Epidemiological studies demonstrate robust correlations between green tea consumption and reduced risk of type 2 diabetes and its cardiovascular complications. However, underlying molecular, ...cellular, and physiological mechanisms remain incompletely understood. Health promoting actions of green tea are often attributed to epigallocatechin gallate (EGCG), the most abundant polyphenol in green tea. Insulin resistance and endothelial dysfunction play key roles in the pathogenesis of type 2 diabetes and its cardiovascular complications. Metabolic insulin resistance results from impaired insulin-mediated glucose disposal in skeletal muscle and adipose tissue, and blunted insulin-mediated suppression of hepatic glucose output that is often associated with endothelial/ vascular dysfunction. This endothelial dysfunction is itself caused, in part, by impaired insulin signaling in vascular endothelium resulting in reduced insulin-stimulated production of NO in arteries, and arterioles that regulate nutritive capillaries. In this review, we discuss the considerable body of literature supporting insulin-mimetic actions of EGCG that oppose endothelial dysfunction and ameliorate metabolic insulin resistance in skeletal muscle and liver. We conclude that EGCG is a promising therapeutic to combat cardiovascular complications associated with the metabolic diseases characterized by reciprocal relationships between insulin resistance and endothelial dysfunction that include obesity, metabolic syndrome and type 2 diabetes. There is a strong rationale for well-powered randomized placebo controlled intervention trials to be carried out in insulin resistant and diabetic populations.
Microvascular Recruitment Is an Early Insulin Effect That Regulates Skeletal Muscle Glucose Uptake In Vivo
Michelle A. Vincent 1 ,
Lucy H. Clerk 1 ,
Jonathan R. Lindner 1 ,
Alexander L. Klibanov 1 ,
...Michael G. Clark 2 ,
Stephen Rattigan 2 and
Eugene J. Barrett 1
1 Divisions of Cardiovascular Medicine and Endocrinology and Metabolism, Department of Internal Medicine, University of Virginia
Health Sciences Center, Charlottesville, Virginia
2 Department of Biochemistry, University of Tasmania, Hobart, Tasmania, Australia
Address correspondence and reprint requests to Eugene J. Barrett, MD, PhD, Department of Internal Medicine, Box 801410, University
of Virginia Health Sciences Center, Charlottesville, VA 22908. E-mail ejb8x{at}virginia.edu
Abstract
Insulin increases glucose disposal into muscle. In addition, in vivo insulin elicits distinct nitric oxide synthase-dependent
vascular responses to increase total skeletal muscle blood flow and to recruit muscle capillaries (by relaxing resistance
and terminal arterioles, respectively). In the current study, we compared the temporal sequence of vascular and metabolic
responses to a 30-min physiological infusion of insulin (3 mU · min −1 · kg −1 , euglycemic clamp) or saline in rat skeletal muscle in vivo. We used contrast-enhanced ultrasound to continuously quantify
microvascular volume. Insulin recruited microvasculature within 5–10 min ( P < 0.05), and this preceded both activation of insulin-signaling pathways and increases in glucose disposal in muscle, as
well as changes in total leg blood flow. Moreover, l -NAME ( N ω -nitro- l -arginine-methyl ester), a specific inhibitor of nitric oxide synthase, blocked this early microvascular recruitment ( P < 0.05) and at least partially inhibited early increases in muscle glucose uptake ( P < 0.05). We conclude that insulin rapidly recruits skeletal muscle capillaries in vivo by a nitric oxide-dependent action,
and the increase in capillary recruitment may contribute to the subsequent glucose uptake.
Footnotes
A G − V G , arterial glucose concentration minus venous glucose concentration; CEU, contrast-enhanced ultrasound; eNOS, endothelium
nitric oxide synthase; l -NAME, N ω - l -nitro-arginine-methyl ester; NOS, nitric oxide synthase; nNOS, neuronal NOS.
Accepted February 26, 2004.
