Variability in fasting lipid and glycogen contents in hepatic and skeletal muscle tissue in subjects with and without type 2 diabetes: a 1H and 13C MRS study.

The measurement of tissue lipid and glycogen contents and the establishment of normal levels of variability are important when assessing changes caused by pathology or treatment. We measured hepatic and skeletal muscle lipid and glycogen levels using (1)H and (13)C MRS at 3 T in groups of subjects with and without type 2 diabetes. Within-visit reproducibility, due to repositioning and instrument errors was determined from repeat measurements made over 1 h. Natural variability was assessed from separate measurements made on three occasions over 1 month. Hepatic lipid content was greater in subjects with diabetes relative to healthy subjects (p = 0.03), whereas levels of hepatic and skeletal muscle glycogen, and of intra- and extra-myocellular lipid, were similar. The single-session reproducibility values (coefficient of variation, CV) for hepatic lipid content were 12% and 7% in groups of subjects with and without diabetes, respectively. The variability of hepatic lipid content over 1 month was greater than the reproducibility, with CV = 22% (p = 0.08) and CV = 44% (p = 0.004) in subjects with and without diabetes, respectively. Similarly, levels of variation in basal hepatic glycogen concentrations (subjects with diabetes, CV = 38%; healthy volunteers, CV = 35%) were significantly larger than single-session reproducibility values (CV = 17%, p = 0.02 and CV = 13%, p = 0.05, respectively), indicating substantial biological changes in basal concentrations over 1 month. There was a decreasing correlation in measurements of both hepatic lipid and glycogen content with increasing time between scans. Levels of variability in intra- and extra-myocellular lipid in the soleus muscle, and glycogen concentrations in the gastrocnemius muscle, tended to be larger than expected from single-session reproducibility, although these did not reach significance.

Effects of saxagliptin and sitagliptin on glycaemic control and pancreatic ß-cell mass in a streptozotocin-induced mouse model of type 2 diabetes.

AIMS: We examined the effects of the oral dipeptidyl peptidase-4 (DPP-4) inhibitor saxagliptin on glycaemic control and pancreatic ß-cell mass and morphology in a mouse model of type 2 diabetes mellitus (T2DM). METHODS: Male C57BL/6 mice (n = 12/group) aged 4 to 6 weeks and weighing >15 g received a high-fat diet throughout this 45-day study. After a 7-day handling period, baseline levels of plasma glucose, plasma insulin and glycated haemoglobin (HbA1c) were assessed. Animals were allocated to one of six groups: compound vehicle control, intraperitoneal streptozotocin (STZ, 50 mg/kg)-treated control and saxagliptin (10 mg/kg) or sitagliptin (10 mg/kg, positive control) initiated 7 days before or 1 day after STZ administration. Endpoints included changes in body weight, food and water consumption, glucose tolerance (approximately 3 weeks post-STZ), fasting glucose and HbA1c and immunohistochemical analyses of the pancreas. RESULTS: Body weight, weight gain and food intake were reduced in STZ versus control mice. DPP-4 inhibitor treatment did not affect these changes, but the increase in water intake observed post-STZ administration was significantly attenuated with DPP-4 inhibitors whether initiated before or after STZ injury. Small but significant improvements in glycaemic control were observed with DPP-4 inhibitors versus the STZ control. Improved ß-cell mass and morphology were observed with saxagliptin given pre- or post-STZ and sitagliptin given post-STZ. CONCLUSIONS: Saxagliptin mitigated damage to ß-cells and improved glycaemic control in this mouse model of T2DM.

Elevation of alanine transaminase and markers of liver fibrosis after a mixed meal challenge in individuals with type 2 diabetes.

