Insulin-mediated Capillary Recruitment Research Articles

Insulin-Mediated Capillary Recruitment Is Absent in the Cutaneous Microvasculature of Women with a History of Gestational Diabetes Mellitus

Women with a history of gestational diabetes mellitus (GDM) have a significantly greater risk of developing cardiovascular disease and type II diabetes compared to women who had an uncomplicated pregnancy (HC). Although the mechanisms underlying this increased risk are unclear, emerging evidence suggests that microvascular endothelial dysfunction, mediated via reduced nitric oxide (NO)-dependent dilation, persists after pregnancy complicated by GDM. Whether this microvascular dysfunction reduces insulin-mediated responses in these women is unexplored. We hypothesized that insulin-mediated capillary recruitment would be attenuated in women with a history of GDM via NO-dependent mechanisms. 24 women (n=12/group; HC: 35±1 years, 24±4 months post-partum; GDM: 33±1 years, 21±3 months postpartum) underwent intradermal microdialysis, which involved the placement of 2 fibers in the ventral forearm for the local delivery of 10−4M insulin alone or 10−4M insulin+15mM NG-nitro-L-arginine methyl ester (L-NAME, NO-synthase inhibitor). The cutaneous blood flow response to post-occlusive reactive hyperemia (PORH; 5 minutes) was assessed before and during insulin perfusion. Red blood cell flux was measured continuously over each microdialysis site by laser-Doppler flowmetry. Cutaneous vascular conductance was calculated (CVC=flux/mean arterial pressure), normalized to maximum (%CVCmax), and presented as PORH area under the curve (AUC, %CVCmax• sec−1). Microvascular capillary recruitment was assessed as ΔAUC (pre-insulin AUC - insulin AUC). Insulin perfusion increased PORH in HC (pre-insulin: 4066±490 vs. insulin: 7848±978 AUC; p=0.03) but had no effect in GDM (pre-insulin: 3765±293 vs. insulin: 6196±779 AUC; p=0.26). Insulin perfusion did not increase PORH at the L-NAME site in either group (p>0.05). Within groups, L-NAME perfusion reduced insulin-mediated capillary recruitment in HC (control: 3781±1109 vs. L-NAME: 470±945 ΔAUC; p=0.04) but not GDM (control: 2431±796 vs. L-NAME: -41±1014 ΔAUC, p=0.11). These data suggest that insulin-mediated capillary recruitment assessed by PORH in the cutaneous circulation is mediated by NO-dependent mechanisms in healthy women with a history of uncomplicated pregnancy and attenuated in healthy women with a history of GDM. This reduction in insulin-mediated microvascular responses may contribute to the increased risk of cardiovascular disease and type II diabetes in these women. UI Fraternal Order of Eagles Diabetes Research Center Catalyst Grant; NIH HL169201. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

GLP-1 and insulin regulation of skeletal and cardiac muscle microvascular perfusion in type 2 diabetes.

Muscle microvasculature critically regulates skeletal and cardiac muscle health and function. It provides endothelial surface area for substrate exchange between the plasma compartment and the muscle interstitium. Insulin fine-tunes muscle microvascular perfusion to regulate its own action in muscle and oxygen and nutrient supplies to muscle. Specifically, insulin increases muscle microvascular perfusion, which results in increased delivery of insulin to the capillaries that bathe the muscle cells and then facilitate its own transendothelial transport to reach the muscle interstitium. In type 2 diabetes, muscle microvascular responses to insulin are blunted and there is capillary rarefaction. Both loss of capillary density and decreased insulin-mediated capillary recruitment contribute to a decreased endothelial surface area available for substrate exchange. Vasculature expresses abundant glucagon-like peptide 1 (GLP-1) receptors. GLP-1, in addition to its well-characterized glycemic actions, improves endothelial function, increases muscle microvascular perfusion, and stimulates angiogenesis. Importantly, these actions are preserved in the insulin resistant states. Thus, treatment of insulin resistant patients with GLP-1 receptor agonists may improve skeletal and cardiac muscle microvascular perfusion and increase muscle capillarization, leading to improved delivery of oxygen, nutrients, and hormones such as insulin to the myocytes. These actions of GLP-1 impact skeletal and cardiac muscle function and systems biology such as functional exercise capacity. Preclinical studies and clinical trials involving the use of GLP-1 receptor agonists have shown salutary cardiovascular effects and improved cardiovascular outcomes in type 2 diabetes mellitus. Future studies should further examine the different roles of GLP-1 in cardiac as well as skeletal muscle function.

