Tissue-specific effects of in vivo adenosine receptor blockade on glucose uptake in Zucker rats

Preclinical and clinical evaluation of a new method to assess cardiac insulin resistance using nuclear imaging

BACKGROUND

Myocardial insulin resistance (IR) could be a predictive factor of cardiovascular events. This study aimed to introduce a new method using 123I-6-deoxy-6-iodo-D-glucose (6DIG), a pure tracer of glucose transport, for the assessment of IR using cardiac dynamic nuclear imaging.

METHODS

The protocol evaluated first in rat-models consisted in two 6DIG injections and one of insulin associated with planar imaging and blood sampling. Compartmental modeling was used to analyze 6DIG kinetics in basal and insulin conditions and to obtain an index of IR. As a part of a translational approach, a clinical study was then performed in 5 healthy and 6 diabetic volunteers.

RESULTS

In rodent models, the method revealed reproducible when performed twice at 7 days apart in the same animal. Rosiglitazone, an insulin-sensitizing drug, induced a significant increase of myocardial IR index in obese Zucker rats from 0.96 ± 0.18 to 2.26 ± 0.44 (P<.05) after 7 days of an oral treatment, and 6DIG IR indexes correlated with the gold standard IR index obtained through the hyperinsulinemic-euglycemic clamp (r=.68, P<.02). In human, a factorial analysis was applied on images to obtain vascular and myocardial kinetics before compartmental modeling. 1.5-fold to 2.2-fold decreases in mean cardiac IR indexes from healthy to diabetic volunteers were observed without reaching statistical significance.

CONCLUSIONS

These preclinical results demonstrate the reproducibility and sensibility of this novel imaging methodology. Although this first in-human study showed that this new method could be rapidly performed, larger studies need to be planned in order to confirm its performance.

Therapeutic potentials of agonist and antagonist of adenosine receptors in type 2 diabetes

Type 2 diabetes has been a global health challenge over the decades and is among the leading causes of death. Several treatment approaches have been developed, but more effective and new therapies are still needed. The role of adenosine in glucose and lipid homeostasis has offered a different therapeutic approach. Adenosine mediates its physiological role through the activation of adenosine receptors. These adenosine receptors have been implicated in glucose and lipid homeostasis. The ability of agonists and antagonists of adenosine receptors to activate or inhibit the adenosine signalling cascade and thereby affecting the balance of glucose and lipid homeostasis has challenged the studies of agonists and antagonists of adenosine receptors, both preclinical and clinical, as potential anti-diabetic drugs. This review provides a background on different anti-diabetic therapeutic approaches, outlining the role of adenosine receptors in glucose and lipid homeostasis, and mechanisms underlying the action of agonists/antagonists of adenosine receptors as a therapeutic potential towards type 2 diabetes.

Caffeine Sources and Consumption among Saudi Adults Living with Diabetes and Its Potential Effect on HbA1c

Information regarding the spread and effect of coffee and caffeine intake by individuals with type II diabetes remains unclear. This study aims to identify the amount and sources of habitual caffeine intake by individuals with type II diabetes and to investigate its association with other health outcomes, especially HbA1c. This is a cross-sectional survey involving 100 people medically defined as having type II diabetes comprising both genders, recruited from a care centre. All participants completed a caffeine semi-quantitative food frequency questionnaire (C-FFQ) to estimate their caffeine consumption, a two day 24-h recall, and a detailed questionnaire. The average caffeine intake was calculated from all sources and the differences in mean by gender were tested using a regression model (adjusted to important confounders). Regression models were used to verify the association between average caffeine intake on HbA1c and other health outcomes with adjustment for important confounders. A p value < 0.05 represented statistical significance. Arabic coffee (gahwa) and tea were the most common sources of caffeine among Saudi adults living with diabetes. Average caffeine intake for the whole sample was 194 ± 165 mg/day, which is 2.3 ± 2 mg/kg. There was an inverse association between caffeine intake and age: difference in mean -3.26 mg/year (95%CI: -5.34, -1.18; p = 0.003). Males had significantly higher consumption of caffeine compared to females: difference in mean 90.7 mg/day (95%CI: 13.8, 167.6; p = 0.021). No association was found between average caffeine intake and HbA1C or any other cardiovascular risk factors. This information can help public health practitioners and policy makers when assessing the risk of caffeine consumption among this vulnerable group.

Adenosine as a Marker and Mediator of Cardiovascular Homeostasis: A Translational Perspective

Adenosine, a purine nucleoside, is produced broadly and implicated in the homeostasis of many cells and tissues. It signals predominantly via 4 purinergic adenosine receptors (ADORs) - ADORA1, ADORA2A, ADORA2B and ADOosine signaling, both through design as specific ADOR agonists and antagonists and as offtarget effects of existing anti-platelet medications. Despite this, adenosine has yet to be firmly established as either a therapeutic or a prognostic tool in clinical medicine to date. Herein, we provide a bench-to-bedside review of adenosine biology, highlighting the key considerations for further translational development of this proRA3 in addition to non-ADOR mediated effects. Through these signaling mechanisms, adenosine exerts effects on numerous cell types crucial to maintaining vascular homeostasis, especially following vascular injury. Both in vitro and in vivo models have provided considerable insights into adenosine signaling and identified targets for therapeutic intervention. Numerous pharmacologic agents have been developed that modulate adenmising molecule.

The involvement of purinergic signalling in obesity

Obesity is a growing worldwide health problem, with an alarming increasing prevalence in developed countries, caused by a dysregulation of energy balance. Currently, no wholly successful pharmacological treatments are available for obesity and related adverse consequences. In recent years, hints obtained from several experimental animal models support the notion that purinergic signalling, acting through ATP-gated ion channels (P2X), G protein-coupled receptors (P2Y) and adenosine receptors (P1), is involved in obesity, both at peripheral and central levels. This review has drawn together, for the first time, the evidence for a promising, much needed novel therapeutic purinergic signalling approach for the treatment of obesity with a 'proof of concept' that hopefully could lead to further investigations and clinical trials for the management of obesity.

Exercise partially reverses the inhibitory effect of caffeine on liver gluconeogenesis in type 1 diabetic rats with hypoglycemia

The purpose was to determine the possible effects of exercise and/or caffeine on hypoglycemia and liver gluconeogenesis in diabetic rats. These were divided into four subgroups: (a) intraperitoneal insulin only, (b) exercise bout before insulin, (c) caffeine after insulin, and (d) exercise bout before and caffeine after insulin. The marked glycemic drop 45 min after insulin (0 min = 229.00, 45 min = 75.75) was considerably reduced (p < 0.05) by caffeine or exercise (45 min: exercise = 127.00, caffeine = 104.78). However, this systemic effect was lost (p > 0.05) when they were combined (45 min: exercise + caffeine = 65.44) (Mean, in mg·dL^-1). Caffeine alone strongly inhibited liver glucose production from 2 mM lactate 45 min after insulin (without caffeine = 3.05, with caffeine = 0.27; p < 0.05), while exercise + caffeine partially re-established the liver gluconeogenic capacity (exercise + caffeine = 1.61; p < 0.05 relative to the other groups) (Mean, in μmol·g^-1). The improved hypoglycemia with caffeine or exercise cannot be explained by their actions on liver gluconeogenesis. As their beneficial effect disappeared when they were combined, such association in diabetic patients should be avoided during the period of hyperinsulinemia due to the risk of severe hypoglycemia.

