline and HWA 285, on cyclic AMP accumulation in rat hippocampal slices and on adenosine uptake in erythrocytes was examined. Pentoxifylline was a weak competitive antagonist of adenosine effects on cyclic AMP accumulation. HWA 285, by contrast, had a small stimulatory effect per se and also potentiated the effect of adenosine (10-30 microM). Neither pentoxifylline nor HWA 285 significantly affected the cyclic AMP accumulation induced by the stable adenosine analogue NECA or by alpha- or beta-adrenoceptor activation. HWA 285 was a much more potent inhibitor of adenosine uptake into human erythrocytes than pentoxifylline and other examined xanthines including thiocaffeine, 8-p-sulphophenyltheophylline, theophylline, caffeine and enprofylline. It is suggested that HWA 285 may potentiate, rather than antagonize, the effects of endogenous as well as exogenous adenosine, partly as a consequence of adenosine uptake inhibition.     

Oxidative tissue damage following regional intestinal ischemia and reperfusion in the cat

Regional intestinal ischemia in cats resulted in an accumulation of hypoxanthine within 2 h, the concentration of which rose from 0.062 to 1.131 nmol/mg protein. A similar rise in AMP content (from 0.5 to 3.2 nmol/mg protein) was observed, but not in the ADP level. In parallel, ATP content decreased from 7.5 to 2.8 nmol/mg protein. Reperfusion of the ischemic tissue was followed by rapid metabolism of the purine metabolites; after 1 h of reperfusion the tissue level of hypoxanthine was 0.186 nmol/mg protein, of AMP 0.7 nmol/mg protein, and of ATP 4.3 nmol/mg protein. The oxidation of hypoxanthine, mediated by xanthine oxidase, is accompanied by the release of superoxide ions. Consequently, the concentration of oxidized glutathione was doubled upon reperfusion, while marked lipid peroxidation took place, as evidenced by the rise in conjugated diene content from 2.8 mumol/g tissue before reperfusion to 5.6 mumol/g tissue after 10 min of reoxygenation. In line with these findings is the fact that histologically observable damage occurred mainly in the presence of oxygen. These data indicate that, at least in our model, rapid reoxygenation is a major cause of "ischemic" tissue damage.