3D analysis of capillary network in skeletal muscle of obese insulin-resistant mice.

In obesity, the skeletal muscle capillary network regresses and the insulin-mediated capillary recruitment is impaired. However, it has been shown that in the early stage of advanced obesity, an increased functional vascular response can partially compensate for other mechanisms of insulin resistance. The present study aimed to investigate the changes in the capillary network around individual muscle fibres during the early stage of obesity and insulin resistance in mice using 3D analysis. Capillaries and muscle fibres of the gluteus maximus muscles of seven high-fat-diet-induced obese and insulin-resistant mice and seven age-matched lean healthy mice were immunofluorescently labelled in thick transverse muscle sections. Stacks of images were acquired using confocal microscope. Capillary network characteristics were estimated by methods of quantitative image analysis. Muscle fibre typing was performed by histochemical analysis of myosin heavy chain isoforms on thin serial sections of skeletal muscle. Capillary length per muscle fibre length and capillary length per muscle fibre surface were increased by 27% and 23%, respectively, around small muscle fibres in obese mice, while there were no significant comparative differences around large fibres of obese and lean mice. Furthermore, the capillarization was larger around small compared to large fibres and there was a shift toward fast type myosin heavy chain isoforms, with no significant changes in muscle fibre diameters, tortuosity and anisotropy in obese mice. Overall, the results show that obese insulin-resistant mice have selective increase in capillarization around small predominantly intermediate muscle fibres, which is most likely related to the impaired glucose metabolism characteristic of type 2 diabetes.

477-P: Inhibiting Myeloperoxidase (MPO) Prevents Onset and Reverses Established High-Fat Diet (HFD)-Induced Microvascular Insulin Resistance (IR)

HFD rapidly recruits neutrophils to insulin target tissues and induces microvascular IR. MPO is abundant in neutrophils, inhibits nitric oxide mediated vaso-relaxation, and associates with increased cardiovascular risk. AZD5904 (AstraZeneca) irreversibly inhibits MPO and is in clinical trials. MPO knockout, or chemical inhibition, improves glucose regulation in HFD-fed mice. Whether MPO affects muscle microvascular or metabolic IR in vivo is unknown. To test whether AZD5904 can prevent onset of HFD-induced vascular and metabolic IR, Sprague-Dawley rats were fed chow or a HFD (60% calories) for 2 weeks. HFD-fed rats received vehicle (n=8) or AZD5904 subcutaneously (n=10) via Alzet pump. Chow-fed rats (n=8) received vehicle alone. On day 15, we measured muscle microvascular perfusion in all rats by contrast-enhanced ultrasound, at 0, 60 and 120 minutes of a 2-hour euglycemic insulin clamp (3 mU/kg/minutes) and glucose disposal was determined. We next tested whether AZD5904 can reverse established vascular IR. Two groups of rats (n = 10, each) were fed the HFD for 2 weeks to induce vascular and metabolic IR, HFD was continued for 2 additional weeks while giving vehicle or AZD5904. Microvascular and metabolic insulin action was measured again using the insulin clamp. HFD for 2 weeks completely blocked insulin-mediated muscle capillary recruitment, decreased glucose utilization by 35%, and inhibited muscle Akt phosphorylation (P<0.05, for each measure). AZD5904 during HFD selectively and completely restored insulin-induced microvascular sensitivity (P<0.05). Furthermore, AZD5904 during the last 2 of 4 weeks of HFD feeding fully restored insulin-mediated microvascular perfusion (P<0.05) again without affecting glucose disposal. We conclude that inhibiting MPO selectively corrects vascular IR. This effect may connote a therapeutic potential for MPO inhibition in the prevention of vascular disease/dysfunction seen in IR humans. Disclosure E. Barrett: Research Support; Self; AstraZeneca. W. Chai: None. K.W. Aylor: None. L. Gan: Employee; Self; AstraZeneca. E. Michaëlsson: Employee; Self; AstraZeneca. Funding University of Virginia; AstraZeneca

1933-P: Acute Hyperglycemia + Hyperinsulinemia Increases Carotid-Femoral Arterial Stiffness and Perfusion of Skeletal Muscle in Healthy Subjects