Molecular implications of adenosine in obesity

Adenosine has broad activities in organisms due to the existence of multiple receptors, the differential adenosine concentrations necessary to activate these receptors and the presence of proteins able to synthetize, degrade or transport this nucleoside. All adenosine receptors have been reported to be involved in glucose homeostasis, inflammation, adipogenesis, insulin resistance, and thermogenesis, indicating that adenosine could participate in the process of obesity. Since adenosine seems to be associated with several effects, it is plausible that adenosine participates in the initiation and development of obesity or may function to prevent it. Thus, the purpose of this review was to explore the involvement of adenosine in adipogenesis, insulin resistance and thermogenesis, with the aim of understanding how adenosine could be used to avoid, treat or improve the metabolic state of obesity. Treatment with specific agonists and/or antagonists of adenosine receptors could reverse the obesity state, since adenosine receptors normalizes several mechanisms involved in obesity, such as lipolysis, insulin sensitivity and thermogenesis. Furthermore, obesity is a preventable state, and the specific activation of adenosine receptors could aid in the prevention of obesity. Nevertheless, for the treatment of obesity and its consequences, more studies and therapeutic strategies in addition to adenosine are necessary.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Strain-dependent differences for suppression of insulin-stimulated glucose uptake in skeletal and cardiac muscle by ethanol

BACKGROUND

Chronic ethanol (EtOH) consumption impairs the ability of insulin to suppress hepatic glucose production in a strain-dependent manner, with hepatic insulin resistance being greater in Long-Evans (LE) than Sprague-Dawley (SD) rats. We assessed whether strain differences exist for whole-body and tissue glucose uptake under basal and insulin-stimulated conditions and whether they were associated with coordinate strain-dependent elevations in muscle cytokines.

METHODS

Male rats (160 g) were provided the Lieber-DeCarli EtOH-containing (36% total energy) diet or pair-fed a control diet for 8 weeks. Rats were studied in the basal state or during a euglycemic hyperinsulinemic clamp, and whole-body glucose flux assessed using (3) H-glucose and in vivo tissue glucose uptake by (14) C-2-deoxyglucose.

RESULTS

EtOH impaired whole-body insulin-mediated glucose uptake (IMGU) more in SD than LE rats. This difference was due to impaired IMGU by gastrocnemius and heart in EtOH-fed SD versus LE rats. However, decreased IMGU in adipose tissue (epididymal and perirenal) produced by EtOH was comparable between strains. EtOH-induced insulin resistance in muscle from SD rats was associated with reduced AKT and AS160 phosphorylation and plasma membrane-localized GLUT4 protein as well as enhanced phosphorylation of c-Jun N-terminal kinase (JNK) and IRS-1 (S307), changes which were absent in muscle from LE rats. EtOH increased tumor necrosis factor alpha (TNFα) mRNA in gastrocnemius and fat under basal conditions in both SD and LE rats; however, hyperinsulinemia decreased TNFα in skeletal muscle from LE, but not SD rats. Interleukin (IL)-6 mRNA in gastrocnemius was increased under basal conditions and increased further in response to insulin in SD rats, but no EtOH- or insulin-induced change was detected in muscle IL-6 of LE rats.

CONCLUSIONS

These data indicate strain-dependent differences in EtOH-induced IMGU in skeletal and cardiac muscle, but not fat, associated with sustained increases in TNFα and IL-6 mRNA and JNK activation and decreased plasma membrane GLUT4 in response to insulin.

Effects of chronic caffeine administration on blood glucose levels and on glucose tolerance in healthy and diabetic rats

OBJECTIVE

To analyse the effect of chronic caffeine use on risk reduction and prognosis of diabetes mellitus.

METHODS

In this 60-day study, five groups of 11 healthy male Wistar rats were selected to receive one of four doses (37.5, 56.2, 75.0 or 93.0 mg/kg per day) of caffeine orally or no caffeine (control). The effect of caffeine on glycaemia and glucose tolerance was evaluated. After 15 days, each group was treated with 60 mg/kg of streptozotocine to induce diabetes mellitus, and glycaemia and glucose tolerance were assessed for a further 45 days.

RESULTS

In nondiabetic rats, caffeine had no effect on blood glucose. Compared with controls, the fasting blood glucose levels declined significantly in two caffeine-treated groups (93.0 mg/kg per day and 56.2 mg/kg per day) during the first 15 days following diabetes induction. Glucose tolerance was significantly improved 120 min after glucose loading in all caffeine-treated groups. The mean ± SE half-maximal effective concentration of caffeine was 35.79 ± 2.44 mg/dl.

CONCLUSIONS

Blood glucose levels decreased, and glucose tolerance improved, in diabetic rats administered increasing doses of caffeine.

Synthesis and hypoglycemic activity of some new theophylline derivatives

Thirty-one new theophylline derivatives have been synthesized and evaluated for their hypoglycemic activity. Compounds 24 (56% reduction) and 31 (57% reduction) showed better hypoglycemic activity than the standard drug glibenclamide which showed 52% reduction in serum glucose level. Compound 27 remarkably reduced serum glucose level by 53%. Ten compounds showed varying degrees of hypoglycemic activity ranging from 20 to 37% reduction in serum glucose level compared to the standard drug. The aromatic amide functionality is the common feature of these theophylline hypoglycemic derivatives. However, anthranilamide and or aliphatic amides proved to be the least active compounds in the present series.

Adenosine, adenosine receptors and their role in glucose homeostasis and lipid metabolism

Adenosine is an endogenous metabolite that is released from all tissues and cells including liver, pancreas, muscle and fat, particularly under stress, intense exercise, or during cell damage. The role of adenosine in glucose homeostasis has been attributed to its ability to regulate, through its membrane receptors, processes such as insulin secretion, glucose release and clearance, glycogenolysis, and glycogenesis. Additionally, adenosine and its multiple receptors have been connected to lipid metabolism by augmenting insulin-mediated inhibition of lipolysis, and the subsequent increase in free fatty acids and glycerol levels. Furthermore, adenosine was reported to control liver cholesterol synthesis, consequently affecting plasma levels of cholesterol and triglycerides, and the amount of fat tissue. Alterations in the balance of glucose and lipid homeostasis have implications in both cardiovascular disease and diabetes. The ability of different adenosine receptors to activate and inhibit the same signaling cascades has made it challenging to study the influence of adenosine, adenosine analogs and their receptors in health and disease. This review focuses on the role and significance of different adenosine receptors in mediating the effect of adenosine on glucose and lipid homeostasis. J. Cell. Physiol. © 2013 Wiley Periodicals, Inc.

Effects of adenosine, exercise, and moderate acute hypoxia on energy substrate utilization of human skeletal muscle

Glucose metabolism increases in hypoxia and can be influenced by endogenous adenosine, but the role of adenosine for regulating glucose metabolism at rest or during exercise in hypoxia has not been elucidated in humans. We studied the effects of exogenous adenosine on human skeletal muscle glucose uptake and other blood energy substrates [free fatty acid (FFA) and lactate] by infusing adenosine into the femoral artery in nine healthy young men. The role of endogenous adenosine was studied by intra-arterial adenosine receptor inhibition (aminophylline) during dynamic one-leg knee extension exercise in normoxia and acute hypoxia corresponding to ∼3,400 m of altitude. Extraction and release of energy substrates were studied by arterial-to-venous (A-V) blood samples, and total uptake or release was determined by the product of A-V differences and muscle nutritive perfusion measured by positron emission tomography. The results showed that glucose uptake increased from a baseline value of 0.2 ± 0.2 to 2.0 ± 2.2 μmol·100 g(-1)·min(-1) during adenosine infusion (P < 0.05) at rest. Although acute hypoxia enhanced arterial FFA levels, it did not affect muscle substrate utilization at rest. During exercise, glucose uptake was higher (195%) during acute hypoxia compared with normoxia (P = 0.058), and aminophylline had no effect on energy substrate utilization during exercise, despite that arterial FFA levels were increased. In conclusion, exogenous adenosine at rest and acute moderate hypoxia during low-intensity knee-extension exercise increases skeletal muscle glucose uptake, but the increase in hypoxia appears not to be mediated by adenosine.