BACKGROUND: Hyperalimentation for 4 weeks is associated with raised liver enzymes and liver fat content (LFC), which are two common features found in individuals with diabetes. AIM: We evaluated the effect of two mixed meal challenges on LFC, liver enzymes and serum bio-markers of liver injury and fibrosis in 16 healthy volunteers (HV) and subjects with type 2 diabetes (T2DM). METHODS: Subjects (HV: 9 male, 7 female, aged 57.9 ± 1.7 years, body mass index (BMI) 27.1 kg/m(2); and T2DM: 11 male, 5 female, aged 62.1 ± 1.3 years, BMI 28.0 ± 0.4 kg/m(2)) consumed two meals at 1 h (884 kcal) and at 6 h (1,096 kcal). LFC determined by (1)H magnetic resonance spectroscopy, serum levels of liver enzymes, hyaluronic acid (HA), procollagen III N-terminal peptide (P3NP) and tissue inhibitor metalloproteinase-1 (TIMP-1) were estimated at time 0 (fasting) and 9 h (postprandial). RESULTS: Fasting LFC was higher in the T2DM group 7.6 % (4.9, 15.4) [median (inter-quartile range)] than in the HV group 2.3 % (0.8, 5.1) (p < 0.05) while levels of HA, P3NP and TIMP-1 were similar. Following the meal challenge there was no significant change in LFC. Subjects with T2DM had higher post-prandial rise in alanine transaminase (ALT) (p = 0.014), serum HA (p = 0.007) and P3NP (p = 0.015) compared with HV. Fasting LFC correlated with a greater post-prandial increase in P3NP levels in all subjects (Pearson correlation r = 0.53, p = 0.001). CONCLUSIONS: In subjects with T2DM, a mixed meal challenge is associated with a significant elevation in the serum levels of ALT, HA and P3NP without significant changes in LFC. These markers should be performed in the fasted state.

A new general glucose homeostatic model using a proportional-integral-derivative controller.

The glucose-insulin system is a challenging process to model due to the feedback mechanisms present, hence the implementation of a model-based approach to the system is an on-going and challenging research area. A new approach is proposed here which provides an effective way of characterising glycaemic regulation. The resulting model is built on the premise that there are three phases of insulin secretion, similar to those seen in a proportional-integral-derivative (PID) type controller used in engineering control problems. The model relates these three phases to a biological understanding of the system, as well as the logical premise that the homeostatic mechanisms will maintain very tight control of the system. It includes states for insulin, glucose, insulin action and a state to simulate an integral function of glucose. Structural identifiability analysis was performed on the model to determine whether a unique set of parameter values could be identified from the available observations, which should permit meaningful conclusions to be drawn from parameter estimation. Although two parameters--glucose production rate and the proportional control coefficient--were found to be unidentifiable, the former is not a concern as this is known to be impossible to measure without a tracer experiment, and the latter can be easily estimated from other means. Subsequent parameter estimation using Intravenous Glucose Tolerance Test (IVGTT) and hyperglycaemic clamp data was performed and subsequent model simulations have shown good agreement with respect to these real data.

The novel sodium glucose transporter 2 inhibitor dapagliflozin sustains pancreatic function and preserves islet morphology in obese, diabetic rats.

AIMS: To investigate whether glucose lowering with the selective sodium glucose transporter 2 (SGLT2) inhibitor dapagliflozin would prevent or reduce the decline of pancreatic function and disruption of normal islet morphology. METHODS: Female Zucker diabetic fatty (ZDF) rats, 7-8 weeks old, were placed on high-fat diet. Dapagliflozin (1 mg/kg/day, p.o.) was administered for ∼33 days either from initiation of high-fat diet or when rats were moderately hyperglycaemic. Insulin sensitivity and pancreatic function were evaluated using a hyperglycaemic clamp in anaesthetized animals (n = 5-6); ß-cell function was quantified using the disposition index (DI) to account for insulin resistance compensation. Pancreata from a matched subgroup (n = 7-8) were fixed and ß-cell mass and islet morphology investigated using immunohistochemical methods. RESULTS: Dapagliflozin, administered from initiation of high-fat feeding, reduced the development of hyperglycaemia; after 24 days, blood glucose was 8.6 ± 0.5 vs. 13.3 ± 1.3 mmol/l (p < 0.005 vs. vehicle) and glycated haemoglobin 3.6 ± 0.1 vs. 4.8 ± 0.26% (p < 0.003 vs. vehicle). Dapagliflozin improved insulin sensitivity index: 0.08 ± 0.01 vs. 0.02 ± 0.01 in obese controls (p < 0.03). DI was improved to the level of lean control rats (dapagliflozin 0.29 ± 0.04; obese control 0.15 ± 0.01; lean 0.28 ± 0.01). In dapagliflozin-treated rats, ß-cell mass was less variable and significant improvement in islet morphology was observed compared to vehicle-treated rats, although there was no change in mean ß-cell mass with dapagliflozin. Results were similar when dapagliflozin treatment was initiated when animals were already moderately hyperglycaemic. CONCLUSION: Sustained glucose lowering with dapagliflozin in this model of type 2 diabetes prevented the continued decline in functional adaptation of pancreatic ß-cells.