Introduction: Chronic hyperglycemia injures both micro-and microvasculature. Conversely, insulin stimulates vascular nitric oxide production and acts as a vasodilator. Whether acute hyperglycemia (AH) inhibits insulin’s vasodilator action in healthy vasculature is unknown. Here we tested whether AH interfered with insulin’s action to recruit microvasculature, enhance endothelial function, or diminish vascular stiffness. Methods: Nine healthy, fasting subjects (ages 18-35 years; BMI 18-25 kg/m2) twice received a 1 mU/kg/minute 2-hour insulin clamp. On one occasion, subjects were euglycemic (EH); and on the second, were hyperglycemic (∼200 mg/dL) (HH) for 2 hours before and throughout the clamp. Octreotide infusion blocked endogenous insulin in both studies. We assessed endothelial function by flow-mediated dilation (FMD) and arterial stiffness with pulse wave velocity (PWV) and augmentation index (AI). We used contrast-enhanced ultrasound to measure insulin-mediated capillary recruitment in skeletal and cardiac muscle. Linear mixed model was used for statistical analysis, with Bonferroni correction for comparison between studies. Results: Insulin increased carotid-femoral PWV (p=0.02 for Δ PWV) but decreased AI (p= 0.02) during HH, with AI significantly reduced in HH compared to EH (p=0.04). Insulin recruited microvasculature (p<0.05) with either EH or HH, and microvascular blood volume (p=0.03) and blood flow (p=0.04) significantly increased in skeletal muscle after HH. No changes were observed in FMD or cardiac microvascular perfusion with either condition. Discussion: This study provides evidence that 4 hours of AH evokes insulin-induced increases of carotid-femoral arterial stiffness in healthy subjects. Insulin’s ability to recruit microvasculature is preserved during AH. Disclosure W.B. Horton: None. L. Jahn: None. L. Hartline: None. J.T. Patrie: None. E. Barrett: Research Support; Self; AstraZeneca. Funding National Institutes of Health (F32HL142304-01)

Interaction of Endothelial Cell Mineralocorticoid Receptor, Noncoding RNA, and Exosomes in Diet-Induced Vascular Stiffness and Insulin Resistance

Insulin-mediated capillary recruitment is important for increasing the vascular surface area for both insulin and glucose delivery and subsequent muscle glucose-uptake. Our recent data suggest Western diet (WD) induces vascular stiffness and impairs insulin-mediated capillary recruitment by activation of cell-specific endothelial cell (EC) mineralocorticoid receptor (MR). Here, we further hypothesized interaction of ECMR, noncoding RNA, and exosome contributes to WD-induced impairment of vascular stiffness and insulin sensitivity. Six week-old ECMR KO and littermate wild type control male mice were fed a mouse chow or WD containing excess fat (46%) and fructose (17.5%) for 16 weeks. Insulin sensitivity and vascular activity were determined by intraperitoneal glucose tolerance test and wire myograph system. ECMR KO mice prevented WD-impaired systemic insulin sensitivity and improved the aortic vasodilation responses to acetylcholine (10-9-10-4 mol/L) and insulin (0.1- 300 ng/ml), which were impaired by WD. The impairment of insulin sensitivity and vascular function was associated with abnormal expression of noncoding RNA including microRNA (miR)-7, -23, -34, -92, -429, -449, -466 as well as long noncoding RNA-kis2, -Mirt1, -Nron, -Cox2 examined by RNA sequencing, which are associated endothelium inflammation and vascular remodeling. In vitro cultured ECs, aldosterone (10-7 mol) also increased EC exosome release and miR-103 expression. Furthermore, application of a miR103 inhibitor to ECs in vitro attenuated aldosterone-induced downregulation of serine (Ser) 1177/activation of endothelial nitric oxide synthase and upregulation of intercellular adhesion molecule 1. These findings suggest that increased ECMR signaling and associated abnormal expression and release of noncoding RNA and exosome play a key role in obesity-induced vascular stiffness and insulin resistance. Disclosure G. Jia: None. A.R. Aroor: None. J. Habibi: None. A. Whaley-Connell: None. J. Sowers: None.

Sodium nitrate co-ingestion with protein does not augment postprandial muscle protein synthesis rates in older, type 2 diabetes patients.

