Vascular and endocrine control of muscle metabolism

Toward a Better Understanding of Muscle Microvascular Perfusion During Exercise in Patients With Peripheral Artery Disease: The Effect of Lower-Limb Revascularization

PURPOSE

Leg muscle microvascular blood flow (perfusion) is impaired in response to maximal exercise in patients with peripheral artery disease (PAD); however, during submaximal exercise, microvascular perfusion is maintained due to a greater increase in microvascular blood volume compared with that seen in healthy adults. It is unclear whether this submaximal exercise response reflects a microvascular impairment, or whether it is a compensatory response for the limited conduit artery flow in PAD. Therefore, to clarify the role of conduit artery blood flow, we compared whole-limb blood flow and skeletal muscle microvascular perfusion responses with exercise in patients with PAD (n=9; 60±7 years) prior to, and following, lower-limb endovascular revascularization.

MATERIALS AND METHODS

Microvascular perfusion (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after a 5 minute bout of submaximal intermittent isometric plantar-flexion exercise using contrast-enhanced ultrasound imaging. Exercise contraction-by-contraction whole-leg blood flow and vascular conductance were measured using strain-gauge plethysmography.

RESULTS

With revascularization there was a significant increase in whole-leg blood flow and conductance during exercise (p<0.05). Exercise-induced muscle microvascular perfusion response did not change with revascularization (pre-revascularization: 3.19±2.32; post-revascularization: 3.89±1.67 aU.s-1; p=0.38). However, the parameters that determine microvascular perfusion changed, with a reduction in the microvascular volume response to exercise (pre-revascularization: 6.76±3.56; post-revascularization: 2.42±0.69 aU; p<0.01) and an increase in microvascular flow velocity (pre-revascularization: 0.25±0.13; post-revascularization: 0.59±0.25 s-1; p=0.02).

CONCLUSION

These findings suggest that patients with PAD compensate for the conduit artery blood flow impairment with an increase in microvascular blood volume to maintain muscle perfusion during submaximal exercise.

CLINICAL IMPACT

The findings from this study support the notion that the impairment in conduit artery blood flow in patients with PAD leads to compensatory changes in microvascular blood volume and flow velocity to maintain muscle microvascular perfusion during submaximal leg exercise. Moreover, this study demonstrates that these microvascular changes are reversed and become normalized with successful lower-limb endovascular revascularization.

Reduced post-exercise muscle microvascular perfusion with compression is offset by increased muscle oxygen extraction: Assessment by contrast-enhanced ultrasound

The microvasculature is important for both health and exercise tolerance in a range of populations. However, methodological limitations have meant changes in microvascular blood flow are rarely assessed in humans during interventions designed to affect skeletal muscle blood flow such as the wearing of compression garments. The aim of this study is, for the first time, to use contrast-enhanced ultrasound to directly measure the effects of compression on muscle microvascular blood flow alongside measures of femoral artery blood flow and muscle oxygenation following intense exercise in healthy adults. It was hypothesized that both muscle microvascular and femoral artery blood flows would be augmented with compression garments as compared with a control condition. Ten recreationally active participants completed two repeated-sprint exercise sessions, with and without lower-limb compression tights. Muscle microvascular blood flow, femoral arterial blood flow (2D and Doppler ultrasound), muscle oxygenation (near-infrared spectroscopy), cycling performance, and venous blood samples were measured/taken throughout exercise and the 1-hour post-exercise recovery period. Compared with control, compression reduced muscle microvascular blood volume and attenuated the exercise-induced increase in microvascular velocity and flow immediately after exercise and 1 hour post-exercise. Compression increased femoral artery diameter and augmented the exercise-induced increase in femoral arterial blood flow during exercise. Markers of blood oxygen extraction in muscle were increased with compression during and after exercise. Compression had no effect on blood lactate, glucose, or exercise performance. We provide new evidence that lower-limb compression attenuates the exercise-induced increase in skeletal muscle microvascular blood flow following exercise, despite a divergent increase in femoral artery blood flow. Decreased muscle microvascular perfusion is offset by increased muscle oxygen extraction, a potential mechanism allowing for the maintenance of exercise performance.

Postprandial microvascular blood flow in skeletal muscle: Similarities and disparities to the hyperinsulinaemic-euglycaemic clamp

Skeletal muscle contributes to ~40% of total body mass and has numerous important mechanical and metabolic roles in the body. Skeletal muscle is a major site for glucose disposal following a meal. Consequently, skeletal muscle plays an important role in postprandial blood glucose homeostasis. Over the past number of decades, research has demonstrated that insulin has an important role in vasodilating the vasculature in skeletal muscle in response to an insulin infusion (hyperinsulinaemic-euglycaemic clamp) or following the ingestion of a meal. This vascular action of insulin is pivotal for glucose disposal in skeletal muscle, as insulin-stimulated vasodilation increases the delivery of both glucose and insulin to the myocyte. Notably, in insulin-resistant states such as obesity and type 2 diabetes, this vascular response of insulin in skeletal muscle is significantly impaired. Whereas the majority of work in this field has focussed on the action of insulin alone on skeletal muscle microvascular blood flow and myocyte glucose metabolism, there is less understanding of how the consumption of a meal may affect skeletal muscle blood flow. This is in part due to complex variations in glucose and insulin dynamics that occurs postprandially-with changes in humoral concentrations of glucose, insulin, amino acids, gut and pancreatic peptides-compared to the hyperinsulinaemic-euglycaemic clamp. This review will address the emerging body of evidence to suggest that postprandial blood flow responses in skeletal muscle may be a function of the nutritional composition of a meal.

Contrast-enhanced ultrasound assessment of impaired adipose tissue and muscle perfusion in insulin-resistant mice

BACKGROUND

In diabetes mellitus, reduced perfusion and capillary surface area in skeletal muscle, which is a major glucose storage site, contribute to abnormal glucose homeostasis. Using contrast-enhanced ultrasound, we investigated whether abdominal adipose tissue perfusion is abnormal in insulin resistance and correlates with glycemic control.

METHODS AND RESULTS

Contrast-enhanced ultrasound perfusion imaging of abdominal adipose tissue and skeletal muscle was performed in obese insulin resistance (db/db) mice at 11 to 12 or 14 to 16 weeks of age and in control lean mice. Time-intensity data were analyzed to quantify microvascular blood flow (MBF) and capillary blood volume (CBV). Blood glucose response for 1 hour was measured after insulin challenge (1 U/kg, IP). Compared with control mice, db/db mice at 11 to 12 and 14 to 16 weeks had a higher glucose concentration area under the curve after insulin (11.8±2.8, 20.6±4.3, and 28.4±5.9 mg·min/dL [×1000], respectively; P=0.0002) and also had lower adipose MBF (0.094±0.038, 0.035±0.010, and 0.023±0.01 mL/min per gram; P=0.0002) and CBV (1.6±0.6, 1.0±0.3, and 0.5±0.1 mL/100 g; P=0.0017). The glucose area under the curve correlated in a nonlinear fashion with both adipose and skeletal muscle MBF and CBV. There were significant linear correlations between adipose and muscle MBF (r=0.81) and CBV (r=0.66). Adipocyte cell volume on histology was 25-fold higher in 14- to 16-week db/db versus control mice.

CONCLUSIONS

Abnormal adipose MBF and CBV in insulin resistance can be detected by contrast-enhanced ultrasound and correlates with the degree of impairment in glucose storage. Abnormalities in adipose tissue and muscle seem to be coupled. Impaired adipose tissue perfusion is in part explained by an increase in adipocyte size without proportional vascular response.

Tissue inflammation and nitric oxide-mediated alterations in cardiovascular function are major determinants of endotoxin-induced insulin resistance

BACKGROUND

Endotoxin (i.e. LPS) administration induces a robust inflammatory response with accompanying cardiovascular dysfunction and insulin resistance. Overabundance of nitric oxide (NO) contributes to the vascular dysfunction. However, inflammation itself also induces insulin resistance in skeletal muscle. We sought to investigate whether the cardiovascular dysfunction induced by increased NO availability without inflammatory stress can promote insulin resistance. Additionally, we examined the role of inducible nitric oxide synthase (iNOS or NOS2), the source of the increase in NO availability, in modulating LPS-induced decrease in insulin-stimulated muscle glucose uptake (MGU).

METHODS

The impact of NO donor infusion on insulin-stimulated whole-body and muscle glucose uptake (hyperinsulinemic-euglycemic clamps), and the cardiovascular system was assessed in chronically catheterized, conscious mice wild-type (WT) mice. The impact of LPS on insulin action and the cardiovascular system were assessed in WT and global iNOS knockout (KO) mice. Tissue blood flow and cardiac function were assessed using microspheres and echocardiography, respectively. Insulin signaling activity, and gene expression of pro-inflammatory markers were also measured.