Received October 15, 2003.
DIABETES
Nitric oxide (NO) is an important vasodilator and regulator in the cardiovascular system, and this link was the subject of a Nobel prize in 1998. However, NO also plays many other regulatory roles, ...including thrombosis, immune function, neural activity, and gastrointestinal function. Low concentrations of NO are thought to have important signaling effects. In contrast, high concentrations of NO can interact with reactive oxygen species, causing damage to cells and cellular components. A less-recognized site of NO production is within skeletal muscle, where small increases are thought to have beneficial effects such as regulating glucose uptake and possibly blood flow, but higher levels of production are thought to lead to deleterious effects such as an association with insulin resistance. This review will discuss the role of NO in skeletal muscle during and following exercise, including in mitochondrial biogenesis, muscle efficiency, and blood flow with a particular focus on its potential role in regulating skeletal muscle glucose uptake during exercise.
Key points
Acute glucagon‐like peptide‐1 (GLP‐1) infusion reversed the high fat diet‐induced microvascular insulin resistance that occurred after both 5 days and 8 weeks of a high fat diet ...intervention.
When GLP‐1 was co‐infused with insulin it had overt effects on whole body insulin sensitivity as well as insulin‐mediated skeletal muscle glucose uptake after 5 days of a high fat diet, but not after 8 weeks of high fat diet intervention.
Acute GLP‐1 infusion did not have an additive effect to that of insulin on microvascular recruitment or skeletal muscle glucose uptake in the control group.
Here we demonstrate that GLP‐1 potently increases the microvascular recruitment in rat skeletal muscle but does not increase glucose uptake in the fasting state. Thus, like insulin, GLP‐1 increased the microvascular recruitment but unlike insulin, GLP‐1 had no direct effect on skeletal muscle glucose uptake.
Acute infusion of glucagon‐like peptide‐1 (GLP‐1) has potent effects on blood flow distribution through the microcirculation in healthy humans and rats. A high fat diet induces impairments in insulin‐mediated microvascular recruitment (MVR) and muscle glucose uptake, and here we examined whether this could be reversed by GLP‐1. Using contrast‐enhanced ultrasound, microvascular recruitment was assessed by continuous real‐time imaging of gas‐filled microbubbles in the microcirculation after acute (5 days) and prolonged (8 weeks) high fat diet (HF)‐induced insulin resistance in rats. A euglycaemic hyperinsulinaemic clamp (3 mU min−1 kg−1), with or without a co‐infusion of GLP‐1 (100 pmol l−1), was performed in anaesthetized rats. Consumption of HF attenuated the insulin‐mediated MVR in both 5 day and 8 week HF interventions which was associated with a 50% reduction in insulin‐mediated glucose uptake compared to controls. Acute administration of GLP‐1 restored the normal microvascular response by increasing the MVR after both 5 days and 8 weeks of HF intervention (P < 0.05). This effect of GLP‐1 was associated with a restoration of both whole body insulin sensitivity and increased insulin‐mediated glucose uptake in skeletal muscle by 90% (P < 0.05) after 5 days of HF but not after 8 weeks of HF. The present study demonstrates that GLP‐1 increases MVR in rat skeletal muscle and can reverse early stages of high fat diet‐induced insulin resistance in vivo.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
1 Department of Biochemistry, Medical School,
University of Tasmania, Hobart 7001, Australia; and
2 Health Sciences Center, University of Virginia,
Charlottesville, Virginia 22908
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.
nutrient and hormone access; nutritive and nonnutritive flow; total
muscle blood flow; muscle glucose uptake
Enhanced microvascular perfusion of skeletal muscle is important for nutrient exchange and contributes ∼40% insulin-mediated muscle glucose disposal. High fat-fed (36% fat wt./wt.) rats are a ...commonly used model of insulin-resistance that exhibit impairment of insulin-mediated microvascular recruitment and muscle glucose uptake, which is accompanied by myocyte insulin-resistance. Distinguishing the contribution of impaired microvascular recruitment and impaired insulin action in the myocyte to decreased muscle glucose uptake in these high-fat models is difficult. It is unclear whether microvascular and myocyte insulin-resistance develop simultaneously. To assess this, we used a rat diet model with a moderate increase (two-fold) in dietary fat.