The age-related anabolic resistance to protein ingestion is suggested to be associated with impairments in insulin-mediated capillary recruitment and postprandial muscle tissue perfusion. The present study investigated whether dietary nitrate co-ingestion with protein improves muscle protein synthesis in older, type 2 diabetes patients. Twenty-four men with type 2 diabetes (72 ± 1 yr, 26.7 ± 1.4 m/kg(2) body mass index, 7.3 ± 0.4% HbA1C) received a primed continuous infusion of l-[ring-(2)H5]phenylalanine and l-[1-(13)C]leucine and ingested 20 g of intrinsically l-[1-(13)C]phenylalanine- and l-[1-(13)C]leucine-labeled protein with (PRONO3) or without (PRO) sodium nitrate (0.15 mmol/kg). Blood and muscle samples were collected to assess protein digestion and absorption kinetics and postprandial muscle protein synthesis rates. Upon protein ingestion, exogenous phenylalanine appearance rates increased in both groups (P < 0.001), resulting in 55 ± 2% and 53 ± 2% of dietary protein-derived amino acids becoming available in the circulation over the 5h postprandial period in the PRO and PRONO3 groups, respectively. Postprandial myofibrillar protein synthesis rates based on l-[ring-(2)H5]phenylalanine did not differ between groups (0.025 ± 0.004 and 0.021 ± 0.007%/h over 0-2 h and 0.032 ± 0.004 and 0.030 ± 0.003%/h over 2-5 h in PRO and PRONO3, respectively, P = 0.7). No differences in incorporation of dietary protein-derived l-[1-(13)C]phenylalanine into de novo myofibrillar protein were observed at 5 h (0.016 ± 0.002 and 0.014 ± 0.002 mole percent excess in PRO and PRONO3, respectively, P = 0.8). Dietary nitrate co-ingestion with protein does not modulate protein digestion and absorption kinetics, nor does it further increase postprandial muscle protein synthesis rates or the incorporation of dietary protein-derived amino acids into de novo myofibrillar protein in older, type 2 diabetes patients.

Liraglutide prevents microvascular insulin resistance and preserves muscle capillary density in high-fat diet-fed rats.

Muscle microvasculature critically regulates endothelial exchange surface area to facilitate transendothelial delivery of insulin, nutrients, and oxygen to myocytes. Insulin resistance blunts insulin-mediated microvascular recruitment and decreases muscle capillary density; both contribute to lower microvascular blood volume. Glucagon-like peptide 1 (GLP-1) and its analogs are able to dilate blood vessels and stimulate endothelial cell proliferation. In this study, we aim to determine the effects of sustained stimulation of the GLP-1 receptors on insulin-mediated capillary recruitment and metabolic insulin responses, small arterial endothelial function, and muscle capillary density. Rats were fed a high-fat diet (HFD) for 4 wk with or without simultaneous administration of liraglutide and subjected to a euglycemic hyperinsulinemic clamp for 120 min after an overnight fast. Insulin-mediated muscle microvascular recruitment and muscle oxygenation were determined before and during insulin infusion. Muscle capillary density was determined and distal saphenous artery used for determination of endothelial function and insulin-mediated vasodilation. HFD induced muscle microvascular insulin resistance and small arterial vessel endothelial dysfunction and decreased muscle capillary density. Simultaneous treatment of HFD-fed rats with liraglutide prevented all of these changes and improved insulin-stimulated glucose disposal. These were associated with a significantly increased AMPK phosphorylation and the expressions of VEGF and its receptors. We conclude that GLP-1 receptor agonists may exert their salutary glycemic effect via improving microvascular insulin sensitivity and muscle capillary density during the development of insulin resistance, and early use of GLP-1 receptor agonists may attenuate metabolic insulin resistance as well as prevent cardiovascular complications of diabetes.