RESULTS

NO donor infusion decreased mean arterial blood pressure, whole-body glucose requirements, and MGU in the absence of changes in skeletal muscle blood flow. LPS lowered mean arterial blood pressure and glucose requirements in WT mice, but not in iNOS KO mice. Lastly, despite an intact inflammatory response, iNOS KO mice were protected from LPS-mediated deficits in cardiac output. LPS impaired MGU in vivo, regardless of the presence of iNOS. However, ex vivo, insulin action in muscle obtained from LPS treated iNOS KO animals was protected.

CONCLUSION

Nitric oxide excess and LPS impairs glycemic control by diminishing MGU. LPS impairs MGU by both the direct effect of inflammation on the myocyte, as well as by the indirect NO-driven cardiovascular dysfunction.

Metabolic actions of angiotensin II and insulin: a microvascular endothelial balancing act

Metabolic actions of insulin to promote glucose disposal are augmented by nitric oxide (NO)-dependent increases in microvascular blood flow to skeletal muscle. The balance between NO-dependent vasodilator actions and endothelin-1-dependent vasoconstrictor actions of insulin is regulated by phosphatidylinositol 3-kinase-dependent (PI3K)--and mitogen-activated protein kinase (MAPK)-dependent signaling in vascular endothelium, respectively. Angiotensin II acting on AT₂ receptor increases capillary blood flow to increase insulin-mediated glucose disposal. In contrast, AT₁ receptor activation leads to reduced NO bioavailability, impaired insulin signaling, vasoconstriction, and insulin resistance. Insulin-resistant states are characterized by dysregulated local renin-angiotensin-aldosterone system (RAAS). Under insulin-resistant conditions, pathway-specific impairment in PI3K-dependent signaling may cause imbalance between production of NO and secretion of endothelin-1, leading to decreased blood flow, which worsens insulin resistance. Similarly, excess AT₁ receptor activity in the microvasculature may selectively impair vasodilation while simultaneously potentiating the vasoconstrictor actions of insulin. Therapeutic interventions that target pathway-selective impairment in insulin signaling and the imbalance in AT₁ and AT₂ receptor signaling in microvascular endothelium may simultaneously ameliorate endothelial dysfunction and insulin resistance. In the present review, we discuss molecular mechanisms in the endothelium underlying microvascular and metabolic actions of insulin and Angiotensin II, the mechanistic basis for microvascular endothelial dysfunction and insulin resistance in RAAS dysregulated clinical states, and the rationale for therapeutic strategies that restore the balance in vasodilator and constrictor actions of insulin and Angiotensin II in the microvasculature.

cGMP phosphodiesterase inhibition improves the vascular and metabolic actions of insulin in skeletal muscle

There is considerable support for the concept that insulin-mediated increases in microvascular blood flow to muscle impact significantly on muscle glucose uptake. Since the microvascular blood flow increases with insulin have been shown to be nitric oxide-dependent inhibition of cGMP-degrading phosphodiesterases (cGMP PDEs) is predicted to enhance insulin-mediated increases in microvascular perfusion and muscle glucose uptake. Therefore, we studied the effects of the pan-cGMP PDE inhibitor zaprinast on the metabolic and vascular actions of insulin in muscle. Hyperinsulinemic euglycemic clamps (3 mU·min(-1)·kg(-1)) were performed in anesthetized rats and changes in microvascular blood flow assessed from rates of 1-methylxanthine metabolism across the muscle bed by capillary xanthine oxidase in response to insulin and zaprinast. We also characterized cGMP PDE isoform expression in muscle by real-time PCR and immunostaining of frozen muscle sections. Zaprinast enhanced insulin-mediated microvascular perfusion by 29% and muscle glucose uptake by 89%, while whole body glucose infusion rate during insulin infusion was increased by 33% at 2 h. PDE2, -9, and -10 were the major isoforms expressed at the mRNA level in muscle, while PDE1B, -9A, -10A, and -11A proteins were expressed in blood vessels. Acute administration of the cGMP PDE inhibitor zaprinast enhances muscle microvascular blood flow and glucose uptake response to insulin. The expression of a number of cGMP PDE isoforms in skeletal muscle suggests that targeting specific cGMP PDE isoforms may provide a promising avenue for development of a novel class of therapeutics for enhancing muscle insulin sensitivity.

Adiponectin opposes endothelin-1-mediated vasoconstriction in the perfused rat hindlimb

Recent studies have shown that adiponectin is able to increase nitric oxide (NO) production by the endothelium and relax preconstricted isolated aortic rings, suggesting that adiponectin may act as a vasodilator. Endothelin-1 (ET-1) is a potent vasoconstrictor, elevated levels of which are associated with obesity, type 2 diabetes, hypertension, and cardiovascular disease. We hypothesized that adiponectin has NO-dependent vascular actions opposing the vasoconstrictor actions of ET-1. We studied the vascular and metabolic effects of a physiological concentration of adiponectin (6.5 μg/ml) on hooded Wistar rats in the constant-flow pump-perfused rat hindlimb. Adiponectin alone had no observable vascular activity; however, adiponectin pretreatment and coinfusion inhibited the increase in perfusion pressure and associated metabolic stimulation caused by low-dose (1 nM) ET-1. Adiponectin was not able to oppose vasoconstriction when infusion was commenced after ET-1. This is in contrast to the NO donor sodium nitroprusside, which significantly reduced the pressure due to established ET-1 vasoconstriction, suggesting dissociation of the actions of adiponectin and NO. In addition, adiponectin had no effect on vasoconstriction caused by either high-dose (20 nM) ET-1 or low-dose (50 nM) norepinephrine. Our findings suggest that adiponectin has specific, apparently NO-independent, vascular activity to oppose the vasoconstrictor effects of ET-1. The hemodynamic actions of adiponectin may be an important aspect of its insulin-sensitizing ability by regulating access of insulin and glucose to myocytes. Imbalance in the relationship between adiponectin and ET-1 in obesity may contribute to the development of insulin resistance and cardiovascular disease.

Perivascular fatty tissue at the brachial artery is linked to insulin resistance but not to local endothelial dysfunction

AIMS/HYPOTHESIS

Different ectopic fat depots, such as visceral or hepatic fat, are known to affect whole body insulin sensitivity. It has recently been hypothesised that differences in perivascular adipose tissue (PVAT) mass around resistance vessels may also contribute to insulin resistance, possibly via direct vascular effects leading to reduced capillary cross-sectional area in the muscle, which in turn affects muscular blood flow and glucose uptake. Based on this, the aim of the present study was to test whether PVAT around conduit arteries (i.e. the brachial artery) influences NO bioavailability, expressed as flow-mediated dilation (FMD), or insulin sensitivity in humans in vivo.

METHODS

Insulin sensitivity was measured by OGTT in all 95 participants (59 women, 36 men; median age 47 years, range 19-66 years) and by the gold standard, a euglycaemic-hyperinsulinaemic clamp, in a randomly selected subgroup of 33 participants. Quantification of the different fat compartments, including PVAT around the brachial artery, was achieved by high-resolution magnetic resonance imaging (1.5 T). Blood flow and FMD were measured at the brachial artery using high-resolution (13 MHz) ultrasound, after 5 min of forearm occlusion.

RESULTS

PVAT was negatively correlated with insulin sensitivity and the post-ischaemic increase in blood flow. The association between PVAT and insulin sensitivity (r = -0.54, beta = -0.37, p = 0.009) was independent of age, sex, visceral adipose tissue, liver fat, BMI and further cardiovascular risk factors. No correlation could be detected between PVAT and local endothelial function. However, we observed an independent association between PVAT and post-ischaemic increase in blood flow (r = -0.241; beta = -1.69; p = 0.02).

CONCLUSIONS/INTERPRETATION

PVAT seems to play an independent role in the pathogenesis of insulin resistance. This may be due to direct vascular effects influencing muscular blood flow.

Impaired microvascular perfusion: a consequence of vascular dysfunction and a potential cause of insulin resistance in muscle

Insulin has an exercise-like action to increase microvascular perfusion of skeletal muscle and thereby enhance delivery of hormone and nutrient to the myocytes. With insulin resistance, insulin's action to increase microvascular perfusion is markedly impaired. This review examines the present status of these observations and techniques available to measure such changes as well as the possible underpinning mechanisms. Low physiological doses of insulin and light exercise have been shown to increase microvascular perfusion without increasing bulk blood flow. In these circumstances, blood flow is proposed to be redirected from the nonnutritive route to the nutritive route with flow becoming dominant in the nonnutritive route when insulin resistance has developed. Increased vasomotion controlled by vascular smooth muscle may be part of the explanation by which insulin mediates an increase in microvascular perfusion, as seen from the effects of insulin on both muscle and skin microvascular blood flow. In addition, vascular dysfunction appears to be an early development in the onset of insulin resistance, with the consequence that impaired glucose delivery, more so than insulin delivery, accounts for the diminished glucose uptake by insulin-resistant muscle. Regular exercise may prevent and ameliorate insulin resistance by increasing "vascular fitness" and thereby recovering insulin-mediated capillary recruitment.

