Characterization of the adenosine receptor modulating insulin action in rat skeletal muscle

Purinergic signaling in diabetes and metabolism

Purinergic signaling, a concept originally formulated by the late Geoffrey Burnstock (1929-2020), was found to modulate pathways in every physiological system. In metabolic disorders there is a role for both adenosine receptors and P2 (nucleotide) receptors, of which there are two classes, i.e. P2Y metabotropic and P2X ionotropic receptors. The individual roles of the 19 receptors encompassed by this family have been dissected - and in many cases the effects associated with specific cell types, including adipocytes, skeletal muscle, liver cells and immune cells. It is suggested that ligands selective for each of the four adenosine receptors (A₁, A_2A, A_2B and A₃), and several of the P2 subtypes (e.g. P2Y₆ or P2X7 antagonists) might have therapeutic potential for treating diabetes and obesity. This is a developing story with some conflicting conclusions relevant to drug discovery, which we summarize here.

A 2 Adenosine Receptors Mediate Whole-Body Insulin Sensitivity in a Prediabetes Animal Model: Primary Effects on Skeletal Muscle

Epidemiological studies showed that chronic caffeine intake decreased the risk of type 2 diabetes. Previously, we described that chronic caffeine intake prevents and reverses insulin resistance induced by hypercaloric diets and aging, in rats. Caffeine has several cellular mechanisms of action, being the antagonism of adenosine receptors the only attained with human coffee consumption. Here, we investigated the subtypes of adenosine receptors involved on the effects of chronic caffeine intake on insulin sensitivity and the mechanisms and sex differences behind this effect. Experiments were performed in male and female Wistar rats fed either a chow or high-sucrose (HSu) diet (35% of sucrose in drinking water) during 28 days, to induce insulin resistance. In the last 15 days of diet the animals were submitted to DPCPX (A₁ antagonist, 0.4 mg/kg), SCH58261 (A_2A antagonist, 0.5 mg/kg), or MRS1754 (A_2B antagonist, 9.5 μg/kg) administration. Insulin sensitivity, fasting glycaemia, blood pressure, catecholamines, and fat depots were assessed. Expression of A₁, A_2A, A_2B adenosine receptors and protein involved in insulin signaling pathways were evaluated in the liver, skeletal muscle, and visceral adipose tissue. UCP1 expression was measured in adipose tissue. Paradoxically, SCH58261 and MRS1754 decreased insulin sensitivity in control animals, whereas they both improved insulin response in HSu diet animals. DPCPX did not alter significantly insulin sensitivity in control or HSu animals, but reversed the increase in total and visceral fat induced by the HSu diet. In skeletal muscle, A₁, A_2A, and A_2B adenosine receptor expression were increased in HSu group, an effect that was restored by SCH58261 and MRS1754. In the liver, A₁, A_2A expression was increased in HSu group, while A_2B expression was decreased, being this last effect reversed by administration of MRS1754. In adipose tissue, A₁ and A_2A block upregulated the expression of these receptors. A₂ adenosine antagonists restored impaired insulin signaling in the skeletal muscle of HSu rats, but did not affect liver or adipose insulin signaling. Our results show that adenosine receptors exert opposite effects on insulin sensitivity, in control and insulin resistant states and strongly suggest that A₂ adenosine receptors in the skeletal muscle are the majors responsible for whole-body insulin sensitivity.

Adenosine Deaminase Activity in the Serum of Type 2 Diabetic Patients

Adenosine deaminase (ADA) is suggested to be an important enzyme for modulating the bioactivity of insulin, but its clinical significance in diabetes mellitus (DM) is not yet characterized. We measured the serum level of ADA in healthy controls (C, n=29) and type 2 diabetic patients (n=42). The mean serum level of ADA in C, and type 2 diabetic patients were 29.81±9.15 and. 20.73±8.42 U/L, respectively (P<0.006 vs. C). ADA levels of patients were significantly correlated with HbA1c (r=0.45, p<0.01). Our findings suggest that ADA may play a role in insulin effect and glycamic control. On the other hand, increased activity of ADA in type 2 DM might be a marker for insulin indication. However, further studies are required for the pathogenic role of elevated ADA activity in type 2 DM.

Adenosine and chronic ischemia of the lower limbs

Adenosine is an endogenous nucleoside with multiple biological properties which plays a central role in the pathophysiology of tissue ischemia. Adenosine signals an imbalance between oxygen demand and supply, and it initiates responses to redress such a discrepancy. Besides its vasodilating properties, adenosine possesses anti-platelet and anti-neutrophil activities and provides cytoprotection. Adenosine is presumably the main mediator of the preconditioning phenomenon. During ischemia of the lower limbs, adenosine plays a physiological role by inducing vasodilatation and by preventing microcirculatory failure. Exercise training prolongs claudication distance possibly by inducing pulse increases of adenosine and consequently skeletal muscle preconditioning. Moreover, the adenosine increase which follows the administration of some drugs, such as buflomedil and propionylcarnitine, opens new perspectives in the management of leg ischemia. In fact, the concept arises of an ischemic (exercise-dependent) or pharmacologic preconditioning in the treatment of patients with claudication.