Endothelium-Derived Hyperpolarizing Factors: A Potential Therapeutic Target for Vascular Dysfunction in Obesity and Insulin Resistance

The endothelium consists of a single layer of cells that serves as a barrier between blood and tissues and actively participates in the regulation of vascular tone and function (1). The influence of the endothelium on blood flow in arterioles and capillaries is modulated by the synthesis and release of a number of endothelial-derived relaxing and constricting substances such as nitric oxide (NO), prostacyclin (PGI2), and endothelium-derived hyperpolarizing factors (EDHF) (2). However, the balance or homeostasis between endothelial relaxing and constricting factors is disrupted in insulin-resistant states including obesity, type 2 diabetes mellitus (T2DM), and hypertension, all of which promote cardiovascular disease (CVD) (1,2). For example, endothelial cell (EC) dysfunction is caused by impairment of endothelial-dependent vasodilation in various vascular beds such as skeletal muscle arterioles and capillaries. Both conduit arteries and microvessels are very important for insulin and glucose metabolism as an increase in capillary surface area prompts both insulin and glucose delivery and subsequent glucose uptake in skeletal muscle tissue (3,4). In healthy individuals, systemic and local infusion of insulin induces NO production to prompt insulin-mediated capillary recruitment and microvascular blood flow, resulting in an increase in forearm blood volume (2). One study demonstrated that endothelial NO synthase inhibition significantly blunted the insulin-stimulated increase of ultrasound-assessed femoral blood flow without any changes in blood pressure and heart rate in rats (5). In parallel, insulin-stimulated capillary recruitment and glucose uptake were completely abolished (5), suggesting that local capillary NO bioavailability plays a key role in the regulation of insulin-stimulated skeletal muscle capillary recruitment and glucose uptake. The …

Hemodynamic actions of insulin: beyond the endothelium

BRIEF HISTORICAL OVERVIEW Insulin’s hemodynamic actions are being progressively unraveled over the last two decades (Baron, 1994; Clark, 2008). Originally, Baron et al (Laakso et al., 1990) reported insulin-mediated increase in total blood flow, suggesting an associated higher delivery of nutrients and insulin to target tissues. Nonetheless, the physiological meaning of that extensive hemodynamic action of insulin is controversial, given that lower insulin concentrations than those required to increase total blood flow, stimulate muscle glucose uptake (Yki-Jarvinen and Utriainen, 1998; Zhang et al., 2004). Afterward, a distinct vascular action of insulin was observed, which resulted in capillary recruitment (Rattigan et al., 1997). Such capillary recruitment occurs at physiological concentrations of insulin, precedes increases in total blood flow and is directly correlated to glucose uptake in skeletal muscle (Rattigan et al., 1997, 1999; Coggins et al., 2001). Provided that insulin transcapillary transport plausibly involves a predominant non-saturable process in skeletal muscle (Steil et al., 1996; Hamilton-Wessler et al., 2002; Majumdar et al., 2012), insulin-mediated capillary recruitment emerges as a crucial action of insulin, coupling vascular and metabolic physiology. An integrative pathophysiological view of the relationships between hemodynamic and metabolic functions of insulin has been presented in detail in previous reviews (Muniyappa et al., 2007; Clark, 2008; Muris et al., 2013). Likewise, it has been recognized a similar vascular and metabolic insulin-signaling via insulin receptor/phosphatidylinositol 3kinase (PI3K) pathways (Kim et al., 2006; Kubota et al., 2011), which are present in both endothelium and vascular smooth muscle (VSM), leading to nitric oxide synthase (NOS) activation, increased nitric oxide (NO) production and subsequent vasodilation (Zeng and Quon, 1996; Anfossi et al., 2002). Most previous research in this field has focused on insulin actions in and/or through the endothelium, assumed as the preponderant sensor/effector organ underlying the vascular effects of insulin (Muniyappa et al., 2007; Kubota et al., 2011). However, recently increased animal and human evidence suggests a leading hemodynamic role for the direct insulin action in VSM (Rossi et al., 2005; Newman et al., 2009; Montero et al., 2013a,c), thus requiring to reconsider widely held conceptions in this field.

Diminished Insulin-Mediated Forearm Blood Flow and Muscle Glucose Uptake in Young Men with Low Birth Weight