Acute responses of muscle protein metabolism to reduced blood flow reflect metabolic priorities for homeostasis

The present experiment was designed to measure the synthetic and breakdown rates of muscle protein in the hindlimb of rabbits with or without clamping the femoral artery. l-[ring-(13)C(6)]phenylalanine was infused as a tracer for measurement of muscle protein kinetics by means of an arteriovenous model, tracer incorporation, and tracee release methods. The ultrasonic flowmeter, dye dilution, and microsphere methods were used to determine the flow rates in the femoral artery, in the leg, and in muscle capillary, respectively. The femoral artery flow accounted for 65% of leg flow. A 50% reduction in the femoral artery flow reduced leg flow by 28% and nutritive flow by 26%, which did not change protein synthetic or breakdown rate in leg muscle. Full clamp of the femoral artery reduced leg flow by 42% and nutritive flow by 59%, which decreased (P < 0.05) both the fractional synthetic rate from 0.19 +/- 0.05 to 0.14 +/- 0.03%/day and fractional breakdown rate from 0.28 +/- 0.07 to 0.23 +/- 0.09%/day of muscle protein. Neither the partial nor full clamp reduced (P = 0.27-0.39) the intracellular phenylalanine concentration or net protein balance in leg muscle. We conclude that the flow threshold to cause a fall of protein turnover rate in leg muscle was a reduction of 30-40% of the leg flow. The acute responses of muscle protein kinetics to the reductions in blood flow reflected the metabolic priorities to maintain muscle homeostasis. These findings cannot be extrapolated to more chronic conditions without experimental validation.

Role of adenosine in regulating the heterogeneity of skeletal muscle blood flow during exercise in humans

Evidence from both animal and human studies suggests that adenosine plays a role in the regulation of exercise hyperemia in skeletal muscle. We tested whether adenosine also plays a role in the regulation of blood flow (BF) distribution and heterogeneity among and within quadriceps femoris (QF) muscles during exercise, measured using positron emission tomography. In six healthy young women, BF was measured at rest and then during three incremental low and moderate intermittent isometric one-legged knee-extension exercise intensities without and with theophylline-induced nonselective adenosine receptor blockade. BF heterogeneity within muscles was calculated from 16-mm3voxels in BF images and heterogeneity among the muscles from the mean values of the four QF compartments. Mean BF in the whole QF and its four parts increased, and heterogeneity decreased with workload both without and with theophylline ( P < 0.001). Adenosine receptor blockade did not have any effect on mean bulk BF or BF heterogeneity among the QF muscles, yet blockade increased within-muscle BF heterogeneity in all four QF muscles ( P = 0.03). Taken together, these results show that BF becomes less heterogeneous with increasing exercise intensity in the QF muscle group. Adenosine seems to play a role in muscle BF heterogeneity even in the absence of changes in bulk BF at low and moderate one-leg intermittent isometric exercise intensities.

Microvascular dysfunction in obesity: a potential mechanism in the pathogenesis of obesity-associated insulin resistance and hypertension

Obesity is an important risk factor for insulin resistance and hypertension and plays a central role in the metabolic syndrome. Insight into the pathophysiology of this syndrome may lead to new treatments. This paper has reviewed the evidence for an important role for the microcirculation as a possible link between obesity, insulin resistance and hypertension.

Insulin and contraction increase nutritive blood flow in rat muscle in vivo determined by microdialysis of L-[14C]glucose

In the present study, a mathematical model using the microdialysis outflow: inflow (O/I) ratio of the novel analogue L-[14C]glucose has been developed which allows the calculation of the nutritive (and non-nutritive) flow in muscle as a proportion of total blood flow. Anaesthetized rats had microdialysis probes carrying L-[14C]glucose inserted through a calf muscle group (tibialis/plantaris/gastrocnemius). The nutritive fraction of total blood flow was determined under basal conditions and in response to contraction (electrical field stimulation), insulin (hyperinsulinaemic euglycaemic clamp with 10 mU min(-1) kg(-1) insulin) or saline control from limb blood flow and the microdialysis O/I ratio of L-[14C]glucose. Both contraction and insulin infusion decreased the O/I ratio of L-[14C]glucose and increased total limb blood flow. Calculations based on mathematical models using L-[14C]glucose O/I and limb blood flow revealed that during basal conditions, the nutritive fraction of total flow was 0.38 +/- 0.06, indicating that basal flow was predominantly non-nutritive. Contraction and insulin increased the nutritive fraction to 0.82 +/- 0.24 (P < 0.05) and 0.52 +/- 0.12 (P < 0.05). Thus the increase in limb blood flow from insulin was fully accommodated by nutritive flow, while contraction increased nutritive flow at the expense of non-nutritive flow. This novel method using microdialysis and the O/I ratio of L-[14C]glucose allows the determination of the nutritive fraction of total flow in muscle as well as the proportion of total flow that may be redistributed in response to contraction and insulin.

Physiological hyperinsulinemia has no detectable effect on access of macromolecules to insulin-sensitive tissues in healthy humans

OBJECTIVE

Physiologically elevated insulin concentrations promote access of macromolecules to skeletal muscle in dogs. We investigated whether insulin has a stimulating effect on the access of macromolecules to insulin-sensitive tissues in humans as well.

RESEARCH DESIGN AND METHODS

In a randomized, controlled trial, euglycemic-hyperinsulinemic clamp (1.2 mU x kg(-1) x min(-1) insulin) and saline control experiments were performed in 10 healthy volunteers (aged 27.5 +/- 4 years, BMI 22.6 +/- 1.6 kg/m(2)). Distribution and clearance parameters of inulin were determined in a whole-body approach, combining primed intravenous infusion of inulin with compartment modeling. Inulin kinetics were measured in serum using open-flow microperfusion in interstitial fluid of femoral skeletal muscle and subcutaneous adipose tissue.

RESULTS

Inulin kinetics in serum were best described using a three-compartment model incorporating a serum and a fast and a slow equilibrating compartment. Inulin kinetics in interstitial fluid of peripheral insulin-sensitive tissues were best represented by the slow equilibrating compartment. Serum and interstitial fluid inulin kinetics were comparable between the insulin and saline groups. Qualitative analysis of inulin kinetics was confirmed by model-derived distribution and clearance parameters of inulin. Physiological hyperinsulinemia (473 +/- 6 vs. 18 +/- 2 pmol/l for the insulin and saline group, respectively; P < 0.001) indicated no effect on distribution volume (98.2 +/- 6.2 vs. 102.5 +/- 5.7 ml/kg; NS) or exchange parameter (217.6 +/- 34.2 vs. 243.1 +/- 28.6 ml/min; NS) of inulin to peripheral insulin-sensitive tissues. All other parameters identified by the model were also comparable between the groups.

CONCLUSIONS

Our data suggest that in contrast to studies performed in dogs, insulin at physiological concentrations does not augment recruitment of insulin-sensitive tissues in healthy humans.

Reduced access to insulin-sensitive tissues in dogs with obesity secondary to increased fat intake

Physiological hyperinsulinemia provokes hemodynamic actions and augments access of macromolecules to insulin-sensitive tissues. We investigated whether induction of insulin resistance by a hypercaloric high-fat diet has an effect on the extracellular distribution of macromolecules to insulin-sensitive tissues. Male mongrel dogs were randomly selected into two groups: seven dogs were fed an isocaloric control diet ( approximately 3,900 kcal, 35% from fat), and six dogs were fed a hypercaloric high-fat diet ( approximately 5,300 kcal, 54% from fat) for a period of 12 weeks. During hyperinsulinemic-euglycemic clamps, we determined transport parameters and distribution volumes of [(14)C]inulin by applying a three-compartment model to the plasma clearance data of intravenously injected [(14)C]inulin (0.8 microCi/kg). In another study with direct cannulation of the hindlimb skeletal muscle lymphatics, we investigated the effect of physiological hyperinsulinemia on the appearance of intravenously injected [(14)C]inulin in skeletal muscle interstitial fluid and compared the effect of insulin between control and high-fat diet groups. The hypercaloric high-fat diet resulted in significant weight gain (18%; P<0.001) associated with marked increases of subcutaneous (140%; P<0.001) and omental (83%; P<0.001) fat depots, as well as peripheral insulin resistance, measured as a significant reduction of insulin-stimulated glucose uptake during clamps (-35%; P<0.05). Concomitantly, we observed a significant reduction of the peripheral distribution volume of [(14)C]inulin (-26%; P<0.05), whereas the vascular distribution volume and transport and clearance parameters did not change as a cause of the diet. The second study directly confirmed our findings, suggesting a marked reduction of insulin action to stimulate access of macromolecules to insulin-sensitive tissues (control diet 32%, P<0.01; high-fat diet 18%, NS). The present results indicate that access of macromolecules to insulin-sensitive tissues is impaired during diet-induced insulin resistance and suggest that the ability of insulin itself to stimulate tissue access is diminished. We speculate that the observed diet-induced defects in stimulation of tissue perfusion contribute to the development of peripheral insulin resistance.