An assessment of the in vivo efficacy of the glycogen phosphorylase inhibitor GPi688 in rat models of hyperglycaemia.

BACKGROUND AND PURPOSE: Studies in cultured hepatocytes demonstrate glycogen synthase (GS) activation with glycogen phosphorylase (GP) inhibitors. The current study investigated whether these phenomena occurred in vivo using a novel GP inhibitor. EXPERIMENTAL APPROACH: An allosteric GP inhibitor, GPi688, was evaluated against both glucagon-mediated hyperglycaemia and oral glucose challenge-mediated hyperglycaemia to determine the relative effects against GP and GS in vivo. KEY RESULTS: In rat primary hepatocytes, GPi688 inhibited glucagons-mediated glucose output in a concentration dependent manner. Additionally GP activity was reduced and GS activity increased seven-fold. GPi688 inhibited glucagon-mediated hyperglycaemia in both Wistar (65%) & obese Zucker (100%) rats and demonstrated a long duration of action in the Zucker rat. The in vivo efficacy in the glucagon challenge model could be predicted by the equation; % glucagon inhibition=56.9+34.3[log ([free plasma]/rat IC50)], r=0.921). GPi688 also reduced the blood glucose of obese Zucker rats after a 7 h fast by 23%. In an oral glucose tolerance test in Zucker rats, however, GPi688 was less efficacious (7% reduction) than a glycogen synthase kinase-3 (GSK-3) inhibitor (22% reduction), despite also observing activation (by 45%) of GS in vivo. CONCLUSIONS AND IMPLICATIONS: Although GP inhibition can inhibit hyperglycaemia mediated by increased glucose production, the degree of GS activation induced by allosteric GP inhibitors in vivo, although discernible, is insufficient to increase glucose disposal. The data suggests that GP inhibitors might be more effective clinically against fasting rather than prandial hyperglycaemic control.

Sensitivity of glycogen phosphorylase isoforms to indole site inhibitors is markedly dependent on the activation state of the enzyme.

BACKGROUND AND PURPOSE: Inhibition of hepatic glycogen phosphorylase is a potential treatment for glycaemic control in type 2 diabetes. Selective inhibition of the liver phosphorylase isoform could minimize adverse effects in other tissues. We investigated the potential selectivity of two indole site phosphorylase inhibitors, GPi688 and GPi819. EXPERIMENTAL APPROACH: The activity of glycogen phosphorylase was modulated using the allosteric effectors glucose or caffeine to promote the less active T state, and AMP to promote the more active R state. In vitro potency of indole site inhibitors against liver and muscle glycogen phosphorylase a was examined at different effector concentrations using purified recombinant enzymes. The potency of GPi819 was compared with its in vivo efficacy at raising glycogen concentrations in liver and muscle of Zucker (fa/fa) rats. KEY RESULTS: In vitro potency of indole site inhibitors depended upon the activity state of phosphorylase a. Both inhibitors showed selectivity for liver phosphorylase a when the isoform specific activities were equal. After 5 days dosing of GPi819 (37.5 micromol kg(-1)), where free compound levels in plasma and tissue were at steady state, glycogen elevation was 1.5-fold greater in soleus muscle than in liver (P < 0.05). CONCLUSIONS AND IMPLICATIONS: The in vivo selectivity of GPi819 did not match that seen in vitro when the specific activities of phosphorylase a isoforms are equal. This suggests T state promoters may be important physiological regulators in skeletal muscle. The greater efficacy of indole site inhibitors in skeletal muscle has implications for the overall safety profile of such drugs.