Sprague Dawley rats fed normal (4.8% fat wt./wt., 5FD) or high (9.0% fat wt./wt., 9FD) fat diets for 4 weeks were subject to euglycaemic hyperinsulinemic clamp (10 mU/min/kg insulin or saline) or isolated hindlimb perfusion (1.5 or 15 nM insulin or saline). Body weight, epididymal fat mass, and fasting plasma glucose were unaffected by diet. Fasting plasma insulin and non-esterified fatty acid concentrations were significantly elevated in 9FD. Glucose infusion rate and muscle glucose uptake were significantly impaired during insulin clamps in 9FD. Insulin-stimulated microvascular recruitment was significantly blunted in 9FD. Insulin-mediated muscle glucose uptake between 5FD and 9FD were not different during hindlimb perfusion.
Impaired insulin-mediated muscle glucose uptake in vivo can be the direct result of reduced microvascular blood flow responses to insulin, and can result from small (two-fold) increases in dietary fat. Thus, microvascular insulin-resistance can occur independently to the development of myocyte insulin-resistance.
In addition to increased glucose uptake, insulin action is associated with increased total and microvascular blood flow, and
vasomotion in skeletal muscle. The aim of this study was to determine the ...effect of acute insulin resistance caused by the
peripheral vasoconstrictor α-methylserotonin (αMT) on microvascular vasomotion in muscle. Heart rate (HR), mean arterial pressure
(MAP), femoral blood flow (FBF), whole body glucose infusion (GIR) and hindleg glucose uptake (HGU) were determined during
control and hyperinsulinaemic euglycaemic clamp conditions in anaesthetized rats receiving αMT infusion. Changes in muscle
microvascular perfusion were measured by laser Doppler flowmetry (LDF) and vasomotion was assessed by applying wavelet analysis
to the LDF signal. Insulin increased GIR and HGU. Five frequency bands corresponding to cardiac, respiratory, myogenic, neurogenic
and endothelial activities were detected in the LDF signal. Insulin infusion alone increased FBF (1.18 ± 0.10 to 1.78 ± 0.12
ml min â1 , P < 0.05), LDF signal strength (by 16% compared to baseline) and the relative amplitude of the myogenic component of vasomotion
(0.89 ± 0.09 to 1.18 ± 0.06, P < 0.05). When infused alone αMT decreased LDF signal strength and the myogenic component of vasomotion by 23% and 27% respectively
compared to baseline, but did not affect HGU or FBF. Infusion of αMT during the insulin clamp decreased the stimulatory effects
of insulin on GIR, HGU, FBF and LDF signal and blocked the myogenic component of vasomotion. These data suggest that insulin
action to recruit microvascular flow may in part involve action on the vascular smooth muscle to increase vasomotion in skeletal
muscle to thereby enhance perfusion and glucose uptake. These processes are impaired with this model of αMT-induced acute
insulin resistance.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Insulin Sensitivity of Muscle Capillary Recruitment In Vivo
Lei Zhang 1 ,
Michelle A. Vincent 2 ,
Stephen M. Richards 1 ,
Lucy H. Clerk 2 ,
Stephen Rattigan 1 ,
Michael G. Clark 1 and
Eugene J. ...Barrett 2
1 Department of Biochemistry, University of Tasmania, Hobart, Tasmania, Australia
2 Department of Internal Medicine, University of Virginia, Charlottesville, Virginia
Address correspondence and reprint requests to Eugene J. Barrett MD, PhD, Box 801410, University of Virginia Health Sciences
Center, Charlottesville, VA 22908. E-mail: ejb8x{at}virginia.edu
Abstract
We have reported that insulin exerts two vascular actions in muscle; it both increases blood flow and recruits capillaries.