Vascular and metabolic effects of methacholine in relation to insulin action in muscle

AIMS/HYPOTHESIS

Methacholine (MC) is a nitric oxide vasodilator, but unlike other vasodilators, it potentiates insulin-mediated glucose uptake by muscle. The present study aimed to resolve whether this action was the result of a vascular effect of MC leading to increased muscle perfusion or a direct effect of MC on the myocytes. We hypothesise that vascular-mediated insulin-stimulated glucose uptake responses to MC occur at lower doses than direct myocyte MC-mediated increases in glucose uptake.

METHODS

The vascular and metabolic effects of this vasodilator were examined in rats in vivo using a novel local infusion technique, and in the pump-perfused rat hindlimb under conditions of constant flow.

RESULTS

Local infusion of low-dose MC (0.3 micromol/l) into the epigastric artery of one leg (test) in vivo markedly increased femoral blood flow and decreased vascular resistance, without effects in the contra-lateral leg. Capillary recruitment, but not glucose uptake, was increased in the test leg. All increases caused by MC were confined to the test leg and blocked by local infusion into the test leg of N-nitro-L-arginine methyl ester (L-NAME), but not by infusion of N-nitro-D-arginine methyl ester (D-NAME). In the constant-flow pump-perfused rat hindlimb, infusion of 0.6 micromol/l MC vasodilated the pre-constriction effected by 70 nmol/l noradrenaline or 300 nmol/l serotonin, and this was blocked by 10 micromol/l L-NAME. 2-Deoxyglucose in muscle was increased by 30 micromol/l MC (p<0.05), but was unaffected by 3 micromol/l MC. All increases in 2-deoxyglucose uptake by 30 micromol/l MC were blocked by 10 micromol/l L-NAME.

CONCLUSIONS/INTERPRETATION

MC has dose-dependent effects both on the vasculature and on muscle metabolism. At low dose (0.3-3 micromol/l), MC is a potent vasodilator in muscle, both in vivo and in vitro, without metabolic effects; at higher doses (> or =30 micromol/l) MC has a direct metabolic effect leading to increased glucose uptake. Both the vascular and metabolic effects are sensitive to L-NAME. The low-dose enhancement of insulin action in vivo by MC, which has been reported previously, thus seems to be attributable to vascular effects.

Metabolic and vascular actions of endothelin-1 are inhibited by insulin-mediated vasodilation in perfused rat hindlimb muscle

Endothelin-1 (ET-1) is a potent endothelium-derived vasoactive peptide and may be involved in the microvascular actions of insulin for the normal delivery of nutrients to muscle, although higher levels may be antagonistic. Our aim was to observe the interaction between ET-1 and insulin. Initially, we attempted to distinguish the vascular from the metabolic effects of ET-1 in the constant-flow pump-perfused rat hindlimb by using various doses of ET-1 and measuring changes in perfusion pressure (PP), oxygen consumption (VO(2)), glucose uptake (GU) and lactate release (LR). Sodium nitroprusside (SNP) was used to block vasoconstriction and to thus assess the relationship between vascular and metabolic effects. Insulin was included in later experiments to determine the interaction between insulin and ET-1 on the above parameters. ET-1 caused a dose-dependent increase in PP. Effects on VO(2) were biphasic, with low doses increasing VO(2), and higher doses leading to a net inhibition. GU and LR were increased at lower doses (ET-1 < or =1 nM), but this effect was lost at higher doses (> or =10 nM ET-1). SNP (50 microM) fully blocked the increase in pressure and metabolism due to low-dose ET-1 and partly blocked both pressure and metabolic responses by the high dose. ET-1 vasodilatory activity was minimal at high or low dose. Insulin (15 nM) alone caused GU, which was not affected by ET-1. Of the other parameters measured, insulin behaved essentially the same as SNP, inhibiting the pressure and oxygen effects. Overall, these results show that ET-1 has a biphasic dose-dependent vasoconstrictor effect on hindlimb blood vessels, able to modulate flow to cause both the stimulation and inhibition of metabolism, although these effects are blocked by insulin, which is able to vasodilate against both low and high doses of ET-1.

The in vivo effects of the Pro12Ala PPARgamma2 polymorphism on adipose tissue NEFA metabolism: the first use of the Oxford Biobank

AIMS/HYPOTHESIS

To investigate the phenotypic effects of common polymorphisms on adipose tissue metabolism and cardiovascular risk factors, we set out to establish a biobank with the unique feature of allowing a prospective recruit-by-genotype approach. The first use of this biobank investigates the effects of the peroxisome proliferator-activated receptor (PPAR) Pro12Ala polymorphism on integrative tissue-specific physiology. We hypothesised that Ala12 allele carriers demonstrate greater adipose tissue metabolic flexibility and insulin sensitivity.

MATERIALS AND METHODS

From a comprehensive population register, subjects were recruited into a biobank, which was genotyped for the Pro12Ala polymorphism. Twelve healthy male Ala12 carriers and 12 matched Pro12 homozygotes underwent detailed physiological phenotyping using stable isotope techniques, and measurements of blood flow and arteriovenous differences in adipose tissue and muscle in response to a mixed meal containing [1,1,1-(13)C]tripalmitin.

RESULTS

Of 6,148 invited subjects, 1,072 were suitable for inclusion in the biobank. Among Pro12 homozygotes, insulin sensitivity correlated with HDL-cholesterol concentrations, and inversely correlated with blood pressure, apolipoprotein B, triglyceride and total cholesterol concentrations. Ala12 carriers showed no such correlations. In the meal study, Ala12 carriers had lower plasma NEFA concentrations, higher adipose tissue and muscle blood flow, and greater insulin-mediated postprandial hormone-sensitive lipase suppression along with greater insulin sensitivity than Pro12 homozygotes.

CONCLUSIONS/INTERPRETATION

This study shows that a recruit-by-genotype approach is feasible and describes the biobank's first application, providing tissue-specific physiological findings consistent with the epidemiological observation that the PPAR Ala12 allele protects against the development of type 2 diabetes.

NOS3 is involved in the increased protein and arginine metabolic response in muscle during early endotoxemia in mice

Sepsis is a severe catabolic condition. The loss of skeletal muscle protein mass is characterized by enhanced release of the amino acids glutamine and arginine, which (in)directly affects interorgan arginine and the related nitric oxide (NO) synthesis. To establish whether changes in muscle amino acid and protein kinetics are regulated by NO synthesized by nitric oxide synthase-2 or -3 (NOS2 or NOS3), we studied C57BL6/J wild-type (WT), NOS2-deficient (NOS2-/-), and NOS3-deficient (NOS3-/-) mice under control (unstimulated) and lipopolysaccharide (LPS)-treated conditions. Muscle amino acid metabolism was studied across the hindquarter by infusing the stable isotopes L-[ring-2H5]phenylalanine, L-[ring-2H2]tyrosine, L-[guanidino-15N2]arginine, and L-[ureido-13C,2H2]citrulline. Muscle blood flow was measured using radioactive p-aminohippuric acid dilution. Under baseline conditions, muscle blood flow was halved in NOS2-/- mice (P < 0.1), with simultaneous reductions in muscle glutamine, glycine, alanine, arginine release and glutamic acid, citrulline, valine, and leucine uptake (P < 0.1). After LPS treatment, (net) muscle protein synthesis increased in WT and NOS2-/- mice [LPS vs. control: 13 +/- 3 vs. 8 +/- 1 (SE) nmol.10 g(-1).min(-1) (WT), 18 +/- 5 vs. 7 +/- 2 nmol.10 g(-1).min(-1) (NOS2-/-); P < 0.05 for LPS vs. control]. This response was absent in NOS3-/- mice (LPS vs. control: 11 +/- 4 vs. 10 +/- 2 nmol.10 g(-1).min(-1)). In agreement, the increase in muscle arginine turnover after LPS was also absent in NOS3-/- mice. In conclusion, disruption of the NOS2 gene compromises muscle glutamine release and muscle blood flow in control mice, but had only minor effects after LPS. NOS3 activity is crucial for the increase in muscle arginine and protein turnover during early endotoxemia.

Major differences in noradrenaline action on lipolysis and blood flow rates in skeletal muscle and adipose tissue in vivo

AIMS/HYPOTHESIS

The regulation of skeletal muscle lipolysis is not fully understood. In the present study, the effects of systemic and local noradrenaline administration on lipolysis and blood flow rates in skeletal muscle and adipose tissue were studied in vivo.

METHODS

First, circulating noradrenaline levels were raised tenfold by a continuous i.v. infusion (n=12). Glycerol levels (an index of lipolysis) were measured in m. gastrocnemius and in abdominal adipose tissue using microdialysis. Local blood flow was determined with the (133)Xe clearance technique and whole-body lipolysis rates assessed with a stable glycerol isotope technique ([(2)H(5)] glycerol). Second, interstitial glycerol levels in m. gastrocnemius, m. vastus and adipose tissue were measured by microdialysis during local perfusion with noradrenaline (10(-8)-10(-6) mol/l) (n=10). Local blood flow was monitored with the ethanol perfusion technique.