Metabolic inertia in contracting skeletal muscle: a novel approach for pharmacological intervention in peripheral vascular disease.

Peripheral vascular disease (PVD) is generally accepted to result in the failure of skeletal muscle blood flow to increase adequately at the onset of muscular work. There are currently no routine pharmacological interventions towards the treatment of PVD, however, recent Phase III trials in the USA have demonstrated the clinical potential of the phosphodiesterase III inhibitor Cilostazol for pain-free and maximal walking distances in patients with intermittent claudication. PVD is characterized by a marked reliance on oxygen-independent routes of ATP regeneration (phosphocreatine hydrolysis and glycolysis) in skeletal muscle during contraction and the rapid onset of muscular pain and fatigue. The accumulation of metabolic by-products of oxygen-independent ATP production (hydrogen and lactate ions and inorganic phosphate) has long been associated with an inhibition in contractile function in both healthy volunteers and PVD patients. Therefore, any strategy that could reduce the reliance upon ATP re-synthesis from oxygen-independent routes, and increase the contribution of oxygen-dependent (mitochondrial) ATP re-synthesis, particularly at the onset of exercise, might be expected to improve functional capacity and be of considerable therapeutic value. Historically, the increased contribution of oxygen-independent ATP re-synthesis to total ATP generation at the onset of exercise has been attributed to a lag in muscle blood flow limiting oxygen delivery during this period. However, recent evidence suggests that limited inertia is present at the level of oxygen delivery, whilst considerable inertia exists at the level of mitochondrial enzyme activation and substrate supply. In support of this latter hypothesis, we have reported on a number of occasions that activation of the pyruvate dehydrogenase complex, using pharmacological interventions, can markedly reduce the dependence on ATP re-synthesis from oxygen-independent routes at the onset of muscle contraction. This review will focus on these findings and will highlight the pyruvate dehydrogenase complex as a novel therapeutic target towards the treatment of peripheral vascular disease, or any other disease state where premature muscular fatigue is prevalent due to metabolite accumulation.

An acetyl group deficit limits mitochondrial ATP production at the onset of exercise.

The oxygen deficit at the onset of submaximal exercise represents a period when the energy demand of contraction cannot be met solely by mitochondrial ATP generation, and as a consequence there is an acceleration of ATP re-synthesis from oxygen-independent routes (phosphocreatine hydrolysis and glycolysis). Historically, the origin of the oxygen deficit has been attributed to a lag in muscle blood flow and oxygen availability at the onset of exercise which limits mitochondrial respiration. However, more recent evidence suggests that considerable inertia exists at the level of mitochondrial enzyme activation and substrate supply. In support of this latter hypothesis, we have reported on a number of occasions that pharmacological activation of the pyruvate dehydrogenase complex (and consequent stockpiling of acetyl groups), using dichloroacetate or exercise interventions, can markedly reduce the degree of ATP re-synthesis from oxygen-independent routes during the rest-to-work transition period. This review will focus on these findings, and will offer the hypothesis that acetyl group delivery to the tricarboxylic acid cycle limits mitochondrial flux at the onset of exercise--the so-called acetyl group deficit.

The role of bradykinin in the regulation of blood flow to hindlimb muscle groups of the anaesthetized cat.