Performance effects and metabolic consequences of caffeine and caffeinated energy drink consumption on glucose disposal

This review documents two opposing effects of caffeine and caffeine-containing energy drinks, i.e., their positive effects on athletic performance and their negative impacts on glucose tolerance in the sedentary state. Analysis of studies examining caffeine administration prior to performance-based exercise showed caffeine improved completion time by 3.6%. Similar analyses following consumption of caffeine-containing energy drinks yielded positive, but more varied, benefits, which were likely due to the diverse nature of the studies performed, the highly variable composition of the beverages consumed, and the range of caffeine doses administered. Conversely, analyses of studies administering caffeine prior to either an oral glucose tolerance test or insulin clamp showed a decline in whole-body glucose disposal of ~30%. The consequences of this resistance are unknown, but there may be implications for the development of a number of chronic diseases. Both caffeine-induced performance enhancement and insulin resistance converge with the primary actions of caffeine on skeletal muscle.

Adenosine signalling in diabetes mellitus--pathophysiology and therapeutic considerations

Adenosine is a key extracellular signalling molecule that regulates several aspects of tissue function by activating four G-protein-coupled receptors, A1, A2A, A2B and A1 adenosine receptors. Accumulating evidence highlights a critical role for the adenosine system in the regulation of glucose homeostasis and the pathophysiology of type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). Although adenosine signalling is known to affect insulin secretion, new data indicate that adenosine signalling also contributes to the regulation of β-cell homeostasis and activity by controlling the proliferation and regeneration of these cells as well as the survival of β cells in inflammatory microenvironments. Furthermore, adenosine is emerging as a major regulator of insulin responsiveness by controlling insulin signalling in adipose tissue, muscle and liver; adenosine also indirectly mediates effects on inflammatory and/or immune cells in these tissues. This Review critically discusses the role of the adenosine-adenosine receptor system in regulating both the onset and progression of T1DM and T2DM, and the potential of pharmacological manipulation of the adenosinergic system as an approach to manage T1DM, T2DM and their associated complications.

Improved blood glucose disposal and altered insulin secretion patterns in adenosine A(1) receptor knockout mice

Type 2 diabetes mellitus (T2DM) is characterized by the inability of the pancreatic β-cells to secrete enough insulin to meet the demands of the body. Therefore, research of potential therapeutic approaches to treat T2DM has focused on increasing insulin output from β-cells or improving systemic sensitivity to circulating insulin. In this study, we examined the role of the A(1) receptor in glucose homeostasis with the use of A(1) receptor knockout mice (A(1)R(-/-)). A(1)R(-/-) mice exhibited superior glucose tolerance compared with wild-type controls. However, glucose-stimulated insulin release, insulin sensitivity, weight gain, and food intake were comparable between the two genotypes. Following a glucose challenge, plasma glucagon levels in wild-type controls decreased, but this was not observed in A(1)R(-/-) mice. In addition, pancreas perfusion with oscillatory glucose levels of 10-min intervals produced a regular pattern of pulsatile insulin release with a 10-min cycling period in wild-type controls and 5 min in A(1)R(-/-) mice. When the mice were fed a high-fat diet (HFD), both genotypes exhibited impaired glucose tolerance and insulin resistance. Increased insulin release was observed in HFD-fed mice in both genotypes, but increased glucagon release was observed only in HFD-fed A(1)R(-/-) mice. In addition, the regular patterns of insulin release following oscillatory glucose perfusion were abolished in HFD-fed mice in both genotypes. In conclusion, A(1) receptors in the pancreas are involved in regulating the temporal patterns of insulin release, which could have implications in the development of glucose intolerance seen in T2DM.

Impaired glucose tolerance in the absence of adenosine A1 receptor signaling

OBJECTIVE

The role of adenosine (ADO) in the regulation of glucose homeostasis is not clear. In the current study, we used A1-ADO receptor (A1AR)-deficient mice to investigate the role of ADO/A1AR signaling for glucose homeostasis.

RESEARCH DESIGN AND METHODS

After weaning, A1AR(-/-) and wild-type mice received either a standard diet (12 kcal% fat) or high-fat diet (HFD; 45 kcal% fat). Body weight, fasting plasma glucose, plasma insulin, and intraperitoneal glucose tolerance tests were performed in 8-week-old mice and again after 12-20 weeks of subsequent observation. Body composition was quantified by magnetic resonance imaging and epididymal fat-pad weights. Glucose metabolism was investigated by hyperinsulinemic-euglycemic clamp studies. To describe pathophysiological mechanisms, adipokines and Akt phosphorylation were measured.

RESULTS

A1AR(-/-) mice were significantly heavier than wild-type mice because of an increased fat mass. Fasting plasma glucose and insulin were significantly higher in A1AR(-/-) mice after weaning and remained higher in adulthood. An intraperitoneal glucose challenge disclosed a significantly slower glucose clearance in A1AR(-/-) mice. An HFD enhanced this phenotype in A1AR(-/-) mice and unmasked a dysfunctional insulin secretory mechanism. Insulin sensitivity was significantly impaired in A1AR(-/-) mice on the standard diet shortly after weaning. Clamp studies detected a significant decrease of net glucose uptake in A1AR(-/-) mice and a reduced glucose uptake in muscle and white adipose tissue. Effects were not triggered by leptin deficiency but involved a decreased Akt phosphorylation.