RESULTS

With regard to systemic noradrenergic stimulation, no change in fractional release of glycerol (difference between tissue and arterial glycerol) was seen in skeletal muscle. In adipose tissue it transiently increased twofold (p<0.0001), and the rate of appearance of glycerol in plasma showed the same kinetic pattern. Blood flow was reduced by 40% in skeletal muscle (p<0.005) and increased by 50% in adipose tissue (p<0.05). After noradrenaline stimulation in situ, a discrete elevation of skeletal muscle glycerol was registered only at the highest concentration of noradrenaline (10(-6) mol/l) (p<0.05). Adipose tissue glycerol doubled already at the lowest concentration (10(-8) mol/l) (p<0.05). In skeletal muscle a decrease in blood flow was seen at the highest noradrenaline concentrations (p<0.05).

CONCLUSIONS/INTERPRETATION

Lipolysis and blood flow rates are regulated differently in adipose tissue and skeletal muscle. Adipose tissue displays a high, but transient (tachyphylaxia) sensitivity to noradrenaline, leading to stimulation of both lipolysis and blood flow rates. In skeletal muscle, physiological concentrations of noradrenaline decrease blood flow but have no stimulatory effect on lipolysis rates.

Axially symmetric semi-infinite domain models of microdialysis and their application to the determination of nutritive flow in rat muscle

Theoretical models for the description of microdialysis outflow:inflow (O/I) ratio for 3H2O and [14C]ethanol were developed, taking into account the nutritive fraction of total blood flow in muscle. The models yielded an approximately exponential decay expression for the O/I ratio, dependent on the physical dimensions of a linear probe (length and radius), the flow rate through the probe, muscle blood flow (including the nutritive fraction) and the diffusion coefficients for the tracer in the probe and muscle. The models compared favourably with experimental data from the constant-flow perfused rat hindlimb. Estimates of the nutritive fraction of total blood flow from experimental data were determined by minimizing the error between model and experimental data. The nutritive fraction was found to be 0.22 +/- 0.04 under basal perfusion conditions. When 70 nM noradrenaline (norepinephrine) was included in the perfusion medium, the nutritive fraction was 0.91 +/- 0.06 (P < 0.05). The inclusion of 300 nM serotonin, decreased the nutritive fraction to 0.05 +/- 0.01 (P < 0.05). This model can be applied to the determination of nutritive fraction of skeletal muscle blood flow in physiologically relevant microvascular conditions such as during exercise and in disease states.

Tetrahydrobiopterin increases insulin sensitivity in patients with type 2 diabetes and coronary heart disease

Tetrahydrobiopterin (BH(4)) is an essential cofactor of nitric oxide synthase that improves endothelial function in diabetics, smokers, and patients with hypercholesterolemia. Insulin resistance has been suggested as a contributing factor in the development of endothelial dysfunction via an abnormal pteridine metabolism. We hypothesized that BH(4) would restore flow-mediated vasodilation (FMD, endothelial-dependent vasodilation), which may affect insulin resistance in type 2 diabetic patients. Thirty-two subjects (12 type 2 diabetic subjects, 10 matched nondiabetic subjects, and 10 healthy unmatched subjects) underwent infusion of BH(4) or saline in a random crossover study. Insulin sensitivity index (S(I)) was measured by hyperinsulinemic isoglycemic clamp. FMD was measured using ultrasonography. BH(4) significantly increased S(I) in the type 2 diabetics [3.6 +/- 0.6 vs. 4.9 +/- 0.7 x 10(-4) dl.kg(-1).min(-1)/(microU/ml), P < 0.05], while having no effects in nondiabetics [8.9 +/- 1.1 vs. 9.0 +/- 0.9 x 10(-4) dl.kg(-1).min(-1)/(microU/ml), P = 0.92] or in healthy subjects [17.5 +/- 1.6 vs. 18 +/- 1.8 x 10(-4) dl.kg(-1).min(-1)/(microU/ml), P = 0.87]. BH(4) did not affect the relative changes in brachial artery diameter from baseline FMD (%) in type 2 diabetic subjects (2.3 +/- 0.8 vs. 1.8 +/- 1.0%, P = 0.42), nondiabetic subjects (5.3 +/- 1.1 vs. 6.6 +/- 0.9%, P = 0.32), or healthy subjects (11.9 +/- 0.6 vs. 11.0 +/- 1.0%, P = 0.48). In conclusion, BH(4) significantly increases insulin sensitivity in type 2 diabetic patients without any discernible improvement in endothelial function.

Physiological hyperinsulinemia in dogs augments access of macromolecules to insulin-sensitive tissues

Pharmacological doses of insulin increase limb blood flow and enhance tissue recruitment for small solutes such as glucose. We investigated whether elevating insulin within the physiological range (68 +/- 6 vs. 425 +/- 27 pmol/l) can influence tissue recruitment of [(14)C]inulin, an inert diffusionary marker of molecular weight similar to that of insulin itself. During hyperinsulinemic-euglycemic clamps, transport parameters and distribution volumes of [(14)C]inulin were determined in conscious dogs by applying a three-compartment model to the plasma clearance data of intravenously injected [(14)C]inulin (0.8 microCi/kg). In a second set of experiments in anesthetized dogs with direct cannulation of the hindlimb skeletal muscle lymphatics, we measured a possible effect of physiological hyperinsulinemia on the response of the interstitial fluid of skeletal muscle to intravenously injected [(14)C]inulin and compared this response with the model prediction from plasma data. Physiological hyperinsulinemia caused a 48 +/- 10% (P < 0.005) and a 35 +/- 15% (P < 0.05) increase of peripheral and splanchnic interstitial distribution volumes for [(14)C]inulin. Hindlimb lymph measurements directly confirmed the ability of insulin to enhance the access of macromolecules to the peripheral interstitial fluid compartment. The present results show that physiological hyperinsulinemia will enhance the delivery of a substance of similar molecular size to insulin to previously less intensively perfused regions of insulin-sensitive tissues. Our data suggest that the delivery of insulin itself to insulin-sensitive tissues could be a mechanism of insulin action on cellular glucose uptake independent of and possibly synergistic with either enhanced blood flow distribution or GLUT4 transporter recruitment to enhance glucose utilization. Because of the differences between inulin and insulin itself, whether delivery of the bioactive hormone is increased remains speculative.

Insulin-mediated capillary recruitment in skeletal muscle: is this a mediator of insulin action on glucose metabolism?

The possibility that insulin stimulates microvascular access for itself and glucose in muscle in vivo is discussed. The application of new techniques suggests that capillary recruitment is a normal part of insulin's action and that this process becomes impaired in insulin resistance. Exercise, which also leads to capillary recruitment, may involve a different mechanism than that used by insulin.

Nonnutritive flow impairs uptake of fatty acid by white muscles of the perfused rat hindlimb

Triglyceride hydrolysis by the perfused rat hindlimb is enhanced with serotonin-induced nonnutritive flow (NNF) and may be due to the presence of nonnutritive route-associated connective tissue fat cells. Here, we assess whether NNF influences muscle uptake of 0.55 mM palmitate in the perfused hindlimb. Comparisons were made with insulin-mediated glucose uptake. NNF induced during 60 nM insulin infusion inhibited hindlimb oxygen uptake from 22.0 +/- 0.5 to 9.7 +/- 0.8 micromol x g(-1) x h(-1) (P < 0.001), 1-methylxanthine metabolism (indicator of nutritive flow) from 5.8 +/- 0.4 to 3.8 +/- 0.4 nmol x min(-1) x g(-1) (P = 0.004), glucose uptake from 29.2 +/- 1.7 to 23.1 +/- 1.8 micromol x g(-1) x h(-1) (P = 0.005) and muscle 2-deoxyglucose uptake from 82.1 +/- 4.6 to 41.6 +/- 6.7 micromol x g(-1) x h(-1) (P < 0.001). Palmitate uptake, unaffected by insulin alone, was inhibited by NNF in extensor digitorum longus, white gastrocnemius, and tibialis anterior muscles; average inhibition was from 13.9 +/- 1.2 to 6.9 +/- 1.4 micromol x g(-1) x h(-1) (P = 0.02). Thus NNF impairs both fatty acid and glucose uptake by muscle by restricting flow to myocytes but, as shown previously, favors triglyceride hydrolysis and uptake into nearby connective tissue fat cells. The findings have implications for lipid partitioning in limb muscles between myocytes and attendant adipocytes.