Background: Low birth weight (LBW) is associated with increased risk of type 2 diabetes and cardiovascular disease. We studied endothelial function and insulin sensitivity in young men with LBW (n = 22) and controls (n = 22). Methods: Insulin sensitivity and endothelial function was studied with venous occlusion plethysmography and intra-arterial infusions of adenosine and acetylcholine, before and during a hyperinsulinemic isoglycemic clamp. Results: Forearm blood flow response to systemic hyperinsulinemia was diminished in LBW compared to controls (p < 0.05). Fractional arteriovenous glucose extraction was similar, and consequently insulin-stimulated forearm glucose clearance was diminished in LBW compared with controls (0.8 ± 0.09 vs. 1.4 ± 0.36 ml·100 ml^–1·min^–1, respectively, p < 0.05). Forearm blood flow response to adenosine and acetylcholine with or without insulin stimulation did not differ between groups. Whole-body glucose uptake was lower in LBW than controls (8.7 ± 0.5 and 9.1 ± 0.6 mg·min^–1·kg^–1 lean body mass); however, this was not significant. Conclusions: Forearm blood flow response to insulin is impaired in LBW, whereas the response to adenosine and acetylcholine is preserved. The impaired insulin-mediated increase in bulk flow in LBW may be due to an impairment of insulin-mediated capillary recruitment independent of – or preceding – whole-body insulin resistance in LBW subjects.

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.

Effects of Intrabrachial Metacholine Infusion on Muscle Capillary Recruitment and Forearm Glucose Uptake during Physiological Hyperinsulinemia in Obese, Insulin-Resistant Individuals

Impairment of insulin-mediated capillary recruitment in skeletal muscle contributes to a hampered glucose uptake in obesity. The objective of this study was to evaluate whether metacholine (MCh), a nitric oxide vasodilator, potentiates muscle capillary recruitment and forearm glucose uptake (FGU) during physiological hyperinsulinemia. The double-forearm technique [i.e. infused vs. control (Ctrl) forearm] was combined with im microdialysis during an oral glucose tolerance test in 15 nondiabetic, obese subjects divided into a group of insulin-resistant (IR) (n = 7) and insulin-sensitive (n = 8) individuals. After the oral glucose tolerance test, forearm blood flow in the Ctrl forearm was unchanged, whereas it increased about 3-fold (P < 0.0001 vs. baseline) in response to MCh. Capillary permeability surface area product for glucose (PS(glu)) (capillary recruitment), FGU, and interstitial insulin concentrations increased significantly over time (P < 0.001) in both forearms. Compared with insulin-sensitive, the IR subjects exhibited lower PS(glu) (P < 0.001) and FGU (P < 0.01) in the Ctrl arm, whereas this difference was insignificant in the MCh arm despite the blunted forearm blood flow increase. Moreover, in IR individuals MCh significantly (P < 0.05) ameliorated the delayed onset of insulin action, i.e. the FGU response to hyperinsulinemia. Finally, we found PS(glu) to be a strong and independent predictor of FGU response (adjusted R(2) 0.72; P < 0.0001). MCh-induced vasodilation may improve the microvascular and metabolic responses to physiological hyperinsulinemia in obese, IR individuals. Further studies are required to unravel whether stimulation of nitric oxide production in skeletal muscle may represent an attractive therapeutic approach to bypassing cellular resistance to glucose disposal.

Counterpoint: There is not capillary recruitment in active skeletal muscle during exercise

The notion that a substantial proportion of capillaries do not contain moving red blood cells (RBCs) in muscle at rest but are “recruited,” i.e., begin flowing with RBCs during contractions, is one basis for our present understanding of blood-muscle exchange during exercise ([20][1], [28][2]).