CONCLUSIONS

ADO/A1AR signaling contributes importantly to insulin-controlled glucose homeostasis and insulin sensitivity in C57BL/6 mice and is involved in the metabolic regulation of adipose tissue.

A study of three polymorphic sites of the ADA gene in children with type 1 diabetes mellitus

BACKGROUND

Adenosine deaminase is a polymorphic enzyme that has an important role in immune functions and in the regulation of intracellular and extracellular concentrations of adenosine and adenosine receptor activity.

AIM

To search for possible association of type 1 diabetes mellitus (DM1) with three loci haplotypes (ADA1, ADA2, ADA6) of the adenosine deaminase gene.

PATIENTS

One hundred and eighty-nine consecutive children with DM1 from Sassari, Sardinia, and a control sample of 239 children from the same area were studied.

METHODS

ADA loci genotypes were determined by DNA analysis.

RESULTS

Compared to controls, diabetic boys show a decrease of the 2(2)/6(1) haplotype while diabetic girls show an increase of the same haplotype. This association was replicated in an independent sample from Continental Italy.

CONCLUSIONS

The 2(2)/6(1) haplotype may exert a protective action in males but may increase susceptibility to DM1 in females: OR = 0.398, 95% CI 0.16-0.96 for males, and OR = 2.31, 95% CI 1.32-4.06 for females.

Angiotensin-converting enzyme in skeletal muscle: sentinel of blood pressure control and glucose homeostasis

Recent evidence suggests a coordinated regulation by the local renin-angiotensin system (RAS) and tissue kallikrein-kinin system (TKKS) of blood flow and substrate supply in oxidative red myofibres of skeletal muscle tissue during endurance exercise. The performance of these myofibres is dependent on the increased oxidation of substrates facilitated by augmenting nutritive blood flow and glucose uptake. Humoral factors released by the contracting fibres, such as adenosine and kinins, are suggested to be responsible for this metabolic adjustment. The considerable drain of blood volume and the enormous consumption of glucose during endurance exercise require a control mechanism for the maintenance of blood pressure (BP) and glucose homeostasis. This is achieved by the sympathetic nervous system and its subordinate RAS, which is located in the nutritive vessels and parenchyma of the red myofibres. The angiotensin-converting enzyme (ACE) is the primary enzyme responsible for kinin degradation during exercise, underscoring the important interrelationship between the RAS and the TKKS in the critical role of kinins in the multifactorial regulation of muscle bioenergetics and glucose and BP homeostasis. Importantly, overactivity of the ACE, as occurs in individuals displaying risk factors such as overweight, causes exaggerated BP response and reduced glucose disposal. If they persist over years, compensatory responses to this ACE overactivity, such as hypersecretion of insulin and compliance of the vessel walls, will inevitably be exhausted, leading ultimately to the manifestation of type 2 diabetes and hypertension. This concept also provides a unifying explanation for the beneficial effects of ACE-inhibitors and Angiotensin II receptor antagonists in the treatment of hypertension and insulin resistance.

Activation of the A1adenosine receptor increases insulin-stimulated glucose transport in isolated rat soleus muscle

The A1adenosine receptor (A1AR) has been suggested to participate in insulin- and contraction-stimulated glucose transport in skeletal muscle, but the qualitative and quantitative nature of the effect are controversial. We sought to determine if A1AR is expressed in rat soleus muscle and then characterize its role in glucose transport in this muscle. A1AR mRNA and protein expression were determined by RT-PCR and Western blotting, respectively. To examine the role of adenosine in 3-O-methylglucose transport, isolated muscles were exposed to adenosine deaminase and α,β-methylene adenosine diphosphate to remove endogenous adenosine and were left unstimulated (basal) or stimulated with insulin. To assess the functional participation of A1AR in 3-O-methylglucose transport, muscles were incubated with A1-selective agonist and (or) antagonist in the absence of endogenous adenosine and with or without insulin. A1AR mRNA was expressed in soleus muscle and A1AR was present at the plasma membrane. Removal of endogenous adenosine reduced glucose transport in response to 100 μU/mL insulin (~50%). The A1-selective agonist, N6-cyclopentyladenosine, increased submaximal (100 μU/mL) insulin-stimulated glucose transport in a dose-dependent manner (0.001–1.0 μmol/L). This stimulatory effect was inhibited by the A1-selective receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine in a concentration-dependent manner (0.001–1.0 μmol/L). However, neither activation nor inhibition of A1AR altered basal or maximal (10 mU/mL) insulin-stimulated glucose transport. Our results suggest that adenosine contributes ~50% to insulin-stimulated muscle glucose transport by activating the A1AR. This effect is limited to increasing insulin sensitivity, but not to either basal or maximal insulin-stimulated glucose uptake in rat soleus muscle.