Blood flow and muscle metabolism: a focus on insulin action

The vascular system controls the delivery of nutrients and hormones to muscle, and a number of hormones may act to regulate muscle metabolism and contractile performance by modulating blood flow to and within muscle. This review examines evidence that insulin has major hemodynamic effects to influence muscle metabolism. Whole body, isolated hindlimb perfusion studies and experiments with cell cultures suggest that the hemodynamic effects of insulin emanate from the vasculature itself and involve nitric oxide-dependent vasodilation at large and small vessels with the purpose of increasing access for insulin and nutrients to the interstitium and muscle cells. Recently developed techniques for detecting changes in microvascular flow, specifically capillary recruitment in muscle, indicate this to be a key site for early insulin action at physiological levels in rats and humans. In the absence of increases in bulk flow to muscle, insulin may act to switch flow from nonnutritive to the nutritive route. In addition, there is accumulating evidence to suggest that insulin resistance of muscle in vivo in terms of impaired glucose uptake could be partly due to impaired insulin-mediated capillary recruitment. Exercise training improves insulin-mediated capillary recruitment and glucose uptake by muscle.

Nutritive blood flow improves interstitial glucose and lactate exchange in perfused rat hindlimb

Microdialysis was used to assess the interstitial concentrations of glucose and lactate in the constant-flow-perfused rat hindlimb under varying levels of nutritive flow controlled by vasoconstrictors. Increased nutritive flow was achieved by norepinephrine (NE) or angiotensin II (ANG II) and decreased nutritive flow by serotonin (5-HT). NE and ANG II increased oxygen and glucose uptake as well as hindlimb lactate release by 50%. 5-HT decreased oxygen uptake by 15% but had no significant effect on glucose uptake or hindlimb lactate release. Microdialysis recovery of glucose and lactate was significantly elevated by NE and ANG II and decreased by 5-HT. The calculated interstitial concentration of glucose was increased by NE and ANG II but decreased by 5-HT. The interstitial concentration of lactate was decreased by NE and ANG II but increased by 5-HT. In all cases, nitroprusside reversed the effects of the vasoconstrictors. These data indicate that increased nutritive blood flow enhances the exchange of glucose and lactate by improving the supply of glucose to and the removal of lactate from the interstitium.

Direct evidence for insulin-induced capillary recruitment in skin of healthy subjects during physiological hyperinsulinemia

It has been proposed that insulin-mediated changes in muscle perfusion modulate insulin-mediated glucose uptake. However, the putative effects of insulin on the microcirculation that permit such modulation have not been studied in humans. We examined the effects of systemic hyperinsulinemia on skin microvascular function in eight healthy nondiabetic subjects. In addition, the effects of locally administered insulin on skin blood flow were assessed in 10 healthy subjects. During a hyperinsulinemic clamp, we measured leg blood flow with venous occlusion plethysmography, skin capillary density with capillaroscopy, endothelium-(in)dependent vasodilatation of skin microcirculation with iontophoresis of acetylcholine and sodium nitroprusside combined with laser Doppler fluxmetry, and skin vasomotion by Fourier analysis of microcirculatory blood flow. To exclude nonspecific changes in the hemodynamic variables, a time-volume control study was performed. Insulin iontophoresis was used to study the local effects of insulin on skin blood flow. Compared to the control study, systemic hyperinsulinemia caused an increase in leg blood flow (-0.54 +/- 0.93 vs. 1.97 +/- 1.1 ml. min(-1). dl(-1); P < 0.01), an increase in the number of perfused capillaries in the resting state (-3.7 +/- 3.0 vs. 3.4 +/- 1.4 per mm(2); P < 0.001) and during postocclusive reactive hyperemia (-0.8 +/- 2.2 vs. 5.1 +/- 3.7 per mm(2); P < 0.001), an augmentation of the vasodilatation caused by acetylcholine (722 +/- 206 vs. 989 +/- 495%; P < 0.05) and sodium nitroprusside (618 +/- 159 vs. 788 +/- 276%; P < 0.05), and a change in vasomotion by increasing the relative contribution of the 0.01- to 0.02-Hz and 0.4- to 1.6-Hz spectral components (P < 0.05). Compared to the control substance, locally administered insulin caused a rapid increase ( approximately 13.5 min) in skin microcirculatory blood flow (34.4 +/- 42.5 vs. 82.8 +/- 85.7%; P < 0.05). In conclusion, systemic hyperinsulinemia in skin 1) induces recruitment of capillaries, 2) augments nitric oxide-mediated vasodilatation, and 3) influences vasomotion. In addition, locally administered insulin 4) induces a rapid increase in total skin blood flow, independent of systemic effects.

Regional measurement of canine skeletal muscle blood flow by positron emission tomography with H2(15)O

Positron emission tomography (PET) with H2(15)O was used as an in vivo, relatively noninvasive, quantitative method for measuring regional blood flow to hindlimb skeletal muscle of anesthetized dogs. A hydrooccluder positioned on the femoral artery was used to reduce flow, and high-flow states were produced by local infusion of adenosine. Three to four measurements were made in each animal. Approximately 40 mCi of H2(15)O were injected intravenously, and serial images and arterial blood samples were acquired over 2.5 min. Data analysis was performed by fitting tissue and arterial blood time-activity curves to a modified, single-compartment Kety model. The model equation was also solved on a pixel-by-pixel basis to yield maps of regional skeletal muscle blood flow. After each PET determination, flow was measured with radioactive microspheres. Results of the PET measurements demonstrated that basal flow to hindlimb skeletal muscle was 3.83 +/- 0.36 ml x min(-1) x 100 g(-1) (mean +/- SE). This value was in excellent agreement with the microsphere data, 3.73 +/- 0.32 ml x min(-1) x 100 g(-1) (P = 0.69, not significant). Adenosine infusion resulted in flows as high as 30 ml x min(-1) x 100 g(-1), and the PET and microsphere data were highly correlated over the entire range of flows (r2 = 0.98, P < 0.0001). We conclude that muscle blood flow can be accurately measured in vivo by PET with H2(15)O and that this approach offers promise for application in human studies of muscle metabolism under varying pathophysiological states.

Angiotensin II regulates oxygen consumption

Previous studies demonstrated that angiotensin II (ANG II) decreases body weight. This study examined whether ANG II regulates body weight through energy expenditure. Acute ANG II administration decreased oxygen consumption. To determine whether this effect was maintained, rats were infused with ANG II or saline for 14 days. Oxygen consumption was transiently decreased on day 1 of ANG II infusion; however, body weight and food intake were reduced for 14 days. In pair-feeding studies, reductions in food intake accounted for 63% of the effect of ANG II on body weight but did not influence systolic pressure, water intake, or oxygen consumption. With 28 days of ANG II infusion, differences in body weight between ANG II and control rats were of greater magnitude. An initial decrease in oxygen consumption was followed by a rebound increase. Coadministration of losartan prevented the effect of ANG II on body weight, food intake, blood pressure, and water intake. However, losartan only partially prevented ANG II reductions in oxygen consumption. These results demonstrate that ANG II transiently decreases oxygen consumption through mechanisms unrelated to food intake. With chronic ANG II exposure, energy expenditure may contribute to sustained reductions in body weight.

Physiologic hyperinsulinemia enhances human skeletal muscle perfusion by capillary recruitment

Despite intensive study, the relation between insulin's action on blood flow and glucose metabolism remains unclear. Insulin-induced changes in microvascular perfusion, independent from effects on total blood flow, could be an important variable contributing to insulin's metabolic action. We hypothesized that modest, physiologic increments in plasma insulin concentration alter microvascular perfusion in human skeletal muscle and that these changes can be assessed using contrast-enhanced ultrasound (CEU), a validated method for quantifying flow by measurement of microvascular blood volume (MBV) and microvascular flow velocity (MFV). In the first protocol, 10 healthy, fasting adults received insulin (0.05 mU. kg(-1). min(-1)) via a brachial artery for 4 h under euglycemic conditions. At baseline and after insulin infusion, MBV and MFV were measured by CEU during continuous intravenous infusion of albumin microbubbles with intermittent harmonic ultrasound imaging of the forearm deep flexor muscles. In the second protocol, 17 healthy, fasting adults received a 4-h infusion of either insulin (0.1 mU. kg(-1). min(-1), n = 9) or saline (n = 8) via a brachial artery. Microvascular volume was assessed in these subjects by an alternate CEU technique using an intra-arterial bolus injection of albumin microbubbles at baseline and after the 4-h infusion. With both protocols, muscle glucose uptake, plasma insulin concentration, and total blood flow to the forearm were measured at each stage. In protocol 2 subjects, tissue extraction of 1-methylxanthine (1-MX) was measured as an index of perfused capillary volume. Caffeine, which produces 1-MX as a metabolite, was administered to these subjects before the study to raise plasma 1-MX levels. In protocol 1 subjects, insulin increased muscle glucose uptake (180%, P < 0.05) and MBV (54%, P < 0.01) and decreased MFV (-42%, P = 0.07) in the absence of significant changes in total forearm blood flow. In protocol 2 subjects, insulin increased glucose uptake (220%, P < 0.01) and microvascular volume (45%, P < 0.05) with an associated moderate increase in total forearm blood flow (P < 0.05). Using forearm 1-MX extraction, we observed a trend, though not significant, toward increasing capillary volume in the insulin-treated subjects. In conclusion, modest physiologic increments in plasma insulin concentration increased microvascular blood volume, indicating altered microvascular perfusion consistent with a mechanism of capillary recruitment. The increases in microvascular (capillary) volume (despite unchanged total blood flow) indicate that the relation between insulin's vascular and metabolic actions cannot be fully understood using measurements of bulk blood flow alone.