Olanzapine promotes fat accumulation in male rats by decreasing physical activity, repartitioning energy and increasing adipose tissue lipogenesis while impairing lipolysis

Olanzapine and other atypical antipsychotics cause metabolic side effects leading to obesity and diabetes; although these continue to be an important public health concern, their underlying mechanisms remain elusive. Therefore, an animal model of these side effects was developed in male Sprague-Dawley rats. Chronic administration of olanzapine elevated fasting glucose, impaired glucose and insulin tolerance, increased fat mass but, in contrast to female rats, did not increase body weight or food intake. Acute studies were conducted to delineate the mechanisms responsible for these effects. Olanzapine markedly decreased physical activity without a compensatory decline in food intake. It also acutely elevated fasting glucose and worsened oral glucose and insulin tolerance, suggesting that these effects are adiposity independent. Hyperinsulinemic-euglycemic clamp studies measuring (14)C-2-deoxyglucose uptake revealed tissue-specific insulin resistance. Insulin sensitivity was impaired in skeletal muscle, but either unchanged or increased in adipose tissue depots. Consistent with the olanzapine-induced hyperglycemia, there was a tendency for increased (14)C-2-deoxyglucose uptake into fat depots of fed rats and, surprisingly, free fatty acid (FFA) uptake into fat depots was elevated approximately twofold. The increased glucose and FFA uptake into adipose tissue was coupled with increased adipose tissue lipogenesis. Finally, olanzapine lowered fasting plasma FFA, and as it had no effect on isoproterenol-stimulated rises in plasma glucose, it blunted isoproterenol-stimulated in vivo lipolysis in fed rats. Collectively, these results suggest that olanzapine exerts several metabolic effects that together favor increased accumulation of fuel into adipose tissue, thereby increasing adiposity.

Links between insulin resistance, adenosine A2B receptors, and inflammatory markers in mice and humans

OBJECTIVE

To determine the mechanisms by which blockade of adenosine A(2B) receptors (A(2B)Rs) reduces insulin resistance.

RESEARCH DESIGN AND METHODS

We investigated the effects of deleting or blocking the A(2B)R on insulin sensitivity using glucose tolerance tests (GTTs) and hyperinsulinemic-euglycemic clamps in mouse models of type 2 diabetes. The effects of diabetes on A(2B)R transcription and signaling were measured in human and mouse macrophages and mouse endothelial cells. In addition, tag single nucleotide polymorphisms (SNPs) in ~42 kb encompassing the A(2B)R gene, ADORA2B, were evaluated for associations with markers of diabetes and inflammation.

RESULTS

Treatment of mice with the nonselective adenosine receptor agonist 5'-N-ethylcarboxamidoadensoine (NECA) increased fasting blood glucose and slowed glucose disposal during GTTs. These responses were inhibited by A(2B)R deletion or blockade and minimally affected by deletion of A(1)Rs or A(2A)Rs. During hyperinsulinemic-euglycemic clamp of diabetic KKA(Y) mice, A(2B)R antagonism increased glucose infusion rate, reduced hepatic glucose production, and increased glucose uptake into skeletal muscle and brown adipose tissue. Diabetes caused a four- to sixfold increase in A(2B)R mRNA in endothelial cells and macrophages and resulted in enhanced interleukin (IL)-6 production in response to NECA due to activation of protein kinases A and C. Five consecutive tag SNPs in ADORA2B were highly correlated with IL-6 and C-reactive protein (CRP). Diabetes had a highly significant independent effect on variation in inflammatory markers. The strength of associations between several ADORA2B SNPs and inflammatory markers was increased when accounting for diabetes status.

CONCLUSIONS

Diabetes affects the production of adenosine and the expression of A(2B)Rs that stimulate IL-6 and CRP production, insulin resistance, and the association between ADORA2B SNPs and inflammatory markers. We hypothesize that increased A(2B)R signaling in diabetes increases insulin resistance in part by elevating proinflammatory mediators. Selective A(2B)R blockers may be useful to treat insulin resistance.

Regulation of leukocyte function by adenosine receptors

The immune system responds to cues in the microenvironment to make acute and chronic adaptations in response to inflammation and injury. Locally produced purine nucleotides and adenosine provide receptor-mediated signaling to all bone-marrow derived cells of the immune system to modulate their responses. This review summarizes recent advances in our understanding of the effects of adenosine signaling through G protein-coupled adenosine receptors on cells of the immune system. Adenosine A(2A) receptors (A(2A)Rs) have a generally suppressive effect on the activation of immune cells. Moreover, their transcription is strongly induced by signals that activate macrophages or dendritic cells through toll-like receptors, or T cells through T cell receptors. A(2A)R induction is responsible for producing a gradual dissipation of inflammatory responses. A(2A)R activation is particularly effective in limiting the activation of invariant NKT (iNKT) cells that play a central role in acute reperfusion injury. A(2A) agonists have clinical promise for the treatment of vaso-occlusive tissue injury. Blockade of A(2A) receptors may be useful to enhance immune-mediated killing of cancer cells. A(2B)R expression also is transcriptionally regulated by hypoxia, cytokines, and oxygen radicals. Acute A(2B)R activation attenuates the production of proinflammatory cytokines from macrophages, but sustained activation facilitates macrophage and dendritic cell remodeling and the production of acute phase proteins and angiogenic factors that may participate in evoking insulin resistance and tissue fibrosis. A(2B)R activation also influences macrophage and neutrophil function by influencing expression of the anti-inflammatory netrin receptor, UNC5B. The therapeutic significance of adenosine-mediated effects on the immune system is discussed.

Mechanisms of glucose homeostasis after Roux-en-Y gastric bypass surgery in the obese, insulin-resistant Zucker rat

OBJECTIVE

Obesity-related diabetes is caused by insulin resistance and beta-cell dysfunction. The current study examines changes in food intake, weight loss, body fat depots, oxygen consumption, insulin sensitivity, and incretin levels as potential mechanisms for improved glucose tolerance after Roux-en-Y gastric bypass (RYGB).

METHODS

Three groups of genetically obese Zucker rats were studied: RYGB, sham surgery pair-fed (PF), and sham surgery ad libitum (AL) fed rats. Changes in body weight, visceral and subcutaneous fat depots, oral glucose tolerance, insulin sensitivity, and the plasma concentrations of insulin, glucagon, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide, and peptide YY (PYY) were measured.

RESULTS

Body weight and subcutaneous fat were decreased after RYGB, compared with the PF and AL groups. The reduction in visceral fat after RYGB appeared largely because of food restriction. Glucose tolerance and insulin sensitivity were significantly improved in only the RYGB group (P < 0.05 vs. AL, PF). Euglycemic, hyperinsulinemic clamp studies indicated RYGB improved the ability of insulin to stimulate peripheral (eg, skeletal muscle) glucose uptake. Fasting total GLP-1, glucose-dependent insulinotropic peptide, and PYY levels were similar between the groups, whereas postprandial plasma levels of intact GLP-1 (7-36) amide, total GLP-1, and PYY were increased in the RYGB group compared with PF and AL controls.