In parallel hyperinsulinemic-euglycemic clamp studies, we compared the insulin dose response of muscle microvascular recruitment
and femoral blood flow as well as hindleg glucose uptake in fed, hooded Wistar and fasted Sprague-Dawley rats. Using insulin
doses between 0 and 30 mU −1 · min −1 · kg −1 , we measured microvascular recruitment at 2 h by 1-methylxanthine (1-MX) metabolism or contrast-enhanced ultrasound (CEU),
and muscle glucose uptake was measured by either arteriovenous differences or using 2-deoxyglucose. We also examined the time
course for reversal of microvascular recruitment following cessation of a 3 mU · min −1 · kg −1 insulin infusion. In both groups, whether measured by 1-MX metabolism or CEU, microvascular recruitment was fully activated
by physiologic hyperinsulinemia and occurred at lower insulin concentrations than those that stimulated glucose uptake or
hindleg total blood flow. The latter processes were insulin dose dependent throughout the entire dose range studied. Upon
stopping the insulin infusion, increases in microvascular volume persisted for 15–30 min after insulin concentrations returned
to basal levels. We conclude that the precapillary arterioles that regulate microvascular recruitment are more insulin sensitive
than resistance arterioles that regulate total flow.
1-MX, 1-methylanthine
2DG, 2-deoxy-d-2,6-3Hglucose
CEU, contrast-enhanced ultrasound
Footnotes
Accepted November 6, 2003.
Received July 29, 2003.
DIABETES
Lipid Infusion Impairs Physiologic Insulin-Mediated Capillary Recruitment and Muscle Glucose Uptake In Vivo
Lucy H. Clerk ,
Stephen Rattigan and
Michael G. Clark
From the Department of Biochemistry, ...School of Medicine, University of Tasmania, Hobart, Tasmania, Australia
Abstract
Infusion of triglycerides and heparin causes insulin resistance in muscle. Because the vascular actions of insulin, particularly
capillary recruitment, may contribute to the increase in glucose uptake by skeletal muscle, we investigated the effects of
Intralipid/heparin infusion on the hemodynamic actions of insulin during clamp conditions. Saline or 10% Intralipid/heparin
(33 U/ml) was infused into anesthetized rats at 20 μl/min for 6 h. At 4 h into the saline infusion, a 2-h hyperinsulinemic
(3 mU · min −1 · kg −1 )-euglycemic clamp was conducted (Ins group). At 4 h into the lipid infusion, a 2-h saline control (Lip group) or 2-h hyperinsulinemic-euglycemic
clamp (Lip + Ins group) was conducted. Arterial blood pressure, heart rate, femoral blood flow (FBF), hindleg vascular resistance,
glucose infusion rate (GIR), hindleg glucose uptake (HGU), and muscle 2-deoxyglucose uptake (R′g) were measured. Capillary
recruitment, as measured by metabolism of infused 1-methylxanthine (1-MX), was also assessed. When compared with either Lip
or Lip + Ins, Ins had no effect on arterial blood pressure, heart rate, FBF, or vascular resistance but increased GIR, HGU,
and R′g of soleus, plantaris, extensor digitorum longus, and gastrocnemius red muscles and hindlimb 1-MX metabolism. GIR,
HGU, and R′g of soleus, plantaris, gastrocnemius red, and the combined muscles and 1-MX metabolism were less in Lip + Ins
than in Ins rats. HGU correlated closely with hindleg capillary recruitment ( r = 0.86, P < 0.001) but not total hindleg blood flow. In conclusion, acute elevation of plasma free fatty acids blocks insulin-mediated
glucose uptake and capillary recruitment.