Exercise training improves insulin-mediated capillary recruitment in association with glucose uptake in rat hindlimb

Exercise training is considered to be beneficial in the treatment and prevention of insulin insensitivity, and much of the effect occurs in muscle. We have recently shown that capillary recruitment by insulin in vivo is associated with and may facilitate insulin action to increase muscle glucose uptake. In the present study, we examined the effect of 14 days of voluntary exercise training on euglycemic-hyperinsulinemic clamped (10 mU. min(-1). kg(-1) for 2 h), anesthetized rats. Whole-body glucose infusion rate (GIR), hindleg glucose uptake, femoral blood flow (FBF), vascular resistance, and capillary recruitment, as measured by metabolism of infused 1-methylxanthine (1-MX), were assessed. In sedentary animals, insulin caused a significant (P < 0.05) increase in FBF (1.6-fold) and capillary recruitment (1.7-fold) but a significant decrease in vascular resistance. In addition, hindleg glucose uptake was increased (4.3-fold). Exercise training increased insulin-mediated GIR (24%), hindleg glucose uptake (93%), and capillary recruitment (62%) relative to sedentary animals. Neither capillary density nor total xanthine-oxidase activity in skeletal muscle were increased as a result of the training regimen used. We concluded that exercise training improves insulin-mediated increases in capillary recruitment in combination with augmented muscle glucose uptake. Increased insulin-mediated glucose uptake may in part result from the improved hemodynamic control attributable to exercise training.

Nutritive blood flow affects microdialysis O/I ratio for [(14)C]ethanol and (3)H(2)O in perfused rat hindlimb

Changes in the microdialysis outflow-to-inflow (O/I) ratio for [(14)C]ethanol and (3)H(2)O were determined in the perfused rat hindlimb after increases and decreases in nutritive flow mediated by the vasoconstrictors norepinephrine (NE) and serotonin (5-HT), respectively. Microdialysis probes (containing 10 mM [(14)C]ethanol and (3)H(2)O pumped at 1 or 2 microl/min) were inserted through the calf of the rat. Hindlimb perfusion flow rate was varied from 6 to 56 ml x min(-1) x 100 g(-1) in the presence of NE, 5-HT, or saline vehicle. The O/I ratios for both tracers were determined at each perfusion flow rate, as was perfusion pressure, oxygen uptake (a surrogate indicator of nutritive flow), and lactate release. Both tracers showed a decreased O/I ratio as hindlimb perfusion flow was increased, with [(14)C]ethanol being higher than (3)H(2)O. NE decreased the O/I ratio compared with vehicle, and 5-HT increased it for both tracers and both microdialysis flow rates. We conclude that the microdialysis O/I ratio, while able to detect changes in total flow, is also sensitive to changes in nutritive and nonnutritive flow, where the latter still extracts tracer, but less than the former.

Alterations in energy metabolism during exercise and heat stress

Much of the research that has examined the interaction between metabolism and exercise has been conducted in comfortable ambient conditions. It is clear, however, that environmental temperature, particularly extreme heat, is a major practical issue one must consider when examining muscle energy metabolism. When exercise is conducted in very high ambient temperatures, the gradient for heat dissipation is significantly reduced which results in changes to thermoregulatory mechanisms designed to promote body heat loss. This can ultimately impact upon hormonal and metabolic responses to exercise which act to alter substrate utilisation. In general, the literature examining metabolic responses to exercise and heat stress has demonstrated a shift towards increased carbohydrate use and decreased fat use. Although glucose production appears to be augmented during exercise in the heat, glucose disposal and utilisation appears to be unaltered. In contrast, glycogen use has been consistently demonstrated to be augmented during exercise in the heat. This increase in glycogenolysis is observed via both aerobic and anaerobic pathways. Although several hypotheses have been proposed as mechanisms for the substrate shift towards greater carbohydrate metabolism during exercise and heat stress, recent work suggests that an augmented sympatho-adrenal response and intramuscular temperature may be responsible for such a phenomenon.

Heterogeneity of laser Doppler flowmetry in perfused muscle indicative of nutritive and nonnutritive flow

Laser Doppler flowmetry (LDF) signal responses have been compared with metabolic changes using both a surface macroprobe and randomly placed implantable microprobes in muscles of the constant-flow-perfused rat hindlimb. Changes in response to total flow and to vasoconstrictors that are known to increase (norepinephrine, NE) or decrease (serotonin, 5-HT) hindlimb oxygen uptake were assessed. The surface macroprobe (anterior end of biceps femoris) identified only one type of LDF response characterized by increased signal in response to NE and decreased signal in response to 5-HT. Implanted microprobes (tibialis, gastrocnemius, vastus, or bicep femoris) identified sites that gave three LDF responses of differing character. These responses were where the LDF signal increased with NE and decreased with 5-HT (56.7%), where the LDF signal decreased with NE and increased with 5-HT (16.5%), or where there was no net response to either vasoconstrictor (24.7%). The data are consistent with discrete regions of nutritive and nonnutritive flow in muscle where flow in each as controlled by vasoconstrictors relates directly to the metabolic behavior of the tissue.

Prevention of hypoinsulinemia modifies catecholamine effects in fetal sheep

Increased epinephrine (Epi) and norepinephrine (NE) production plays an important role in fetal adaptation to reduced oxygen and/or nutrient availability, inhibiting insulin secretion and slowing growth to support more essential processes. To assess the importance of hypoinsulinemia for the efficacy of catecholamines, normoinsulinemia was restored by intravenous insulin infusion (0.18 mU. kg(-1). min(-1)) during prolonged infusion of either Epi (0.25-0. 35 microgram. kg(-1). min(-1) for 12 days, n = 7) or NE (0.5-0.7 microgram. kg(-1). min(-1) for 7 days, n = 6) into normoxemic fetuses in twin-pregnant ewes, from 125-127 days of gestation. Insulin infusion for 8 days during Epi infusion or for 4 days during NE infusion decreased arterial blood pressure, O(2) content, and plasma glucose, but increased heart rate significantly (all P <0.05), despite continuation of Epi or NE infusion. Cessation of insulin infusion reversed these changes. Estimated growth of fetuses infused with insulin during Epi or NE infusion (55 +/- 13.9 and 83 +/- 15.2 g/day) did not differ significantly from that of untreated controls (72 +/- 15.4 g/day, n = 6). Growth of selected muscles and hindlimb bones was not altered either. Restoration of normoinsulinemia evidently counteracts the redistribution of metabolic activity and decreased anabolism brought about by Epi or NE in the fetus. Inhibition of insulin secretion by Epi and NE, therefore, appears essential for the efficacy of catecholamine action in the fetus.

Methysergide reduces nonnutritive blood flow in normal and scalded skin

Methysergide is a serotonin antagonist and has been demonstrated to reduce wound blood flow and edema formation. We have determined the effect of methysergide on protein kinetics in normal and scalded skin of anesthetized rabbits. L-[ring-(13)C(6)]- or L-[ring-(2)H(5)]phenylalanine was used to reflect skin protein kinetics by use of an ear model, and L-[1-(13)C]leucine was used to reflect whole body protein kinetics. The results were that infusion of methysergide (2-3 mg. kg(-1). h(-1)) reduced the blood flow rate in normal skin by 50% without changing skin or whole body protein kinetics. After scald injury on the ear, administration of methysergide for 48 h reduced the weight of scalded ears (43 +/- 4 vs. 30 +/- 5 g, P < 0.01) and ear blood flow rate (42.6 +/- 4.9 vs. 5.8 +/- 1.0 ml. 100 g(-1). min(-1), P < 0.0001) and did not change wound protein kinetics. Methysergide reduced arteriovenous shunting and maintained inward phenylalanine transport from the blood to the skin pool. Using the microsphere technique, we found that the infusion of methysergide decreased blood perfusion by 33-36% in both normal and scalded ear skin. We conclude that methysergide administration reduces nonnutritive, as opposed to nutritive, blood flow in normal and scalded skin.