Adenosine A1 receptors mediate mobilization of calcium in human bronchial smooth muscle cells

Adenosine stimulates contraction of airway smooth muscle, but the mechanism is widely considered indirect, depending on release of contractile agonists from mast cells and nerves. The goal was to determine whether adenosine, by itself, directly regulates calcium signaling in human bronchial smooth muscle cells (HBSMC). Primary cultures of HBSMC from normal subjects were loaded with fura 2-AM, and cytosolic calcium concentrations ([Ca(2+)](i)) were determined ratiometrically by imaging single cells. The nonselective adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA), and the adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine (CPA), both stimulated rapid, transient increases in [Ca(2+)](i). In contrast, there were no calcium responses to 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido-adenosine (100 nM) or N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (100 nM), selective agonists at adenosine A(2A) receptors and adenosine A(3) receptors, respectively. Calcium responses to NECA and CPA were inhibited by 8-cyclopentyl-1,3-dipropylxanthine, an adenosine A(1) receptor antagonist, and by pertussis toxin (PTX). In other experiments, NECA stimulated calcium transients in the absence of extracellular calcium, but not when cells were preincubated in cyclopiazonic acid or thapsigargin to empty intracellular calcium stores. Calcium responses were attenuated by xestospongin C and 2-aminoethoxydiphenylborane, inhibitors of inositol trisphosphate (IP(3)) receptors, and by U73122, an inhibitor of phospholipase C. It was concluded that stimulation of adenosine A(1) receptors on HBSMC rapidly mobilizes intracellular calcium stores by a mechanism dependent on PTX-sensitive G proteins, and IP(3) signaling. These findings suggest that, in addition to its well-established indirect effects on HBSMC, adenosine also has direct effects on contractile signaling pathways.

Functional characterization of the adenosine receptor contributing to glycogenolysis and gluconeogenesis in rat hepatocytes

The adenosine receptor subtype mediating glucose production by glycogenolysis and gluconeogenesis was studied in primary cultured rat hepatocytes. Adenosine and adenosine agonists caused cyclic AMP accumulation in rat hepatocytes. The order of potency was 5'-N-ethylcarboxamidoadenosine (NECA)>R(-)-N(6)-(2-phenylisopropyl)adenosine (RPIA)>adenosine>2-[p-(carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680). Furthermore, adenosine agonists stimulated glycogenolysis and gluconeogenesis. The order of potency was NECA>RPIA>CGS21680. The rank order of potency is typical for adenosine A(2B) receptors. Glycogenolysis stimulated by NECA was fully inhibited by nonselective adenosine antagonists, 9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine (CGS15943). However, the adenosine A(2A) receptor-selective antagonist, 8-(3-chlorostyryl)caffeine (CSC), and the adenosine A(1) receptor-selective antagonist, (+)-(R)-[(E)-3-(2-phenylpyrazolo[1,5-alpha]pyridin-3-yl)acryloyl]-2-piperidine ethanol (FK453), had a low inhibitory potency. A strong correlation was found between the inhibitory effect of adenosine antagonists on NECA-induced glucose production and that on intracellular cyclic AMP generation in rat hepatocytes. Our results suggest that adenosine stimulates cyclic AMP formation and regulates glycogenolysis and gluconeogenesis, most likely through the adenosine A(2B) receptor subtype in rat hepatocytes.

A1 receptor activation decreases fatigue in mammalian slow-twitch skeletal muscle in vitro

To test the hypothesis that adenosine improves skeletal muscle cell function, we exposed curarized mouse soleus and extensor digitorum longus (EDL) to a range of concentrations of adenosine (10–9 M to 10–5 M). Muscles contracted in Krebs-Henseleit bicarbonate buffer (27°C, 95% O2 and 5% CO2) for 500 ms at 50 Hz once every 90 s. Soleus fatigued significantly less with adenosine present at concentrations of 10–8 M and higher than with the Krebs-Henseleit vehicle control. Adenosine significantly improved force generation or delayed fatigue of EDL only with the initial adenosine challenge. To investigate the receptor population involved, we exposed soleus to agonists specific for A1 receptors (N6-cyclopentyladenosine, CPA), or A2 receptors (CGS 21680 hydrochloride, CGS), or A3 receptors (N6-benzyl-5'-N-ethylcarboxamidoadenosine, BNECA). CPA (A1) significantly decreased fatigue compared with the Krebs-Henseleit vehicle control at concentrations of 10–9 M and higher. Muscles exposed to the A2 and A3 agonists did not differ from a Krebs-Henseleit plus methanol control. Phenylephrine (10–6 M), an alpha-adrenergic agonist that increases the concentration of inositol triphosphate (IP3), significantly improved developed force in soleus. Neither a permeable cAMP analog, 8-bromo-cAMP (10–5 M), nor a beta1 agonist, isoproterenol (10–6 M), had an effect on force generation in the soleus when compared with a saline control. Thus adenosine slowed fatigue in slow-twitch skeletal muscle through A1 receptors.Key words: N6-cyclopentyladenosine, CGS 21680 hydrochloride, N6-benzyl-5'-N-ethylcarboxamidoadenosine, mouse skeletal muscle in vitro.

Distribution of adenosine A1, A2A and A2B receptors in human skeletal muscle

Many important physiological functions of skeletal muscle, such as glucose uptake, contraction and blood flow, have been proposed to be regulated via the action of adenosine on adenosine receptors. The cellular location of adenosine receptors in skeletal muscle is however, not known. The present study examined the distribution of A1, A2A and A2B adenosine receptors in human skeletal muscle using immunohistochemistry. All three receptor types were localized to vascular smooth muscle and endothelial cells, only the adenosine A2A and A2B receptors were observed in the plasma membrane and cytosol of the skeletal muscle. The finding was supported by results from western-blotting analysis. The cytosolic staining of the adenosine A2A receptor was slightly more intense in the type I muscle fibres, whereas the A2B receptor was almost absent in type I fibres. The present findings demonstrate for the first time, direct evidence for the existence of A2A and A2B adenosine receptors but absence of the A1 receptor in the sarcolemma and cytosol of skeletal muscle cells. The data also show existence of all three of the A1, A2A and A2B adenosine receptors in vascular cells of skeletal muscle tissue.