1. Male cats were anaesthetized with alphaxalone-alphadolone and breathed spontaneously following tracheotomy, Using coloured microspheres, muscle blood flow was measured at rest and during two periods of contraction elicited by simultaneous stimulation of the left sciatic and femoral nerves at 3 Hz for 10 min. In one group, the hindlimb blood flow was allowed to increase during muscle contraction and in another group the perfusion of the hindlimb was limited by a stenosis on the left external iliac artery. 2. In the absence of flow restriction, soleus muscle blood flow increased from 18.9 +/- 3.8 to 30.4 +/- 3.3 ml min-1 (100 g tissue)-1 (n = 6; P < 0.02) and gastrocnemius muscle blood flow increased from 24.8 +/- 5.9 to 61.6 +/- 12.8 ml min-1 (100 g tissue)-1 (n = 6; P < 0.01) during contraction. The bradykinin (BK) antagonists HOE 140 (1 mg kg-1, i.v.) did not affect the response in either the soleus (37.7 +/- 7.3 ml min-1 (100 g tissue)-1; n = 6; n.s.) or the gastrocnemius (62.9 +/- 7.9 ml min-1 (100 g tissue)-1; n = 6; n.s.) muscles. 3. In the stenosis group, soleus muscle blood flow increased from 9.8 +/- 2.3 to 22.9 +/- 4.9 ml min-1 (100 g tissue)-1 (n = 6; P < 0.01) and gastrocnemius muscle blood flow increased from 15.8 +/ 3.4 to 36.4 +/- 5.5 ml min-1 (100 g tissue)-1 (n = 6; P < 0.01) during contraction. Following administration of HOE 140, functional hyperaemia in the soleus muscle was unaffected (blood flow, 17.8 +/- 2.2 ml min-1 (100 g tissue)-1, n = 6; n.s.) white blood flow in the gastrocnemius muscle was reduced to 21.8 +/- 6.0 ml min-1 (100 g tissue)-1 (n = 6; P < 0.05). 4. The results show that BK does not contribute ot functional hyperaemia associated with twitch contraction at 3 Hz when blood flow is unrestricted, but may contribute up to 40% of the vasodilation of predominantly glycolytic muscle groups such as the gastrocnemius when flow is restricted. BK plays no role in hyperaemia associated with twitch contraction of oxidative muscle groups such as the soleus.

Regulation of skeletal muscle carbohydrate oxidation during steady-state contraction.

Pyruvate dehydrogenase complex (PDC) activation status has been described as being central in the regulation of tissue substrate oxidation as outlined by the glucose fatty-acid cycle. In the present study we examined the effects of reduced lipolysis, with use of nicotinate, and increased PDC activation, with use of dichloroacetate (DCA), on substrate utilization during 20 min of submaximal steady-state contraction (approximately 80% of maximal O2 uptake) in canine gracilis skeletal muscle. At rest, PDC activation was unchanged by nicotinate but was approximately 2.5-fold higher in the DCA group than in the control group (P < 0.05). During contraction, PDC activation status increased to 3.5 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the control group, remained at 4.5 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the DCA group, but only increased to 2.2 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the nicotinate group (P < 0.05). However, the estimated amount of carbohydrate oxidized during the 20-min contraction was similar across groups and did not follow the degree of PDC activation (81.2 +/- 22.9, 95.9 +/- 11.7, and 89.3 +/- 18.9 mmol glucosyl units/kg dry muscle for control, nicotinate, and DCA, respectively). Thus it would appear that, during steady-state contraction, PDC activation status does not determine the rate of carbohydrate oxidation in skeletal muscle.

Muscle contraction-induced steal model in the anesthetized cat.

The current study describes a model in the anesthetized cat which may be used to investigate pharmacological interventions for the treatment of vascular "steal" during muscle contraction. An artificial stenosis was placed around both the descending abdominal aorta and the left external iliac artery. Arterial blood flow was measured in the right and left external iliac and right femoral arteries. Contraction of the left hindquarters, induced by electrical stimulation of the left sciatic and femoral nerves, resulted in an increase of left iliac artery blood flow from 14.7+/-2.0 to 30.5+/-2.8 ml min-1 (p<0.001). A simultaneous reduction of blood flow measured at the level of the right femoral artery (6.7+/-0.1 to 4.5+/-0.7 ml min(-1), p<0.01) was observed in the noncontracting hindlimb (y = 7.76 - 0.10x, correlation coefficient = -0.865). The magnitude of each response was reproducible within the same animal. Administration of 8-phenyltheophylline, a nonselective antagonist of adenosine receptors, reduced the functional hyperemia response within the left external iliac artery by 32% and reduced the "steal" from the vascular bed perfused by the right femoral artery by 21%.

Beta-adrenoceptors in vascular capacitance responses to unloading of carotid baroreceptors in anesthetized dogs.