Sodium nitroprusside increases human skeletal muscle blood flow, but does not change flow distribution or glucose uptake

1. The role of blood flow as a determinant of skeletal muscle glucose uptake is at present controversial and results of previous studies are confounded by possible direct effects of vasoactive agents on glucose uptake. Since increase in muscle blood flow can be due to increased flow velocity or recruitment of new capillaries, or both, it would be ideal to determine whether the vasoactive agent affects flow distribution or only increases the mean flow. 2. In the present study blood flow, flow distribution and glucose uptake were measured simultaneously in both legs of 10 healthy men (aged 29 +/- 1 years, body mass index 24 +/- 1 kg m-2) using positron emission tomography (PET) combined with [15O]H2O and [18F]fluoro-2-deoxy-D-glucose (FDG). The role of blood flow in muscle glucose uptake was studied by increasing blood flow in one leg with sodium nitroprusside (SNP) and measuring glucose uptake simultaneously in both legs during euglycaemic hyperinsulinaemia (insulin infusion 6 pmol kg-1 min-1). 3. SNP infusion increased skeletal muscle blood flow by 86 % (P < 0.01), but skeletal muscle flow distribution and insulin-stimulated glucose uptake (61.4 +/- 7. 5 vs. 67.0 +/- 7.5 micromol kg-1 min-1, control vs. SNP infused leg, not significant), as well as flow distribution between different tissues of the femoral region, remained unchanged. The effect of SNP infusion on blood flow and distribution were unchanged during infusion of physiological levels of insulin (duration, 150 min). 4. Despite a significant increase in mean blood flow induced by an intra-arterial infusion of SNP, glucose uptake and flow distribution remained unchanged in resting muscles of healthy subjects. These findings suggest that SNP, an endothelium-independent vasodilator, increases non-nutritive, but not nutritive flow or capillary recruitment.

Effects of noradrenaline and flow on lactate uptake in the perfused rat hindlimb

Skeletal muscle can release or take up lactate depending on the lactate concentration gradient across the cell membrane. In the perfused rat hindlimb without arterial lactate, both noradrenaline (NA) infusion and increased flow promote lactate release and oxygen consumption (VO2). However, it is unclear whether NA or increased flow rate have similar effects on lactate uptake. The present study compares these effects in the rat hindlimb perfused at a basal flow rate of 0.33 mL min-1 g-1 and 25 degrees C in the presence of added arterial lactate. When 10 mmol L-1 L-(+)-lactate was added to the arterial perfusate, lactate was taken up (16 +/- 1.0 mumol g-1 h-1, n = 13) by the hindlimb with a 35% higher VO2 than that without added lactate. Doubling perfusion flow rate enhanced lactate uptake and VO2 by 120% and 40%, respectively. Glucose uptake was also increased (by 253%) with increased flow. Infusion of NA increased perfusion pressure, VO2 and glucose uptake similarly to those induced by increased flow rate. However, lactate uptake was inhibited by NA. This inhibition was not altered by the beta-adrenergic antagonist propranolol. Vasopressin also showed similar effects to NA to decrease lactate uptake associated with increased VO2 and vasoconstriction. These data indicate that in the presence of a high arterial lactate concentration, NA has opposite effects from increased flow rate on skeletal muscle lactate uptake although both have similar effects on lactate release in the absence of arterial lactate. Inhibition of lactate uptake may relate to the vasoconstrictive action of NA.

Intact insulin stimulation of skeletal muscle blood flow, its heterogeneity and redistribution, but not of glucose uptake in non-insulin-dependent diabetes mellitus

We tested the hypothesis that defects in insulin stimulation of skeletal muscle blood flow, flow dispersion, and coupling between flow and glucose uptake contribute to insulin resistance of glucose uptake in non-insulin-dependent diabetes mellitus (NIDDM). We used positron emission tomography combined with [15O]H2O and [18F]-2-deoxy--glucose and a Bayesian iterative reconstruction algorithm to quantitate mean muscle blood flow, flow heterogeneity, and their relationship to glucose uptake under normoglycemic hyperinsulinemic conditions in 10 men with NIDDM (HbA1c 8.1+/-0.5%, age 43+/-2 yr, BMI 27.3+/-0.7 kg/m2) and in 7 matched normal men. In patients with NIDDM, rates of whole body (35+/-3 vs. 44+/-3 micromol/kg body weight.min, P < 0.05) and femoral muscle (71+/-6 vs. 96+/-7 micromol/kg muscle.min, P < 0.02) glucose uptake were significantly decreased. Insulin increased mean muscle blood flow similarly in both groups, from 1.9+/-0.3 to 2.8+/-0.4 ml/100 g muscle.min in the patients with NIDDM, P < 0.01, and from 2.3+/-0.3 to 3.0+/-0.3 ml/100 g muscle.min in the normal subjects, P < 0.02. Pixel-by-pixel analysis of flow images revealed marked spatial heterogeneity of blood flow. In both groups, insulin increased absolute but not relative dispersion of flow, and insulin-stimulated but not basal blood flow colocalized with glucose uptake. These data provide the first evidence for physiological flow heterogeneity in human skeletal muscle, and demonstrate that insulin increases absolute but not relative dispersion of flow. Furthermore, insulin redirects flow to areas where it stimulates glucose uptake. In patients with NIDDM, these novel actions of insulin are intact, implying that muscle insulin resistance can be attributed to impaired cellular glucose uptake.

Measurement of muscle protein synthesis by positron emission tomography with L-[methyl-11C]methionine

Positron emission tomography (PET) with L-[methyl-11C]methionine was explored as an in vivo, noninvasive, quantitative method for measuring the protein synthesis rate (PSR) in paraspinal and hind limb muscles of anesthetized dogs. Approximately 25 mCi (1 Ci = 37 GBq) of L-[methyl-11C]methionine was injected intravenously, and serial images and arterial blood samples were acquired over 90 min. Data analysis was performed by fitting tissue- and metabolite-corrected arterial blood time-activity curves to a three-compartment model and assuming insignificant transamination and transmethylation in this tissue. PSR was calculated from fitted parameter values and plasma methionine concentrations. PSRs measured by PET were compared with arterio-venous (A-V) difference measurements across the hind limb during primed constant infusion (5-6 h) of L-[1-13C, methyl-2H3]methionine. Results of PET measurements demonstrated similar PSRs for paraspinal and hind limb muscles: 0.172 +/- 0.062 vs. 0.208 +/- 0.048 nmol-1.min-1.(g of muscle)-1 (P = not significant). PSR determined by the stable isotope technique was 0.27 +/- 0.050 nmol-1.min-1.(g of leg tissue)-1 (P < 0.07 from PET) and indicated that the contribution of transmethylation to total hind limb methionine utilization was approximately 10%. High levels of L-[methyl-11C]methionine utilization by bone marrow were observed. We conclude that muscle PSR can be measured in vivo by PET and that this approach offers promise for application in human metabolic studies.

NG-monomethyl-L-arginine inhibits the blood flow but not the insulin-like response of forearm muscle to IGF- I: possible role of nitric oxide in muscle protein synthesis

In human skeletal muscle, insulin-like growth factor-I (IGF-I) exerts both growth hormone-like (increase in protein synthesis) and insulin-like (decrease in protein degradation and increase in glucose uptake) actions and augments forearm blood flow two- to threefold. This study was designed to address whether (a) the increase in blood flow due to IGF-I could be blocked by an inhibitor of nitric oxide synthase; and (b) the metabolic actions of IGF-I were altered by use of a nitric oxide synthase inhibitor. Forearm blood flow, glucose, lactate, oxygen, nitrite, and phenylalanine balances and phenylalanine kinetics were studied in a total of 17 healthy, adult volunteers after an overnight fast in two different protocols. In protocol 1, after basal samples IGF-I was infused alone for 4 h with samples repeated during the last 30 min. After the 4-h sample period, NG-monomethyl-L-arginine (L-NMMA) was infused into the brachial artery for 2 h to bring flow back to baseline and repeat samples were taken (6 h). In response to IGF-I alone, forearm blood flow rose from 3.8 +/- 1.0 (bas) to 7.9 +/- l.9 (4 h) ml/min/100 ml (P < 0.01) and was reduced back to baseline by L-NMMA at 6 h (P < 0.01). In protocol 1, IGF-I alone increased forearm nitrite release at 4 h (P < 0.03), which was reduced back to baseline by L-NMMA at 6 h (P < 0.05). Despite the reduction in flow with L-NMMA, IGF+L-NMMA yielded increases in glucose uptake (P < 0.005), lactate release (P < 0.04), oxygen uptake (P < 0.01), and a positive shift in phenylalanine balance (P < 0.01) due to both an increase in muscle protein synthesis (P < 0.02) and a decrease in protein degradation (P < 0.03). In protocol 2, L-NMMA was coinfused with IGF-I for 6 h, with the dose titrated to keep blood flow +/- 25% of baseline. Coinfusion of L-NMMA restrained blood flow to baseline and also yielded the same, significant metabolic effects, except that no significant increase in muscle protein synthesis was detected. These observations suggest: (a) that IGF-I increases blood flow through a nitric oxide-dependent mechanism; (b) that total blood flow does not affect the insulin-like response of muscle to IGF-I; and (c) that nitric oxide may be required for the protein synthetic (growth hormone-like) response of muscle to IGF-I.