Role of adenosine in regulation of carbohydrate metabolism in contracting muscle

Adenosine production from AMP in the sarcoplasm and interstitial space of muscle is markedly enhanced during contractions. The produced adenosine may act as a 'local hormone' by binding to various types of adenosine receptors present in the membrane of adjacent cells, including skeletal muscle, vascular smooth muscle and neurons. Thus, interstitial adenosine may significantly contribute to regulation of muscle carbohydrate metabolism, both by adjusting metabolism and local blood flow to the energy needs imposed by a given degree of contratile activity on the muscle cell. The studies presented here demonstrate that endogenous adenosine via A1-adenosine receptors is able to directly stimulate insulin-mediated glucose transport in oxidative muscle cells during contractions. In addition, adenosine may further contribute to stimulation of muscle glucose uptake during contractions by increasing blood flow and thereby targetting glucose and insulin delivery to active muscle fibres. Furthermore, our findings demonstrate that adenosine via A1- and A2-receptors may inhibit glycogen breakdown in oxidative muscle tissue which during contractions is simultaneously exposed to insulin and beta-adrenergic stimulation. It is concluded that adenosine importantly contributes to regulation of carbohydrate metabolism in oxidative muscle fibers during contractions.

Caffeine ingestion and metabolic responses of tetraplegic humans during electrical cycling

Normally, caffeine ingestion results in a wide spectrum of neural and hormonal responses, making it difficult to evaluate which are critical regulatory factors. We examined the responses to caffeine (6 mg/kg) ingestion in a group of spinal cord-injured subjects [7 tetraplegic (C5-7) and 2 paraplegic (T4) subjects] at rest and during functional electrical stimulation of their paralyzed limbs to the point of fatigue. Plasma insulin did not change, caffeine had no effect on plasma epinephrine, and there was a slight increase (P < 0. 05) in norepinephrine after 15 min of exercise. Nevertheless, serum free fatty acids were increased (P < 0.05) after caffeine ingestion after 60 min of rest and throughout the first 15 min of exercise, but the respiratory exchange ratio was not affected. The exercise time was increased (P < 0.05) by 6% or 1.26 +/- 0.57 min. These data suggest that caffeine had direct effects on both the adipose tissue and the active muscle. It is proposed that the ergogenic action of caffeine is occurring, at least in part, by a direct action of the drug on muscle.

A1 adenosine receptor antagonism improves glucose tolerance in Zucker rats

The A1 adenosine receptor (A1ar) antagonist 1,3-dipropyl-8-(p-acrylic)-phenylxanthine (BW-1433) was administered to lean and obese Zucker rats to probe the influence of endogenously activated A1ars on whole body energy metabolism. The drug induced a transient increase in lipolysis as indicated by a rise in serum glycerol in obese rats. The disappearance of the response by day 7 of chronic studies was accompanied by an increase in A1ar numbers. Glucose tolerance tests were administered to rats treated with BW-1433. Peak serum insulin levels and areas under glucose curves (AUGs) were 34 and 41% lower in treated obese animals than in controls, respectively, and 19 and 39% lower in lean animals. With chronic administration (6 wk), AUGs decreased 47 and 33% in obese and lean animals, respectively. There was no effect of BW-1433 in either lean or obese rats on weight gain or percent body fat. Thus the major sustained influence of whole body A1ar antagonism in both lean and obese animals was an increase in whole body glucose tolerance at lower levels of insulin.

Short-term metabolic and haemodynamic effects of GR79236 in normal and fructose-fed rats

The adenosine (A1) receptor agonist, GR79236 (N-[(1S,trans)-2-hydroxycyclopentyl]adenosine), inhibits catecholamine-induced lipolysis in vitro, but the short-term metabolic and haemodynamic effects have not been previously reported in the fructose fed model of insulin resistance, dyslipidaemia and hypertension. This study reports the effects of GR79236 (1 mg/kg/day for 8 days) on nonesterified free fatty acid and triglyceride metabolism, oral and i.v. glucose tolerance, blood pressure and heart rate, and insulin sensitivity, in normal rats and rats fed a fructose-enriched diet. In normal rats, GR79236 significantly reduced fasting glucose (25%), free fatty acid (50%) and triglyceride (55%) concentrations, and improved glucose tolerance (AUC[glu] 21.2 +/- 1.3 vs. 16.5 +/- 1.1 mmol h/l, p < 0.05). Fructose feeding induced a state of insulin resistance and dyslipidaemia, as shown by an increase in steady-state plasma glucose levels (7.1 vs. 6.1 mmol/l), impaired i.v. glucose tolerance and a 3-fold rise in fasting triglyceride levels; fructose-fed rats also developed a significant increase in blood pressure. GR79236 ameliorated the effects of fructose feeding on fatty acid and triglyceride levels, and blood pressure, and improved i.v. glucose tolerance in fructose-fed rats. The hypotriglyceridaemic effect was due to a reduction in triglyceride secretion rate (17.3 +/- 1.7 vs. 30.2 +/- 1.1). Thus, in normal rats and in a dietary-induced rodent model of insulin resistance, dyslipidaemia and hypertension, GR79236 has lipid-lowering and glucose-lowering activity, as well as haemodynamic effects, which are potentially useful for treating both the metabolic and haemodynamic features of insulin resistance and NIDDM in humans.