The role of beta- and alpha-adrenoceptors in the total vascular capacitance responses to changing pressure in vascularly isolated carotid sinuses of anesthetized and atropinized dogs was investigated. A change in vascular capacitance was determined by measuring the shift of blood in and out of a reservoir that was connected to the aorta and maintained at a constant pressure. Changes in carotid sinus pressure from 135 to 57 mmHg and back to 137 mmHg resulted in a rapid vascular capacitance response of approximately 30 ml in the absence of adrenoceptor antagonists. Administration of a beta2-adrenoceptor antagonist (ICI-118551) caused a significant enhancement of the capacitance responses to similar decreases and increases in carotid sinus pressure (approximately 130%). Administration of a beta1-adrenoceptor antagonist (CGP-20712A) did not cause any further enhancement of the responses. However, an alpha-blocker (phentolamine) reduced the responses by 75%. The results suggest that in the presence of a beta2-adrenoceptor antagonist vascular capacitance responses to loading and unloading of baroreceptors are greatly enhanced and that patients suffering from orthostatic syncope may benefit from this kind of drug.

Metabolic responses from rest to steady state determine contractile function in ischemic skeletal muscle.

Skeletal muscle contraction during ischemia, such as that experienced by peripheral vascular disease patients, is characterized by rapid fatigue. Using a canine gracilis model, we tested the hypothesis that a critical factor determining force production during ischemia is the metabolic response during the transition from rest to steady state. Dichloroacetate (DCA) administration before gracilis muscle contraction increased pyruvate dehydrogenase complex activation and resulted in acetylation of 80% of the free carnitine pool to acetylcarnitine. After 1 min of contraction, phosphocreatine (PCr) degradation in the DCA group was approximately 50% lower than in the control group (P < 0.05) during conditions of identical force production. After 6 min of contraction, steady-state force production was approximately 30% higher in the DCA group (P < 0.05), and muscle ATP, PCr, and glycogen degradation and lactate accumulation were lower (P < 0.05 in all cases). It appears, therefore, that an important determinant of contractile function during ischemia is the mechanisms by which ATP regeneration occurs during the period of rest to steady-state transition.

Ischaemic skeletal muscle hyperaemia in the anaesthetized cat: no contribution of A2A adenosine receptors.

1. The present study investigated the contribution of the A2A adenosine receptor subtype to the functional hyperaemia response evoked by muscle contraction in anaesthetized cats when muscle blood flow was limited. 2. Application of a stenosis reduced the hindlimb blood flow at rest from 9.67 +/- 1.80 to 5.53 +/- 0.91 ml min(-1) (kg body mass)(-1) and during muscle contraction from 36.80 +/- 2.55 to 11.11 +/- 1.19 ml min(-1) (kg body mass)(-1) (P < 0.001). The force produced by the extensor digitorum longus and tibialis anterior (EDL-TA) muscle groups was also reduced, from 9.66 +/- 0.56 to 4.10 +/- 0.4 N (kg muscle mass)(-1) (P < 0.01). 3. The selective A2A adenosine receptor antagonist ZM241385 (3 mg kg(-1), I.V.) had no effect upon the hindlimb vascular conductance or muscle contraction responses in the presence of the flow-limiting stenosis. 4. In contrast, in the absence of the flow restriction the vascular conductance response was reduced by 27.5 +/- 5.0% (P < 0.05), whilst the isometric force produced by the EDL-TA muscle group was unaffected (pre- vs. post-contraction, 5.8 +/- 0.8 vs. 4.6 +/- 1.0 N (kg muscle mass)(-1) contraction). Oxygen consumption by the contracting hindlimb muscles was maintained (1.71 +/- 0.25 vs. 1.69 +/- 0.26 ml min(-1) (kg body mass)(-1)) by an increase in the oxygen extraction (51.9 +/- 4.9 vs. 66.2 +/- 6.1%; P< 0.01). 5. These results confirm previous data showing that adenosine, acting at the A2A receptor subtype, can contribute up to 30% of the functional hyperaemia response in the hindlimb of anaesthetized cats under free flow conditions. However, when blood flow is limited by a stenosis, antagonism of the A2A adenosine receptor does not affect functional hyperaemia.

