Aims/hypothesis The 5′-AMP-activated protein kinase (AMPK) pathway is intact in type 2 diabetic patients and is seen as a target for diabetes treatment. In this study, we aimed to assess the impact of the AMPK activator 5-aminoimidazole-4-carboxamide riboside (AICAR) on both glucose and fatty acid metabolism in vivo in type 2 diabetic patients.Methods Stable isotope methodology and blood and muscle biopsy sampling were applied to assess blood glucose and fatty acid kinetics following continuous i.v. infusion of AICAR (0.75 mg kg −1 min −1 ) and/or NaCl (0.9%) in ten male type 2 diabetic patients (age 64±2 years; BMI 28±1 kg/m 2 ). Results Plasma glucose rate of appearance (R a ) was reduced following AICAR administration, while plasma glucose rate of disappearance (R d ) was similar in the AICAR and control test. Consequently, blood glucose disposal (R d expressed as a percentage of R a ) was increased following AICAR infusion (p<0.001). Accordingly, a greater decline in plasma glucose concentration was observed following AICAR infusion (p<0.001). Plasma NEFA R a and R d were both significantly reduced in response to AICAR infusion, and were accompanied by a significant decline in plasma NEFA concentration. Although AMPK phosphorylation in skeletal muscle was not increased, we observed a significant increase in acetyl-CoA carboxylase phosphorylation (p<0.001). Conclusions/interpretation The i.v. administration of AICAR reduces hepatic glucose output, thereby lowering blood glucose concentrations in vivo in type 2 diabetic patients. Furthermore, AICAR administration stimulates hepatic fatty acid oxidation and/or inhibits whole body lipolysis, thereby reducing plasma NEFA concentration.

endocrine and metabolic systems on any level of organization. It is published 12 times a year (monthly) by the publishes results of original studies about Bosselaar M, Boon H, van Loon LJ, van den Broek PH, Smits P, Tack CJ. Intra-arterial AICA-riboside administration induces NOdependent vasodilation in vivo in human skeletal muscle. In animal models, administration of the adenosine analog AICA-riboside has shown beneficial effects on ischemia-reperfusion injury and glucose homeostasis. The vascular and/or metabolic effects of AICA-riboside administration in humans remain to be established. AICA-riboside was infused intra-arterially in four different dosages up to 8 mg ⅐ min Ϫ1 ⅐ dl Ϫ1 in 24 healthy subjects. Forearm blood flow (FBF) and glucose uptake and plasma glucose, free fatty acid, and AICAriboside concentrations were assessed. We also combined AICAriboside infusion (2 mg⅐min Ϫ1 ⅐dl Ϫ1 ) with the intra-arterial administration of the adenosine receptor antagonist caffeine (90 g⅐min Ϫ1 ⅐dl Ϫ1 ; n ϭ 6) and with the endothelial NO synthase inhibitor L-NMMA (0.4 mg ⅐ min Ϫ1 ⅐ dl Ϫ1 ; n ϭ 6). Additional in vitro experiments were performed to explain our in vivo effects of AICA-riboside in humans. AICA-riboside increased FBF dose dependently from 2.0 Ϯ 0.2 to 13.2 Ϯ 1.9 ml ⅐ min Ϫ1 ⅐ dl Ϫ1 maximally (P Ͻ 0.05 for all dosages). The latter was not reduced by caffeine administration but was significantly attenuated by L-NMMA infusion. Despite high plasma AICA-riboside concentrations, forearm glucose uptake did not change. In vitro experiments showed rapid uptake of AICA-riboside by the equilibrative nucleoside transporter in erythrocytes and subsequent phosphorylation to AICA-ribotide. We conclude that AICA-riboside induces a potent vasodilator response in humans that is mediated by NO. Despite high local plasma concentrations, AICA-riboside does not increase skeletal muscle glucose uptake.