Inhibition of human tumour cell proliferation by analogues of adenosine

The effects of adenosine and several structural analogues of adenosine upon thymidine incorporation into human tumour cells and rat cervical lymphocytes were investigated. The analogue NECA, which has equal specificity for the A1 and A2 receptor, had the most inhibitory effect on lymphocyte proliferation while the A1 agonists had limited effects, suggesting that these cells possess principally A2 adenosine receptors. In the case of human tumour cells, however, the most inhibitory effect on proliferation was obtained with the A1-specific analogues. The general order of inhibitory effects of adenosine analogues on thymidine incorporation in human tumour cells was: S-ENBA > CPA = R-PIA > S-PIA > NECA. These findings suggest that in the cells presently studied the A1 adenosine receptor predominates. Removal of exogenous adenosine by growth in the presence of adenosine deaminase inhibited thymidine incorporation. The effect of adenosine removal lends further support to the proposal that adenosine has some, as yet unidentified, regulatory role in the control of human tumour cell proliferation.

Adenosine exerts a glycogen-sparing action in contracting rat skeletal muscle

The role of adenosine in regulating glycogen breakdown during electrically induced muscle contractions was investigated in isolated rat hindquarters perfused with a standard medium either lacking or containing 100 microU/ml insulin and/or 1.67 nM isoprenaline. Nonselective A1/A2-adenosine receptor antagonism via caffeine enhanced (P < 0.05) glycogen breakdown in contracting fast-oxidative (FO) fibers by 40%, provided they were exposed to both insulin and isoprenaline. Combined A1/A2-receptor antagonism by 8-cyclopentyl-1,3-dipropylxanthine (CPDPX) plus 3,7-dimethyl-1-proparglyxanthine (DMPX) fully reproduced (P < 0.05) this stimulatory effect. Furthermore, CPDPX plus DMPX also enhanced (P < 0.05) glycogenolysis during contractions in soleus but not in white gastrocnemius muscle. In contrast, CPDPX or DMPX alone did not affect glycogenolysis in either fiber type. Muscle adenosine 3',5'-cyclic monophosphate concentration during contractions was increased (P < 0.05) by CPDPX plus DMPX in both fiber types, whereas glycogen synthase fractional activity was depressed (P < 0.05). Phosphorylase activity was not changed by CPDPX plus DMPX. It is concluded that adenosine exerts a glycogen-sparing action in oxidative skeletal muscle exposed to both insulin and beta-adrenergic stimulation during contraction, presumably via stimulation of glycogen synthase activity.

Activation of A2-purinoceptors by adenosine stimulates L-arginine transport (system y+) and nitric oxide synthesis in human fetal endothelial cells

1. Human umbilical vein endothelial cells were challenged acutely with adenosine and its analogues to examine whether adenosine modulates L-arginine transport (system y+) and synthesis of nitric oxide (NO) and prostacyclin (PGI2). 2. L-Arginine transport was stimulated by adenosine (10 microM, 2 min) and the A2-receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680; 100 nM), but not by the A1-receptor agonist N6-cyclopentyladenosine (CPA). 3. Activation of L-arginine transport was inhibited by the A2-receptor antagonists ZM-241385 and 3,7-dimethyl-1-propargylxanthine (DMPX), but unaffected by the A1-receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine and 8-phenyltheophylline or the adenosine transport inhibitor nitrobenzylthioinosine. 4. Adenosine and CGS-21680 evoked a rapid membrane hyperpolarization. 5. Adenosine and CGS-21680 induced increases in intracellular cGMP levels, whereas release of PGI2 was unaffected. NG-nitro-L-arginine methyl ester (an NO synthase inhibitor) and the A2-receptor antagonists ZM-241385 and DMPX prevented increases in cGMP accumulation. 6. Our findings provide the first evidence that activation of human fetal endothelial cell A2-purinoceptors, but not A1-purinoceptors, leads to a membrane hyperpolarization and stimulation of basal rates of L-arginine transport and NO biosynthesis.

Metabolic catecholamine, and endurance responses to caffeine during intense exercise

This study examined the possible effects of caffeine ingestion on muscle metabolism and endurance during brief intense exercise. We tested 14 subjects after they ingested placebo or caffeine (6 mg/kg) with an exercise protocol in which they cycled for 2 min, rested 6 min, cycled 2 min, rested 6 min, and then cycled to voluntary exhaustion. In each exercise the intensity required the subject's maximal O2 consumption. Eight subjects had muscle and venous blood samples taken before and after each exercise period. The caffeine ingestion resulted in a significant increase in endurance (4.12 +/- 0.36 and 4.93 +/- 0.60 min for placebo and caffeine, respectively) and resulted in a significant increase in plasma epinephrine concentration throughout the protocol but not in norepinephrine concentration. During the first two exercise bouts, the power and work output were not different; blood lactate concentrations were not affected significantly by caffeine ingestion, but during the exercise bouts muscle lactate concentration was significantly increased by caffeine. The net decrease in muscle glycogen was not different between treatments at any point in the protocol, and even at the time of fatigue there was at least 50% of the original glycogen concentration remaining. the data demonstrated that caffeine ingestion can be an effective ergogenic aid for exercise that is as brief as 4-6 min. However, the mechanism is not associated with muscle glycogen sparing. It is possible that caffeine is exerting actions directly on the active muscle and/or the neural processes that are involved in the activity.