Effect of the potassium channel opener ZM260384 on skeletal muscle function during restricted blood flow in the anaesthetized cat.

The aim of the present experiment was to determine whether the potassium channel opener 2-(2,2-bis(difluoromethyl)-6-nitro-3,4-dihydro-2H-1, 4-benzoxazine-4-yl)pyridine-N-oxide (ZM260384) was capable of accelerating the decline in skeletal muscle function during restricted blood flow in vivo. Cats (3.0-4.5 kg body weight) were anaesthetized with alphaxalone-alphadalone and breathed spontaneously following tracheotomy. Isometric tension was measured in the extensor digitorum longus-anterior tibialis (EDL-TA) muscle group. Ischaemia was induced by perfusing the hindlimb with the animal's own blood at a rate of 12.5 ml min-1 using a roller pump and stimulating the common peroneal nerve to induce repetitive submaximal tetanic contractions in the EDL-TA. The number of stimulation voltage increments required each minute to maintain a constant level of submaximal mechanical output and the time to exhaustion were used as indices of the rate of tension decline. The rate of tension decline in the ischaemic EDL-TA in the presence of ZM260384 at 3 mg kg-1, a maximally hypotensive dose predicted to be within the dose range required to exert direct effects on skeletal muscle, was measured and compared with the rate of tension decline in the presence of ZM260384 at 0.03 mg kg-1, also maximally hypotensive dose but below the predicted dose range for skeletal muscle effects. The number of voltage increments per minute was 1.93 +/- 0.07 and 1.48 +/- 0.14 (P < 0.05) in the presence of 3 and 0.03 mg kg-1 ZM260384, respectively. Time to exhaustion was 17.5 +/- 4.2 and 7.2 +/- 0.8 min (P < 0.05) in the presence of 3 and 0.03 mg kg-1 ZM260384, respectively. Given that there was no difference between these two groups in any haemodynamic variable measured, the results of the present study suggest that ZM260384 (3 mg kg-1) increases the rate of isometric force loss in ischemic skeletal muscle in vivo.

Pharmacodynamics of ZM 241385, a potent A2a adenosine receptor antagonist, after enteric administration in rat, cat and dog.

4-(2-[7-Amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5] triazin-5-ylamino]ethyl)phenol (ZM 241385) is currently the most selective for the A2a adenosine receptor antagonist. This paper describes the in-vivo activity of ZM 241385 after administration by both oral and intraduodenal routes. In conscious spontaneously hypertensive rats, ZM 241385 (1-10 mg kg-1) selectively attenuated the mean arterial blood pressure response produced by exogenous adenosine (1 mg kg-1 min-1, i.v.) by up to 45% after oral administration. Activity of ZM 241385 was maintained for at least 6 h after 3 and 10 mg kg-1 (p.o.). In conscious normotensive cats, ZM 241385 attenuated the blood pressure responses to adenosine (0.6-1.0 mg kg-1, i.v.) by 94% after 10 mg kg-1 (p.o.) and by up to 74% after 0.3 mg kg-1 (i.v.). Duration of action of ZM 241385 up to 12 h (36% inhibition) was observed after 3 mg kg-1 (p.o.). In anaesthetized dogs and cats, ZM 241385, after intraduodenal administration (1-10 mg kg-1), produced a rapid (dose ratio 100-fold 15 min after administration of 10 mg kg-1 in the cat) and prolonged (dose ratio of 14 at 6 h after administration of 10 mg kg-1) attenuation of the vasodilatation responses to adenosine receptor stimulation. When administered by this route ZM 241385 was six times more potent than theophylline in the cat and at least twice as potent as theophylline in the dog. In conclusion, ZM 241385 is a potent, selective A2a adenosine receptor antagonist which is orally active, with a good duration of action by the enteric route in cat, rat and dog. It could therefore be used to evaluate the role of adenosine A2a receptors in the action of adenosine in-vivo.

The effects of pharmacological modulation of KATP on the guinea-pig isolated diaphragm.