CONCLUSIONS

Glucose homeostasis after RYGB is associated with decreased subcutaneous fat, increased postprandial PYY, GLP-1, and insulin, as well as improved insulin sensitivity/action. Changes in food intake and visceral fat do not seem to explain improvements in insulin action after RYGB in the Zucker rat model.

Modulation of insulin release by adenosine A1 receptor agonists and antagonists in INS-1 cells: the possible contribution of 86Rb+ efflux and 45Ca2+ uptake

Due to the lack of specific agonists and antagonists the role of adenosine receptor subtypes with respect to their effect on the insulin secretory system is not well investigated. The A1 receptor may be linked to different 2nd messenger systems, i.e. cAMP, K+- and 45Ca2+ channel activity. Partial A1 receptor agonists are going to be developed in order to improve diabetes (increase in insulin sensitivity, lowering of FFA and triglycerides). In this study newly synthesized selective A1 receptor agonists and antagonists were investigated thereby integrating three parameters, insulin release (RIA), 45Ca2+ uptake and 86Rb+ efflux (surrogate for K+ efflux) of INS-1 cells, an insulin secretory cell line. The presence of A1-receptors was demonstrated by Western blotting. The receptor nonselective adenosine analogue NECA (5-N-ethylcarboxyamidoadenosine) at high concentration (10 microM) had no effect on insulin release and 45Ca2+ uptake which could be interpreted as the sum of effects mediated by mutual antagonistic adenosine receptor subtypes. However, an inhibitory effect mediated by A1 receptor agonism was detected at 10 nM NECA and could be confirmed by adding the A1 receptor antagonist PSB-36 (1-butyl-8-(3-noradamantyl)-3-(3-hydroxy-propyl)xanthine). NECA inhibited 86Rb+ efflux which, however, did not fit with the simultaneous inhibition of insulin secretion. The selective A1 receptor agonist CHA (N6-cyclohexyladenosine) inhibited insulin release; the simultaneously increased Ca2+ uptake (nifedipine dependent) and inhibition of 86Rb+ efflux did not fit the insulin release data. The CHA effect (even the maximum effect at 50 microM) can be increased by 10 microM NECA indicating that CHA and NECA have nonspecific and physiologically non-relevant effects on 86Rb+ efflux in addition to their A1-receptor interaction. Since PSB-36 did not influence the NECA-induced inhibition of 86Rb+ efflux, the NECA effect is not mediated by potassium channel-linked A1 receptors. The nonselective adenosine receptor antagonist caffeine increased insulin release which was reversed by CHA as expected when hypothesizing that both act via A1 receptors in this case. In conclusion, stimulation of A1 receptors by receptor selective and nonselective compounds reduced insulin release which is not coupled to opening of potassium channels (86Rb+ efflux experiments) or inhibition of calcium channels (45Ca2+ uptake experiments). It may be expected that of all pleiotropic 2nd messengers, the cAMP system (not tested here) is predominant for A1 receptor effects and the channel systems (K+ and Ca2+) are of minor importance and do not contribute to insulin release though being coupled to the receptor in other tissues.

Adenosine A1 and A2a receptors modulate insulinemia, glycemia, and lactatemia in fetal sheep

Adenosine A(1) and A(2A) receptor subtypes modulate metabolism in adult mammals. This study was designed to determine the role of these receptors in regulating plasma levels of insulin, glucose, and lactate in 20 chronically catheterized fetal sheep (>0.8 term). In normoxic fetuses (Pa(O(2)) approximately 24 Torr), systemic blockade of A(1) receptors with DPCPX (n = 6) increased plasma concentrations of insulin, glucose, and lactate, but antagonism of A(2A) receptors with ZM-241385 (n = 5) had no significant effects. Intravascular administration of adenosine (n = 9) reduced insulin concentrations and elevated glucose and lactate levels. DPCPX (n = 6) augmented the glycemic and lactatemic responses of adenosine. In contrast, ZM241385 (n = 5) virtually abolished adenosine-induced hyperglycemia and hyperlactatemia. Isocapnic hypoxia (Pa(O(2)) approximately 13 Torr) suppressed insulinemia and enhanced glycemia and lactatemia, but only the hyperglycemia was blunted by blockade of A(1) (n = 6) or A(2A) (n = 6) receptors. We conclude that 1) endogenous adenosine via A(1) receptors depresses plasma concentrations of insulin, glucose, and lactate; 2) exogenous adenosine via A(2A) receptors increases glucose and lactate levels, but these responses are dampened by stimulation of A(1) receptors; and 3) hypoxia, which increases endogenous adenosine concentrations, induces hyperglycemia that is partly mediated by activation of A(1) and A(2A) receptors. We predict that adenosine, via A(1) receptors, facilitates at least 12% of glucose uptake and utilization in normoxic fetuses.

Consumption of dietary caffeine and coffee in physically active populations: physiological interactions

Caffeine is a proven ergogenic aid, increasing athletic performance, endurance, and mental chronometry at doses as low as 1–3 mg·kg–1. As coffee is a readily available and commonly ingested form of caffeine, the two are often equated. However, coffee also contains hundreds of other biologically active compounds, many of which are metabolically distinct from caffeine. The purpose of this review was to examine the prevalence of coffee and (or) caffeine consumption among elite Canadian athletes, and to delineate the effects of coffee and caffeine on physical activity, weight maintenance, performance, and metabolism. A total of 270 self-reported 3-day food records were examined for caffeine intake from athletes registered with Canadian Sport Centres in 2005 and 2006. Athletes ranged in age from 16–45 years, and competed in 38 different sports. Results showed that 30% of athletes ingested >1 mg·kg–1·day–1 from a variety of sources. Average daily intake was 0.85 ± 13 mg·kg–1. Caffeine intake was not correlated with any 1 sport; the 10 highest caffeine users were athletes from 9 different sports, including skill, endurance, and power sports. No differences were noted for average caffeine ingestion between summer and winter sports. High caffeine intakes corresponded to coffee ingestion, with the 25 highest individual intakes (193–895 mg·day–1) from coffee drinkers. In summary, it can be concluded that the majority of high-level Canadian athletes consume dietary caffeine primarily in the form of coffee. However, levels consumed are insufficient to elicit performance enhancement. Potential detrimental effects of caffeine consumption on exercise performance include gastric upset, withdrawal, sleep disturbance, and interactions with other dietary supplements.

Heart lipid accumulation in obese non-diabetic rats: effect of weight loss

BACKGROUND AND AIM

The aim of this study was to investigate lipid content and expression of genes involved in lipid metabolism, in lean and obese non-diabetic rats and in obese rats undergoing food restriction and weight loss.

METHODS AND RESULTS

We studied lean and genetically obese Zucker rats (fa/fa). Another group of obese rats were food restricted to lose 20% of initial body weight. We measured expression of genes involved in lipid oxidation and synthesis. Tissue triglyceride content, cell apoptosis and tissue fibrosis were also evaluated. The hearts of obese rats have higher triglyceride content compared to lean controls despite an increased expression of genes involved in fatty acid oxidation (PPAR alpha, PGC-1 alpha, CPT-I, ACO, UCP3). No differences were found in apoptosis and tissue fibrosis between the two phenotypes. Weight loss resulted in a significant reduction in heart lipid content, while the expression of genes involved in fatty acid oxidation remained elevated.