AMPK activation may stimulate glucose uptake in skeletal muscle, but the results in humans have so far been inconclusive. The authors investigated whether infusion of the AMPK activator, 5-aminoimidazole-4-carboxamide-riboside (AICAR), increased whole-body glucose infusion rate (GIR) and forearm skeletal muscle glucose uptake (FGU) during hyperin-sulinemia in vivo in healthy humans. Ten participants (paired data: n = 8) underwent 2 euglycemic hyperinsulinemic clamps (60 mU·m(-2)·min(-1), 120 minutes) with concomitant AICAR (67 mg·kg(-1)) or placebo (saline) administration over the last 60 minutes. The authors also measured forearm blood flow (FBF; plethysmography), heart rate, blood pressure, and AICAR and AICA-ribotide (ZMP) concentrations in plasma and erythrocytes. FGU and GIR (T = 95-120 min) did not differ between insulin + AICAR and insulin + placebo. Compared with insulin + placebo, insulin + AICAR raised heart rate more profoundly (T = 60-120 minutes: from 58 ± 3 to 70 ± 3 vs 60 ± 4 to 63 ± 4 bpm for placebo; P < .05 between treatments) and lowered blood pressure significantly. AICAR plasma concentrations increased significantly during AICAR infusion; AICAR was rapidly taken up by erythrocytes and phosphorylated to ZMP. In conclusion, AICAR does not seem to have a direct effect on systemic or local glucose uptake in humans. AICAR increases heart rate and decreases blood pressure, most likely by systemic vasodilation.

Plasma NEFA (non-esterified fatty acid) concentrations are elevated in patients with obesity. In the present study we first aimed to provide an integral haemodynamic profile of elevated plasma NEFAs by the simultaneous assessment of blood pressure, pulse wave velocity, FBF (forearm blood flow) and sympathetic nervous system activity during acute elevation of NEFAs. Secondly, we hypothesized that NEFA-induced vasodilation is mediated by adenosine receptor stimulation. In a randomized cross-over trial in healthy subjects, Intralipid was infused for 2 h to elevate plasma NEFAs. Glycerol was administered as the Control infusion. We assessed blood pressure, pulse wave velocity, FBF (using venous occlusion plethysmography) and sympathetic nervous system activity by measurement of noradrenaline and adrenaline. During the last 15 min of Intralipid/Control infusion, the adenosine receptor antagonist caffeine (90 microg x min(-1) x dl(-1)) was administered into the brachial artery of the non-dominant arm. Compared with Control infusion, Intralipid increased pulse wave velocity, SBP (systolic blood pressure) and pulse pressure, as well as FBF (from 1.8+/-0.2 to 2.7+/-0.6 and from 2.3+/-0.2 to 2.7+/-0.6 ml x min(-1) x dl(-1) for Intralipid compared with Control infusion; P<0.05, n=9). Although in a positive control study caffeine attenuated adenosine-induced forearm vasodilation (P<0.01, n=6), caffeine had no effect on Intralipid-induced vasodilation (P=0.5). In conclusion, elevation of plasma NEFA levels increased pulse wave velocity, SBP and pulse pressure. FBF was also increased, either by baroreflex-mediated inhibition of the sympathetic nervous system or by a direct vasodilating effect of NEFAs. As the adenosine receptor antagonist caffeine could not antagonize the vasodilator response, this response is not mediated by adenosine receptor stimulation.

Ischaemia, like muscle contraction, has been reported to induce skeletal muscle glucose uptake in in vitro models. This stimulating effect appears independent of insulin and is probably mediated by activation of AMPK (AMP-activated protein kinase). In the present study, we hypothesized that in vivo in humans ischaemia- and insulin-induced glucose uptake are additive, and that the combined impact of ischaemia and contraction on glucose uptake is of a similar magnitude when each is applied separately. We assessed the effects of ischaemia with and without euglycaemic-hyperinsulinaemia (clamp; protocol 1) and with and without muscle contraction (protocol 2) on muscle FGU (forearm glucose uptake) in healthy subjects. Furthermore, we assessed the impact of ischaemia on FBF (forearm blood flow; plethysmography). In protocol 1, ischaemia increased FGU from 0.6+/-0.1 at baseline to 5.5+/-1.9 micromol x min(-1) x dl(-1), and insulin increased FGU to 1.6+/-0.3 micromol x min(-1) x dl(-1) (P<0.05 for both). The combination of ischaemia+insulin increased FGU to 15.5+/-2.2 micromol x min(-1) x dl(-1) (P<0.05 compared with each stimulus alone). Maximal FBF obtained after ischaemia was similar with and without hyperinsulinaemia. In protocol 2, isometric contraction increased FGU from 0.3+/-0.1 to 2.7+/-0.8 micromol x min(-1) x dl(-1) (P<0.05), but FGU was not significantly different from ischaemia compared with ischaemia+contraction. However, combined ischaemia+contraction resulted in a greater increase in FBF. In summary, ischaemia and insulin independently stimulate skeletal muscle glucose uptake in vivo in humans, whereas ischaemia and contraction do not. The observed differential effects of these stimuli on glucose uptake appear to be unrelated to changes in muscle blood flow.