Footnotes
Address correspondence and reprint requests to Michael G. Clark, Biochemistry, School of Medicine, University of Tasmania,
GPO Box 252-58, Hobart, Tasmania, Australia, 7001. E-mail: michael.clark{at}utas.edu.au .
Received for publication 15 August 2001 and accepted in revised form 17 December 2001.
1-MX, 1-methylxanthine; 2DG, 2-deoxyglucose; eNOS, endothelial nitric oxide synthase; FBF, femoral arterial blood flow; HGU,
hindleg glucose uptake; IMGU, insulin-mediated glucose uptake; PKCθ, protein kinase Cθ; R′g, muscle 2DG uptake; TNF, tumor
necrosis factor; XO, xanthine oxidase.
DIABETES
Local Nitric Oxide Synthase Inhibition Reduces Skeletal Muscle Glucose Uptake but Not Capillary Blood Flow During In Situ
Muscle Contraction in Rats
Renee M. Ross 1 ,
Glenn D. Wadley 2 ,
Michael G. ...Clark 1 ,
Stephen Rattigan 1 and
Glenn K. McConell 2
1 Menzies Research Institute, University of Tasmania, Hobart, Tasmania, Australia
2 Department of Physiology, University of Melbourne, Parkville, Victoria, Australia
Address correspondence and reprint requests to Dr. Glenn McConell, Department of Physiology, University of Melbourne, Parkville,
Victoria, 3010, Australia. E-mail: mcconell{at}unimelb.edu.au
Abstract
OBJECTIVE— We have previously shown in humans that local infusion of a nitric oxide synthase (NOS) inhibitor into the femoral artery
attenuates the increase in leg glucose uptake during exercise without influencing total leg blood flow. However, rodent studies
examining the effect of NOS inhibition on contraction-stimulated skeletal muscle glucose uptake have yielded contradictory
results. This study examined the effect of local infusion of an NOS inhibitor on skeletal muscle glucose uptake (2-deoxyglucose)
and capillary blood flow (contrast-enhanced ultrasound) during in situ contractions in rats.
RESEARCH DESIGN AND METHODS— Male hooded Wistar rats were anesthetized and one hindleg electrically stimulated to contract (2 Hz, 0.1 ms) for 30 min while
the other leg rested. After 10 min, the NOS inhibitor N G -nitro- l -arginine methyl ester ( l -NAME) (arterial concentration of 5 μmol/l) or saline was infused into the epigastric artery of the contracting leg.
RESULTS— Local NOS inhibition had no effect on blood pressure, heart rate, or muscle contraction force. Contractions increased ( P < 0.05) skeletal muscle NOS activity, and this was prevented by l -NAME infusion. NOS inhibition caused a modest significant ( P < 0.05) attenuation of the increase in femoral blood flow during contractions, but importantly there was no effect on capillary
recruitment. NOS inhibition attenuated ( P < 0.05) the increase in contraction-stimulated skeletal muscle glucose uptake by ∼35%, without affecting AMP-activated protein
kinase (AMPK) activation.
CONCLUSIONS— NOS inhibition attenuated increases in skeletal muscle glucose uptake during contraction without influencing capillary recruitment,
suggesting that NO is critical for part of the normal increase in skeletal muscle fiber glucose uptake during contraction.
14C2-DG, 2-deoxy-d-1-14Cglucose
AMPK, AMP-activated protein kinase
CEU, contrast-enhanced ultrasound
eNOS, endothelial nitric oxide synthase
FBF, femoral blood flow
l-NAME, NG-nitro-l-arginine methyl ester
l-NMMA, NG-monomethyl-l-arginine
nNOS, neuronal nitric oxide synthase
NOS, nitric oxide synthase
NOx, nitrate and nitrite
Footnotes
Published ahead of print at http://diabetes.diabetesjournals.org on 19 September 2007. DOI: 10.2337/db07-0745.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore
be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Accepted September 15, 2007.
Received June 1, 2007.
DIABETES
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