CONCLUSION

In contrast to data reported in diabetic rats, pathways of lipid oxidation are increased rather than decreased in insulin-resistant obese rats. Food restriction reduced heart triglyceride content while lipid-oxidizing genes remained elevated, probably as a consequence of lipid oversupply coming from the endogenous source.

A1 receptor deficiency causes increased insulin and glucagon secretion in mice

Adenosine influences metabolism and the adenosine receptor antagonist caffeine decreases the risk of type 2 diabetes. In this study the metabolic role of one adenosine receptor subtype, the adenosine A(1)R, was evaluated in mice lacking this receptor [A(1)R (-/-)]. The HbA1c levels and body weight were not significantly different between wild type [A(1)R (+/+)] and A(1)R (-/-) mice (3-4 months) fed normal lab chow. At rest, plasma levels of glucose, insulin and glucagon were similar in both genotypes. Following glucose injection, glucose tolerance was not appreciably altered in A(1)R (-/-) mice. Glucose injection induced sustained increases in plasma insulin and glucagon levels in A(1)R (-/-) mice, whereas A(1)R (+/+) control mice reacted with the expected transient increase in insulin and decrease in glucagon levels. Pancreas perfusion experiments showed that A(1)R (-/-) mice had a slightly higher basal insulin secretion than A(1)R (+/+) mice. The first phase insulin secretion (initiated with 16.7 mM glucose) was of the same magnitude in both genotypes, but the second phase was significantly enhanced in the A(1)R (-/-) pancreata compared with A(1)R (+/+). Insulin- and contraction-mediated glucose uptake in skeletal muscle were not significantly different between in A(1)R (-/-) and A(1)R (+/+) mice. All adenosine receptors were expressed at mRNA level in skeletal muscle in A(1)R (+/+) mice and the mRNA A(2A)R, A(2B)R and A(3)R levels were similar in A(1)R (-/-) and A(1)R (+/+) mice. In conclusion, the A(1)R minimally affects muscle glucose uptake, but is important in regulating pancreatic islet function.

Glucose homeostasis remains altered by acute caffeine ingestion following 2 weeks of daily caffeine consumption in previously non-caffeine-consuming males

Acute caffeine ingestion increases serum NEFA and plasma adrenaline and decreases insulin sensitivity. Although frequently suggested, it is not known if a tolerance to these alterations in glucose homeostasis is developed in habitual caffeine consumers. Our objective was to determine whether acute caffeine ingestion continued to alter insulin, glucose, NEFA and adrenaline during an oral glucose tolerance test (OGTT) following 14 d of caffeine consumption. Twelve caffeine-naive young males underwent four OGTTs over a 4-week period. Subjects ingested a gelatin-filled placebo (PLA) capsule on the first trial day and 5 mg caffeine/kg body weight on the remaining three trial days (day 0, day 7, day 14) before a 2 h OGTT. Following day 0 and day 7, subjects were given six dosages of 5 mg caffeine/kg to consume per d between trials. Serum insulin and blood glucose area under the curve (AUC) were significantly elevated (P < 0·05) v. PLA on day 0 (36 and 103 %, respectively) and were not different from PLA on day 7. On day 14, insulin AUC was 29 % greater than PLA (P < 0·05), and glucose was greater (P < 0·05) during the first hour, although the 50 % elevation in glucose AUC was not different from PLA. Before the OGTT, caffeine resulted in greater (P < 0·05) serum NEFA and plasma adrenaline concentrations in all three caffeine trials, but both NEFA and adrenaline concentrations were decreased (P < 0·05) on day 14 v. day 0. Although 14 d of caffeine consumption by previously caffeine-naive subjects reduced its impact on glucose homeostasis, carbohydrate metabolism remained disrupted.

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.

Acute caffeine ingestion does not impair glucose tolerance in persons with tetraplegia

Acute caffeine (Caf) ingestion impairs glucose tolerance in able-bodied humans during an oral glucose tolerance test (OGTT). The mechanism responsible for this effect remains unclear, however, it is suggested to be due to the accompanying increase in epinephrine concentration. We examined whether or not Caf would elicit a glucose intolerance in persons with tetraplegia (TP) who do not exhibit an increased epinephrine response following Caf ingestion. All TP [n = 14; 9 incomplete (Inc) lesion, 5 complete (Com) lesion] completed two OGTT 1 h after consuming either gelatin (Pl) or Caf capsules (dose = 4 mg/kg). Blood samples were collected at baseline (time = 0 min), 1 h after capsule ingestion (time = 60 min), and every 30 min during the OGTT (time = 90-180 min). Glucose, insulin, proinsulin, and C-peptide responses were similar (P > 0.05) between treatments, demonstrating no effect of Caf on glucose tolerance. This lack of a Caf effect may be due to the low epinephrine concentration that remained unchanged (P > 0.05) throughout all experiments. Interestingly, the Com exhibited a 50% higher glucose response (P <or= 0.05) and a 46% (P > 0.05) lower insulin response (vs. Inc), suggesting a more pronounced glucose intolerance within this subgroup. Furthermore, nine TP (5 Com, 4 Inc) had glucose levels of >or= 7.8 mM at the end of the OGTT (time = 180 min), classifying them as glucose intolerant. In summary, acute Caf ingestion does not increase epinephrine concentration or impair glucose tolerance in TP.

Influence of caffeine on perception of effort, metabolism and exercise performance following a high-fat meal

This study examined the effects of caffeine, co-ingested with a high fat meal, on perceptual and metabolic responses during incremental (Experiment 1) and endurance (Experiment 2) exercise performance. Trained participants performed three constant-load cycling tests at approximately 73% of maximal oxygen uptake (VO2max) for 30 min at 20 degrees C (Experiment 1, n = 8) and to the limit of tolerance at 10 degrees C (Experiment 2, n = 10). The 30 min constant-load exercise in Experiment 1 was followed by incremental exercise (15 W . min-1) to fatigue. Four hours before the first test, the participants consumed a 90% carbohydrate meal (control trial); in the remaining two tests, the participants consumed a 90% fat meal with (fat + caffeine trial) and without (fat-only trial) caffeine. Caffeine and placebo were randomly assigned and ingested 1 h before exercise. In both experiments, ratings of perceived leg exertion were significantly lower during the fat + caffeine than fat-only trial (Experiment 1: P < 0.001; Experiment 2: P < 0.01). Ratings of perceived breathlessness were significantly lower in Experiment 1 (P < 0.01) and heart rate higher in Experiment 2 (P < 0.001) on the fat + caffeine than fat-only trial. In the two experiments, oxygen uptake, ventilation, blood [glucose], [lactate] and plasma [glycerol] were significantly higher on the fat + caffeine than fat-only trial. In Experiment 2, plasma [free fatty acids], blood [pyruvate] and the [lactate]:[pyruvate] ratio were significantly higher on the fat + caffeine than fat-only trial. Time to exhaustion during incremental exercise (Experiment 1: control: 4.9, s = 1.8 min; fat-only: 5.0, s = 2.2 min; fat + caffeine: 5.0, s = 2.2 min; P > 0.05) and constant-load exercise (Experiment 2: control: 116 (88 - 145) min; fat-only: 122 (96 - 144) min; fat + caffeine: 127 (107 - 176) min; P > 0.05) was not different between the fat-only and fat + caffeine trials. In conclusion, while a number of metabolic responses were increased during exercise after caffeine ingestion, perception of effort was reduced and this may be attributed to the direct stimulatory effect of caffeine on the central nervous system. However, this caffeine-induced reduction in effort perception did not improve exercise performance.