In vitro effect of adenosine agonist GR79236 on the insulin sensitivity of glucose utilisation in rat soleus and human rectus abdominus muscle

The dose-response effects of a new adenosine agonist, GR79236, were examined in isolated rat soleus muscle strips and human rectus abdominus muscle strips. Effects on the insulin sensitivity of carbohydrate metabolism were examined, in particular upon insulin stimulated glycogen synthesis and glycolytic flux. In the presence of adenosine deaminase (ADA), GR79236 increased insulin sensitivity of pyruvate release from rat soleus muscle strips by 24% from 82.5 +/- 10.0 to 102.5 +/- 10.0 (P < 0.01), by 27% to 105.0 +/- 12.5 (P < 0.01) and by 24% to 102.5 +/- 10.0 (P < 0.01) nmol/25 mg per h at 0.1 and 10 microM GR79236, respectively. Rates of lactate release followed a similar but non-significant trend. Addition of GR79236 in the presence of ADA had no effect on rates of glycogen synthesis. Insulin stimulated rates of pyruvate or lactate release or of glycogen synthesis were unaffected by the addition of adenosine deaminase or GR79236 in human rectus abdominus muscle strips. Adenosine agonists may act indirectly to modulate insulin sensitivity of carbohydrate metabolism.

Inhibition of insulin receptor phosphorylation by PC-1 is not mediated by the hydrolysis of adenosine triphosphate or the generation of adenosine

Individuals with insulin resistance show increased levels of PC-1 expression in skeletal muscle and fibroblasts, and in transfected cell lines that overexpress PC-1 there is a reduction in the insulin-stimulated insulin receptor tyrosine phosphorylation. As PC-1 is a type II transmembrane protein with extracellular phosphodiesterase and pyrophosphatase activity, increased expression of PC-1 at the cell surface will decrease extracellular adenosine triphosphate levels and increase extracellular adenosine levels. Consequently it is possible that PC-1-mediated insulin resistance could be caused either by a decrease in adenosine triphosphate or an indirect increase in adenosine levels. We have tested this hypothesis and find that the PC-1-mediated inhibition of insulin-stimulated insulin receptor autophosphorylation is not altered by agents that alter the level or action of adenosine. Further, a mutated PC-1 with a single amino acid change that abolishes the phosphodiesterase and pyrophosphatase activities is still able to inhibit insulin-stimulated insulin receptor phosphorylation. The results of these experiments indicate that the phosphodiesterase activity of PC-1 is not involved in the inhibition of insulin receptor autophosphorylation.

Vascular and endocrine control of muscle metabolism

Important differences exist between perfused and incubated (or perifused) skeletal muscle preparations with regard to their metabolism and control. A growing body of evidence suggests that the differences may be due to the role played by the vascular system. In the constant-flow perfused rat hindlimb preparation, a group of vasoconstrictors has been identified that enhance muscle metabolism and aerobic contractility. Another group of vasoconstrictors decrease muscle metabolism and aerobic contractility even though perfusate flow remains constant. All effects of both groups of vasoconstrictors are opposed by vasodilators. Because none of the vasoconstrictor effects is evident when isolated muscles are incubated or perifused, involvement of an active vascular system is indicated. Although some hormones may act directly on muscle by purely endocrine effects, a vascular component of their actions is now emerging. Mechanisms to account for vascular control of perfused skeletal muscle metabolism may involve 1) functional vascular shunts where the proportion of flow processed by these is regulated by site-specific vasomodulators, 2) a direct response to a change in the rate of supply of nutrients and removal of products, and 3) a signal substance released by vascular tissue in association with vasoconstriction that interacts with surrounding skeletal muscle cells. Impaired control at the level of the vascular system may have implications for long-term access of nutrients and hormones and therefore the control of skeletal muscle metabolism and contractile performance.

Aminophylline enhances resting Ca2+ concentrations and twitch tension by adenosine receptor blockade in Rana pipiens

1. We hypothesized that the xanthine aminophylline acts to block adenosine receptors on the surface of skeletal muscle fibres, thereby inhibiting a depressant action of endogenous adenosine. We further hypothesized that this action results in increased concentrations of intracellular resting Ca2+ and enhanced twitch tension upon muscle stimulation. 2. Peak twitch tension (Pt) of the semitendinosus muscle in normal frog Ringer solution (NFR) ranged from 6.8 to 9.4 g. Intracellular Ca2+ concentrations in control resting fibres ranged from 67 to 70 nM. Aminophylline at 100 microM produced increases of 26 and 22% in Pt and Ca2+ concentrations, respectively. 3. The adenosine receptor antagonists 8-phenyltheophylline (8-PT) and 1,3-dipropyl-7-methylxanthine (1,3-d-7-M) both increased Pt by 32% over values in NFR. In addition, 1,3-d-7-M increased resting Ca2+ concentrations by 29% over control levels. 4. Adenosine deaminase increased twitch tension and resting intracellular Ca2+ concentrations by 22 and 26% over controls, respectively. 5. N6-(2-phenylisopropyl)adenosine (R-PIA, 1 microM), a potent adenosine analogue, partially blocked both the increase in Pt and intracellular Ca2+ concentrations induced by the xanthines, possibly by competing for the adenosine receptor. 6. The data herein provide support for the existence of adenosine receptors on the membranes of skeletal muscle fibres and suggest a possible role for adenosine receptors in the regulation of twitch tension.