The purpose of the present study was to investigate the functional consequences of KATP modulation in the normal and the metabolically inhibited guinea-pig isolated diaphragm using the K+ channel openers cromakalim, pinacidil, RP49356 (N-methyl-2-(3-pyridil)-tetrahydrothiopyran-2-carbothiami de-1-oxide) and ZM260384 (2-(2,2-bis(difluoromethyl)-6-nitro-3,4-dihydro-2H-1,4-benzoxazine -4-yl)pyridine-N-oxide) and the K+ channel inhibitors glibenclamide, phentolamine and ciclazindol. All K+ channel openers accelerated the decline in function induced by intermittent tetanic contractions following metabolic inhibition and delayed the development of contracture. Cromakalim also improved the recovery of twitch tension following 10 min intermittent tetanic stimulation in the hypoxic guinea-pig diaphragm preparation. Of the K+ channel inhibitors tested, only ciclazindol, at the highest concentration tested (10 microM), significantly delayed the decline in tetanic tension following metabolic inhibition in the guinea-pig isolated diaphragm. None of the inhibitors significantly accelerated the development of contracture. All inhibitors however, antagonised the actions of the K+ channel opener, cromakalim. The results indicate that opening of KATP can accelerate the decline in function following metabolic inhibition in the guinea-pig isolated diaphragm. In the absence of K+ channel openers however, KATP does not appear to contribute to this decline under the conditions of the present study.

The role of the A(2A) adenosine receptor subtype in functional hyperaemia in the hindlimb of anaesthetized cats.

1. The present study was designed to investigate the contribution of the A(2A) adenosine receptor subtype in the functional hyperaemia response during muscle contraction. 2. In cats anaesthetized with sodium pentobarbitone and breathing spontaneously following tracheotomy, the left sciatic and femoral nerves were electrically stimulated at 3 Hz for 20 min to induce muscle contraction, and hindlimb blood flow was measured with a flow probe. The contribution of the A(2A) adenosine receptor subtype was assessed using ZM 241385, a potent and selective A(2A) adenosine receptor antagonist. 3. In a control group, the muscle isometric tension measured in the extensor digitorum longus-tibialis anterior muscle group was 6.64 +/- 0.66 kg (100 g muscle mass)(-1) and hindlimb vascular conductance was 0.22 +/- 0.03 ml mmHg(-1)(kg body mass)(-1) at 20 min of contraction. Administration of vehicle did not affect these parameters upon a second contraction period: 6.31 +/- 0.61 kg (100 g muscle mass)(-1) and 0.23 +/- 0.03 ml mmHg(-1) (kg body mass)(-1), respectively. Total hindlimb conductance during contraction was unaffected (5.5 +/- 3.7% decrease). 4. ZM 241385 (1.0 mg kg(-1)) did not alter the amount of force produced by the muscle at 20 min of contraction. Hindlimb conductance response was reduced by 27.1 +/- 4.8% following the A(2A) selective adenosine receptor antagonist, similar to that observed with the non-selective antagonist 8-phenyltheophylline. 5. These results show that adenosine acting at the A(2A) subtype receptor can contribute up to 30% of the functional hyperaemia response in the hindlimb of anaesthetized cats.

In vivo characterisation of ZM 241385, a selective adenosine A2A receptor antagonist.

The in vivo characterisation of ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-+ ++ylamino] ethyl)phenol), a novel, non-xanthine, selective adenosine A2A antagonist is described. In anaesthetised dogs ZM 241385 (i.v.) was 140-fold more potent in attenuating vasodilator responses to exogenous adenosine in the constant flow perfused hind limb than the bradycardic effects. In pithed rats in which blood pressure was supported by angiotensin II infusion, ZM 241385 (10 mg kg-1, i.v.) did not inhibit the hypotensive or bradycardic effects of the A3/A1 receptor agonist N(6)-2-(4-amino-3-iodophenyl)ethyladenosine (APNEA). In conscious spontaneously hypertensive rats, ZM 241385 (3-10 mg kg-1, p.o.) selectively attenuated the mean arterial blood pressure response produced by exogenous adenosine. No inhibition of the bradycardic effects of adenosine was observed following these doses of ZM 241385. The results indicate that ZM 241385 can be used to evaluate the role of adenosine A2A receptors in the action of adenosine in vivo.