The effect of caffeine on glucose kinetics in humans--influence of adrenaline

While caffeine impedes insulin-mediated glucose disposal in humans, its effect on endo-genous glucose production (EGP) remains unknown. In addition, the mechanism involved in these effects is unclear, but may be due to the accompanying increase in adrenaline concentration. We studied the effect of caffeine on EGP and glucose infusion rates (GIR), and whether or not adrenaline can account for all of caffeine's effects. Subjects completed three isoglycaemic-hyperinsulinaemic clamps (with 3-[(3)H]glucose infusion) 30 min after ingesting: (1) placebo capsules (n= 12); (2) caffeine capsules (5 mg kg(-1)) (n= 12); and either (3) placebo plus a high-dose adrenaline infusion (HAdr; adrenaline concentration, 1.2 nM; n= 8) or (4) placebo plus a low-dose adrenaline infusion (LAdr; adrenaline concentration, 0.75 nM; n= 6). With caffeine, adrenaline increased to 0.6 nM but no effect on EGP was observed. While caffeine and HAdr decreased GIR by 13 (P < 0.05) and 34% (P < 0.05) versus the placebo, respectively, LAdr did not result in a significant reduction (5%) in GIR versus the placebo. Due to the fact that both caffeine and LAdr resulted in similar adrenaline concentrations, but resulted in different decreases in GIR, it is concluded that adrenaline alone does not account for the effects of caffeine and additional mechanisms must be involved.

Caffeine ingestion does not impede the resynthesis of proglycogen and macroglycogen after prolonged exercise and carbohydrate supplementation in humans

The purpose of this study was to examine the effects of caffeine (Caf) ingestion on pro- (PG) and macroglycogen (MG) resynthesis in 10 healthy men. Subjects completed two trials, consisting of a glycogen-depleting exercise, while ingesting either Caf or placebo capsules. Throughout recovery, biopsies were taken at 0 (exhaustion), 30, 120, and 300 min, and 75 g of carbohydrate were ingested at 0, 60, 120, 180, and 240 min. Whereas Caf ingestion resulted in a higher blood glucose concentration and decreased glycogen synthase fractional velocity (P <or= 0.05), no effect was observed in either the amount or rate of PG and MG resynthesis. PG concentration increased significantly at each time point during recovery, whereas MG concentration remained unchanged until 120 min. The net rate of PG resynthesis was 115 mmol x kg dw(-1) x h(-1) during the first 30 min of recovery, and then it significantly decreased by 62% throughout the remaining 4.5 h of recovery. The net rate of MG resynthesis was 77% lower than the net rate of PG resynthesis during the first 30 min of recovery and remained constant throughout 5 h of recovery despite increasing levels of insulin. In conclusion, Caf ingestion does not impede the resynthesis of PG or MG after an extensive depletion of muscle glycogen and with the provision of exogenous dietary carbohydrate.

Increase in adenosine A1 receptor gene expression in the liver of streptozotocin-induced diabetic rats

BACKGROUND

Adenosine A1 receptor (A1-AR) activation can lower plasma glucose in diabetic rats lacking insulin. We investigated the change in A1-AR gene expression in diabetic rats.

METHODS

The incorporation of [U-(14)C]-glucose into glycogen was carried out to evaluate the effect of N(6)-cyclopentyladenosine (CPA) on glucose utilization in vitro. The plasma glucose concentration was assessed by the glucose oxidase method. The mRNA and protein levels of A1-AR in isolated liver were detected by Western blotting analysis and Northern blotting analysis, respectively.

RESULTS

The effect of CPA, an agonist of A1-AR, on glycogen incorporation in hepatocytes isolated from streptozotocin-induced diabetic rats (STZ-diabetic rats) was more marked than that from the normal rats. However, similar glycogen synthesis was not modified by 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, in the isolated hepatocytes from both the normal and the STZ-diabetic rats. A change in response at the receptor level can thus be considered. The mean level of liver mRNA transcripts encoding A1-AR was increased in STZ-diabetic rats to about 250% of that in normal rats. Exogenous insulin at a dose sufficient to normalize the plasma glucose of STZ-diabetic rats reversed the mRNA level of A1-AR in the liver after a four-day treatment. Similar results were also observed in STZ-diabetic rats that received treatment with phlorizin for four days. Moreover, the protein level of A1-AR was higher in the liver of STZ-diabetic rats than that in the normal rats. Similar treatment with exogenous insulin or phlorizin reversed the elevated protein level of A1-AR in the liver of STZ-diabetic rats to near the normal level. Therefore, correction of hyperglycemia in STZ-diabetic rats can reverse the higher gene expression of A1-AR in liver.

CONCLUSIONS

The obtained results suggest that an increase in plasma glucose is responsible for the higher gene expression of A1-AR in the liver of STZ-diabetic rats.

Insulin-mediated hemodynamic changes are impaired in muscle of Zucker obese rats

Insulin-mediated hemodynamic effects in muscle were assessed in relation to insulin resistance in obese and lean Zucker rats. Whole-body glucose infusion rate (GIR), femoral blood flow (FBF), hindleg glucose extraction (HGE), hindleg glucose uptake (HGU), 2-deoxyglucose (DG) uptake into muscles of the lower leg (R(g)), and metabolism of infused 1-methylxanthine (1-MX) to measure capillary recruitment were determined for isogylcemic (4.8 +/- 0.2 mmol/l, lean; 11.7 +/- 0.6 mmol/l, obese) insulin-clamped (20 mU. min(-1). kg(-1) x 2 h) and saline-infused control anesthetized age-matched (20 weeks) lean and obese animals. Obese rats (445 +/- 5 g) were less responsive to insulin than lean animals (322 +/- 4 g) for GIR (7.7 +/- 1.4 vs. 22.2 +/- 1.1 mg. min(-1). kg(-1), respectively), and when compared with saline-infused controls there was no increase due to insulin by obese rats in FBF, HGE, HGU, and R(g) of soleus, plantaris, red gastrocnemius, white gastrocnemius, extensor digitorum longus (EDL), or tibialis muscles. In contrast, lean animals showed marked increases due to insulin in FBF (5.3-fold), HGE (5-fold), HGU (8-fold), and R(g) ( approximately 5.6-fold). Basal (saline) hindleg 1-MX metabolism was 1.5-fold higher in lean than in obese Zucker rats, and insulin increased in only that of the lean. Hindleg 1-MX metabolism in the obese decreased slightly in response to insulin, thus postinsulin lean was 2.6-fold that of the postinsulin obese. We conclude that muscle insulin resistance of obese Zucker rats is accompanied by impaired hemodynamic responses to insulin, including capillary recruitment and FBF.

Purinergic regulation of glucose and glutamine synthesis in isolated rabbit kidney-cortex tubules

The effects of extracellular purinergic agonists and their breakdown products on glucose and glutamine synthesis in rabbit kidney-cortex tubules incubated with aspartate + glycerol or alanine + glycerol + octanoate were investigated. A rapid extracellular degradation of ATP was accompanied by an accumulation of AMP, inosine, and hypoxanthine. Extracellular ATP and its breakdown products accelerated glucose synthesis in renal tubules, while ammonium released from adenine-containing compounds enhanced glutamine synthesis and diminished the degree of gluconeogenesis stimulation. In contrast to AMP and inosine, ATP evoked calcium signals, while both ATP and inosine decreased intracellular cAMP content and accelerated the flux through fructose-1,6-bisphosphatase as concluded from changes in gluconeogenic intermediates. Since (i) the activity of partially purified renal fructose-1,6-bisphosphatase was increased upon protein phosphatase-1 treatment and decreased following treatment of previously dephosphorylated enzyme with protein kinase A catalytic subunit and (ii) both 8-bromoadenosine 3',5'-cyclic monophosphate and 8-(4-chlorophenyltio)-cAMP inhibited renal glucose synthesis, it seems likely that in rabbit renal tubules ATP and inosine stimulate gluconeogenesis via cAMP decrease, which favors the appearance of a more active, dephosphorylated form of fructose-1,6-bisphosphatase, a key gluconeogenic enzyme.