Adenosine transport in cultured human umbilical vein endothelial cells is reduced in diabetes

Adenosine transport in cultured human umbilical vein endothelial cells (HUVEC) was characterized and shown to be mediated by a single facilitated diffusion mechanism. Initial rates of adenosine influx at 22 degrees C were saturable [apparent Michaelis constant, 69 +/- 10 microM; maximum velocity (Vmax), 600 +/- 70 pmol.10(6) cells-1.s-1] and inhibited by nitrobenzylthioinosine (NBMPR). Formycin B had an unusually high affinity [inhibitory constant (Ki), 18 +/- 4.3 microM], whereas inosine had a low affinity (Ki, 440 +/- 68 microM) and nucleobases were without effect on adenosine influx. The number of transporters (1.2 x 10(6) sites/cell) was estimated by NBMPR equilibrium binding (apparent dissociation constant, 0.11 +/- 0.01 nM; maximum binding, 2.0 +/- 0.15 pmol/10(6) cells). In addition, we compared these endothelial cells with those obtained from cords from pregnancies complicated by diabetes (HUVEC-D), since embriopathy may occur in these conditions. HUVEC-D exhibited a 2.3-fold reduction in both the Vmax for adenosine influx and the maximum number of NBMPR binding sites (260 +/- 40 pmol.10(6) cells-1.s-1 and 0.86 +/- 0.08 pmol/10(6) cells, respectively). However, the turnover number for each nucleoside transporter in normal and diabetic HUVEC was similar (approximately 300 adenosine molecules/s). Adenosine metabolism at 10 microM in HUVEC-D was modified compared with normal cells. Intracellular phosphorylation (> 90%) was the predominant pathway in normal HUVEC, whereas in HUVEC-D, substantial levels of adenine and adenosine were detected. The present results demonstrate therefore the downregulation of the NBMPR-sensitive nucleoside transporter and changes in adenosine metabolism in HUVEC from diabetic pregnancies.

Caffeine and exercise: metabolism and performance

Caffeine ingestion prior to prolonged exercise delays fatigue. However, the mechanisms involved are very unclear. Caffeine is associated with elevated plasma epinephrine but the metabolic impact of this is uncertain. Glycogen sparing occurs in active muscle, at least in the first few minutes, but studies have generally failed to demonstrate enhanced fat metabolism. The demethylation of caffeine by the hepatic cytochrome P-450 oxygenases begins within minutes and dimethylxanthines (especially paraxanthine) are generated. These compounds appear in the plasma within an hour of caffeine ingestion and may have effects on tissues that have been attributed to caffeine and/or epinephrine. While the most widely supported theory is that caffeine and other methylxanthines are adenosine receptor antagonists, this action alone cannot explain all of the observed responses. Nevertheless, habituation to and withdrawal from caffeine are associated with up and down regulation of adenosine receptors. One study demonstrated marked differences in the effects of caffeine on the plasma concentrations of epinephrine and dimethylxanthines between caffeine users and nonusers. Caffeine is clearly a very active drug that has many effects on humans including increasing exercise endurance. This can be associated with muscle glycogen sparing and elevated plasma epinephrine, but the underlying mechanisms are unknown.

Adenosine receptors mediate synergistic stimulation of glucose uptake and transport by insulin and by contractions in rat skeletal muscle

The role of adenosine receptors in the regulation of muscle glucose uptake by insulin and contractions was studied in isolated rat hindquarters that were perfused with a standard medium containing no insulin or a submaximal concentration of 100 microU/ml. Adenosine receptor antagonism was induced by caffeine or 8-cyclopentyl-1,3-dipropylxantine (CPDPX). Glucose uptake and transport were measured before and during 30 min of electrically induced muscle contractions. Caffeine nor CPDPX affected glucose uptake in resting hindquarters. In contrast, the contraction-induced increase in muscle glucose uptake was inhibited by 30-50% by caffeine, as well as by CPDPX, resulting in a 20-25% decrease in the absolute rate of glucose uptake during contractions, compared with control values. This inhibition was independent of the rate of perfusate flow and only occurred in hindquarters perfused with insulin added to the medium. Thus, adenosine receptor antagonism inhibited glucose uptake during simultaneous exposure to insulin and contractions only. Accordingly, caffeine inhibited 3-O-methylglucose uptake during contractions only in oxidative muscle fibers that are characterized by a high sensitivity to insulin. In conclusion, the present data demonstrate A1 receptors to regulate insulin-mediated glucose transport in contracting skeletal muscle. The findings provide evidence that stimulation of sarcolemmic adenosine receptors during contractions is involved in the synergistic stimulation of muscle glucose transport by insulin and by contractions.