Long-term caffeine consumption exacerbates renal failure in obese, diabetic, ZSF1 (fa-fa(cp)) rats

BACKGROUND

Our preliminary studies indicate that chronic caffeine consumption has adverse renal effects in nephropathy associated with high blood pressure and insulin resistance. The purpose of this study was to investigate the effects of early (beginning at 8 weeks of age) and long-term (30 weeks) caffeine treatment (0.1% solution) on renal function and structure in obese, diabetic ZSF1 rats.

METHODS

Metabolic and renal function measurements were performed at six-week intervals and in a subset of animals (N = 6 per group) heart rate (HR) and mean arterial blood pressure (MABP) were monitored by a radiotelemetric technique. At the end of the study acute, measurements of renal hemodynamics and excretory function were conducted in anesthetized animals.

RESULTS

Caffeine produced a very mild increase (4 to 5%) of MABP and HR, but greatly augmented proteinuria (P < 0.001), reduced creatinine clearance (P < 0.05) and had a mixed effect on metabolic status in obese ZSF1 rats. Caffeine significantly reduced body weight, glycosuria, fasting glucose and insulin levels and improved glucose tolerance, had no effect on elevated plasma triglycerides levels and significantly increased plasma cholesterol level (P < 0.001). Acute measurements of renal function revealed increased renal vascular resistance (95.1 +/- 11 vs. 50.7 +/- 2.4 mm Hg/mL/min/g kidney, P < 0.01) and decreased inulin clearance (0.37 +/- 0.11 vs. 0.97 +/- 0.13 mL/min/g kidney, P < 0.002) in caffeine-treated versus control animals, respectively. Caffeine potentiated the development of more severe tubulointerstitial changes (P < 0.05) and increased focal glomerulosclerosis (14.7 +/- 1.7 vs. 6.5 +/- 0.9%, caffeine vs. control, P < 0.002).

CONCLUSION

The present study provides the first evidence that caffeine (despite improving insulin sensitivity) exacerbates renal failure in obese, diabetic ZSF1 rats. Further mechanistic studies of adverse renal effects of caffeine in chronic renal failure associated with metabolic syndrome are warranted.

Activation of muscarinic M1 receptors by acetylcholine to increase glucose uptake into cultured C2C12 cells

Regulation of glucose metabolism by cholinergic nervous activation has been demonstrated. In an attempt to evaluate the role of cholinergic receptor subtype in this regulation of glucose metabolism, we employed cultured myoblast C2C12 cells to investigate the glucose uptake in the present study. Acetylcholine (ACh) enhanced the uptake of radioactive glucose into C2C12 cells at the concentration range of 0.001 to 1.0 micromol/l. This effect was suppressed by the muscarinic antagonist atropine. Effect of ACh on muscarinic receptors was further supported by the blockade of scopolamine, another classical antagonist. Thus, activation of muscarinic receptors to enhance the radioactive glucose uptake into C2C12 cells can be considered. Moreover, pirenzepine, the antagonist of muscarinic M1 receptors, competitively antagonized the action of ACh in C2C12 cells. However, methoctramine at concentration sufficient to inhibit the muscarinic M2 receptors failed to produce similar effect. Similarly, 4-DAMP at effective concentration to block muscarinic M3 receptors lacked the influence. An activation of muscarinic M1 receptors seems responsible for the action of ACh in C2C12 cells. Pharmacological inhibition of phospholipase C by U73312 resulted in a concentration-dependent decrease in ACh-stimulated uptake of radioactive glucose into C2C12 cells. However, treatment with U73343, the inactive congener, failed to block the action of ACh. Moreover, both chelerythrine and GF 109203X diminished the action of ACh at concentrations sufficient to inhibit protein kinase C. Therefore, the obtained data suggest that increase of the glucose uptake evoked by ACh is mainly due to the activation of muscarinic M1 receptors in cultured myoblast C2C12 cells.

Impaired glucose metabolism in the heart of obese Zucker rats after treatment with phorbol ester

OBJECTIVE

To investigate the influence of obesity on the regulation of myocardial glucose metabolism following protein kinase C (PKC) activation in obese (fa/fa) and lean (Fa/?) Zucker rats.

DESIGN

Isolated hearts obtained from 17-week-old lean and obese Zucker rats were perfused with 200 nM phorbol 12-myristate 13-acetate (PMA) for different time periods prior to the evaluation of PKC and GLUT-4 translocation. For metabolic studies isolated hearts from 48 h starved Zucker rats were perfused with an erythrocytes-enriched buffer containing increased concentrations (10-100 nM) of PMA.

MEASUREMENTS

Immunodetectable PKC isozymes and GLUT-4 were determined by Western blots. Glucose oxidation and glycolysis were evaluated by measuring the myocardial release of 14CO2 and 3H2O from [U-14C]glucose and [5-3H]glucose, respectively.

RESULTS

PMA (200 nM) induced maximal translocation of ventricular PKCalpha from the cytosol to the membranes within 10 min. This translocation was 2-fold lower in the heart from obese rats when compared to lean rats. PMA also induced a significant translocation of ventricular GLUT-4 from the microsomal to the sarcolemmal fraction within 60 min in lean but not in obese rats. Rates of basal cardiac glucose oxidation and glycolysis in obese rats were approximately 2-fold lower than those of lean rats. Perfusion with increasing concentrations of PMA (10-100 nM) led to a significant decrease of cardiac glucose oxidation in lean but not in obese rats.

CONCLUSION

Our results show that in the heart of the genetically obese Zucker rat, the impairment in PKCalpha activation is in line with a diminished activation of GLUT-4 as well as with the lack of PMA effect on glucose oxidation.

Caffeine can decrease insulin sensitivity in humans

OBJECTIVE

Caffeine is a central stimulant that increases the release of catecholamines. As a component of popular beverages, caffeine is widely used around the world. Its pharmacological effects are predominantly due to adenosine receptor antagonism and include release of catecholamines. We hypothesized that caffeine reduces insulin sensitivity, either due to catecholamines and/or as a result of blocking adenosine-mediated stimulation of peripheral glucose uptake.

RESEARCH DESIGN AND METHODS

Hyperinsulinemic-euglycemic glucose clamps were used to assess insulin sensitivity. Caffeine or placebo was administered intravenously to 12 healthy volunteers in a randomized, double-blind, crossover design. Measurements included plasma levels of insulin, catecholamines, free fatty acids (FFAs), and hemodynamic parameters. Insulin sensitivity was calculated as whole-body glucose uptake corrected for the insulin concentration. In a second study, the adenosine reuptake inhibitor dipyridamole was tested using an identical protocol in 10 healthy subjects.

RESULTS

Caffeine decreased insulin sensitivity by 15% (P < 0.05 vs. placebo). After caffeine administration, plasma FFAs increased (P < 0.05) and remained higher than during placebo. Plasma epinephrine increased fivefold (P < 0.0005), and smaller increases were recorded in plasma norepinephrine (P < 0.02) and blood pressure (P < 0.001). Dipyridamole did not alter insulin sensitivity and only increased plasma norepinephrine (P < 0.01).

CONCLUSIONS

Caffeine can decrease insulin sensitivity in healthy humans, possibly as a result of elevated plasma epinephrine levels. Because dipyridamole did not affect glucose uptake, peripheral adenosine receptor antagonism does not appear to contribute to this effect.

Caffeine ingestion decreases glucose disposal during a hyperinsulinemic-euglycemic clamp in sedentary humans

The purpose of this investigation was to examine the effect of caffeine (an adenosine receptor antagonist) on whole-body insulin-mediated glucose disposal in resting humans. We hypothesized that glucose disposal would be lower after the administration of caffeine compared with placebo. Healthy, lean, sedentary (n = 9) men underwent two trial sessions, one after caffeine administration (5 mg/kg body wt) and one after placebo administration (dextrose) in a double-blind randomized design. Glucose disposal was assessed using a hyperinsulinemic-euglycemic clamp. Before the clamp, there were no differences in circulating levels of methylxanthines, catecholamines, or glucose. Euglycemia was maintained throughout the clamp with no difference in plasma glucose concentrations between trials. The insulin concentrations were also similar in the caffeine and placebo trials. After caffeine administration, glucose disposal was 6.38 +/- 0.76 mg/kg body wt compared with 8.42 +/- 0.63 mg/kg body wt after the placebo trial. This represents a significant (P < 0.05) decrease (24%) in glucose disposal after caffeine ingestion. In addition, carbohydrate storage was 35% lower (P < 0.05) in the caffeine trial than in the placebo trial. Furthermore, even when the difference in glucose disposal was normalized between the trials, there was a 23% difference in the amount of carbohydrate stored after caffeine administration compared with placebo administration. Caffeine ingestion also resulted in higher plasma epinephrine levels than placebo ingestion (P < 0.05). These data support our hypothesis that caffeine ingestion decreases glucose disposal and suggests that adenosine plays a role in regulating glucose disposal in resting humans.

Caffeine ingestion elevates plasma insulin response in humans during an oral glucose tolerance test

We tested the hypothesis that caffeine ingestion results in an exaggerated response in blood glucose and (or) insulin during an oral glucose tolerance test (OGTT). Young, fit adult males (n = 18) underwent 2 OGTT. The subjects ingested caffeine (5 mg/kg) or placebo (double blind) and 1 h later ingested 75 g of dextrose. There were no differences between the fasted levels of serum insulin, C peptide, blood glucose, or lactate and there were no differences within or between trials in these measures prior to the OGTT. Following the OGTT, all of these parameters increased (P [Formula: see text] 0.05) for the duration of the OGTT. Caffeine ingestion resulted in an increase (P [Formula: see text] 0.05) in serum fatty acids, glycerol, and plasma epinephrine prior to the OGTT. During the OGTT, these parameters decreased to match those of the placebo trial. In the caffeine trial the serum insulin and C peptide concentrations were significantly greater (P [Formula: see text] 0.001) than for placebo for the last 90 min of the OGTT and the area under the curve (AUC) for both measures were 60 and 37% greater (P [Formula: see text] 0.001), respectively. This prolonged, increased elevation in insulin did not result in a lower blood glucose level; in fact, the AUC for blood glucose was 24% greater (P = 0.20) in the caffeine treatment group. The data support our hypothesis that caffeine ingestion results in a greater increase in insulin concentration during an OGTT. This, together with a trend towards a greater rather than a more modest response in blood glucose, suggests that caffeine ingestion may have resulted in insulin resistance.Key words: adenosine, skeletal muscle, methylxanthines, glucose uptake, diabetes.

Stimulatory effect of isoferulic acid on alpha1A-adrenoceptor to increase glucose uptake into cultured myoblast C2C12 cell of mice

In an attempt to elucidate the effect of isoferulic acid on alpha1-adrenoceptor (AR), the myoblast C2C12 cells of mice were employed to investigate the change of glucose uptake in the present study. Isoferulic acid enhanced the uptake of radioactive glucose into C2C12 cells in a concentration-dependent manner, which were abolished by pretreatment with prazosin. Effect of isoferulic acid on alpha1-AR was further characterized using the displacement of [3H]YM617 binding in C2C12 cells. The radioactive glucose uptake increasing action of isoferulic acid was abolished by tamsulosin or WB 4101 at concentration sufficient to block alpha1A-adrenoceptor (alpha1A-AR) but it was not modified by chlorethylclonidine (CEC) at the concentration sufficient to abolish alpha1B-AR. An activation of alpha1A-AR by isoferulic acid in C2C12 cells can thus be considered. Pharmacological inhibition of phospholipase C (PLC) by U73312 resulted in a concentration-dependent reduction of isoferulic acid-stimulated glucose uptake in C2C12 cells. This inhibition by U73112 was specific because the inactive congener, U73343, failed to modify the action of isoferulic acid. Also, chelerythrine and GF 109203X diminished the action of isoferulic acid at concentration sufficient to inhibit the activity of protein kinase C (PKC). The obtained data suggest that an activation of alpha1A-AR by isoferulic acid may increase the glucose uptake via PLC-PKC pathway in C2C12 cells.

Nitrogen shuttling between neurons and glial cells during glutamate synthesis

The relationship between neuronal glutamate turnover, the glutamate/glutamine cycle and de novo glutamate synthesis was examined using two different model systems, freshly dissected rat retinas ex vivo and in vivo perfused rat brains. In the ex vivo rat retina, dual kinetic control of de novo glutamate synthesis by pyruvate carboxylation and transamination of alpha-ketoglutarate to glutamate was demonstrated. Rate limitation at the transaminase step is likely imposed by the limited supply of amino acids which provide the alpha-amino group to glutamate. Measurements of synthesis of (14)C-glutamate and of (14)C-glutamine from H(14)CO(3) have shown that (14)C-amino acid synthesis increased 70% by raising medium pyruvate from 0.2 to 5 mM. The specific radioactivity of (14)C-glutamine indicated that approximately 30% of glutamine was derived from (14)CO(2) fixation. Using gabapentin, an inhibitor of the cytosolic branched-chain aminotransferase, synthesis of (14)C-glutamate and (14)C-glutamine from H(14)CO(3)(-) was inhibited by 31%. These results suggest that transamination of alpha-ketoglutarate to glutamate in Müller cells is slow, the supply of branched-chain amino acids may limit flux, and that branched-chain amino acids are an obligatory source of the nitrogen required for optimal rates of de novo glutamate synthesis. Kinetic analysis suggests that the glutamate/glutamine cycle accounts for 15% of total neuronal glutamate turnover in the ex vivo retina. To examine the contribution of the glutamate/glutamine cycle to glutamate turnover in the whole brain in vivo, rats were infused intravenously with H(14)CO(3)(-). (14)C-metabolites in brain extracts were measured to determine net incorporation of (14)CO(2) and specific radioactivity of glutamate and glutamine. The results indicate that 23% of glutamine in the brain in vivo is derived from (14)CO(2) fixation. Using published values for whole brain neuronal glutamate turnover, we calculated that the glutamate/glutamine cycle accounts for approximately 60% of total neuronal turnover. Finally, differences between glutamine/glutamate cycle rates in these two model systems suggest that the cycle is closely linked to neuronal activity.