Rebuttal from Drs. Clark, Rattigan, Barrett, and Vincent

POINT-COUNTERPOINTRebuttal from Drs. Clark, Rattigan, Barrett, and VincentPublished Online:01 Mar 2008https://doi.org/10.1152/japplphysiol.00779.2007bMoreSectionsPDF (39 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat We congratulate David Poole and his associates for a clear and thoughtful argument (6). The accompanying intravital microscopy video of the spinotrapezius muscle is equally instructive where over 80% of the capillaries are supporting RBC flow at rest.However, the data are only as good as the techniques used to acquire them, and the authors themselves (6) acknowledge some of the problems associated with exposing a muscle such as the spinotrapezius for viewing under a microscope. These problems, which could alter the extent of capillary recruitment, include the effect of manipulations required to expose the muscle; the question of whether the thin muscles that can be exposed are representative of the cylindrical load-bearing muscles; the effects of anesthesia; and the superfusion Po2. Since the weight of argument against exercise-mediated capillary recruitment is based on studies using intravital microscopy it is essential that the findings from this system can be extrapolated to in vivo. Not least among the problems is the superfusion Po2 where suffusate buffer for intravital microscopy of isolated muscles is invariably gassed with 5% CO2 in 95% N2 [e.g. tenuissimus (4); spinotrapezius (3); cremaster (2)]. Comparison with a suffusate Po2 dose curve (7) suggests this could have a marked impact to increase the number of capillaries perfused at rest.In contrast, our endeavours in this field have focused on noninvasive estimations of microvascular perfusion in bulk load-bearing muscle in vivo that has not been manipulated nor perturbed by invasive conditions. Ultrasound imaging of a region of interest comprising ∼15 g of human forearm muscle of conscious healthy humans has indicated capillary recruitment to occur in response to insulin infusion under euglycemic conditions, to a mixed meal, and to exercise (8), and that the insulin response is impaired in obese insulin-resistant patients (1).Finally, despite the differences in our two approaches, an impaired blood-muscle exchange has been identified in animal models of type 2 diabetes (5, 9). The percentage of RBC-perfused capillaries is decreased in the Goto-Kakizaki rat (5) and we find that the obese Zucker rat has impaired insulin-mediated capillary recruitment (9). Future experiments may reveal that the vascular dysfunction that is responsible for the impaired insulin response in vivo (9) is also that which prevents capillary recruitment when the muscle is exposed, irrigated with N2 buffer, and viewed microscopically (5). “The voyage of discovery is not in seeking new landscapes but in having new eyes.”—Marcel ProustREFERENCES1 Clerk LH, Vincent MA, Jahn LA, Liu Z, Lindner JR, Barrett EJ. Obesity blunts insulin-mediated microvascular recruitment in human forearm muscle. Diabetes 55: 1436–1442, 2006.Crossref | PubMed | ISI | Google Scholar2 Gorczynski RJ, Klitzman B, Duling BR. Interrelations between contracting striated muscle and precapillary microvessels. Am J Physiol Heart Circ Physiol 235: H494–H504, 1978.Link | ISI | Google Scholar3 Kindig CA, Richardson TE, Poole DC. Skeletal muscle capillary hemodynamics from rest to contractions: implications for oxygen transfer. J Appl Physiol 92: 2513–2520, 2002.Link | ISI | Google Scholar4 Lindbom L. Microvascular blood flow distribution in skeletal muscle. Acta Physiol Scand Suppl 525: 1–40, 1983.PubMed | Google Scholar5 Padilla DJ, McDonough P, Behnke BJ, Kano Y, Hageman KS, Musch TI, Poole DC. Effects of Type II diabetes on capillary hemodynamics in skeletal muscle. Am J Physiol Heart Circ Physiol 291: H2439–H2444, 2006.Link | ISI | Google Scholar6 Poole DC, Brown MD, Hudlicka O.Counterpoint: There is not capillary recruitment in active skeletal muscle during exercise. J Appl Physiol; doi:10.1152/japplphysiol.00779.2007a.Link | ISI | Google Scholar7 Shibata M, Ichioka S, Togawa T, Kamiya A. Arterioles' contribution to oxygen supply to the skeletal muscles at rest. Eur J Appl Physiol 97: 327–331, 2006.Crossref | PubMed | ISI | Google Scholar8 Vincent MA, Clerk LH, Lindner JR, Price WJ, Jahn LA, Leong-Poi H, Barrett EJ. Mixed meal and light exercise each recruit muscle capillaries in healthy humans. Am J Physiol Endocrinol Metab 290: E1191–E1197, 2006.Link | ISI | Google Scholar9 Wallis MG, Wheatley CM, Rattigan S, Barrett EJ, Clark ADH, Clark MG. Insulin-mediated hemodynamic changes are impaired in muscle of Zucker obese rats. Diabetes 51: 3492–3498, 2002.Crossref | PubMed | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 104Issue 3March 2008Pages 893-893 Copyright & PermissionsCopyright © 2008 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00779.2007bHistory Published online 1 March 2008 Published in print 1 March 2008 Metrics Downloaded 133 times

Skeletal muscle capillary responses to insulin are abnormal in late-stage diabetes and are restored by angiogensin-converting enzyme inhibition

Acute physiological hyperinsulinemia increases skeletal muscle capillary blood volume (CBV), presumably to augment glucose and insulin delivery. We hypothesized that insulin-mediated changes in CBV are impaired in type 2 diabetes mellitus (DM) and are improved by angiotensin-converting enzyme inhibition (ACE-I). Zucker obese diabetic rats (ZDF, n = 18) and control rats (n = 9) were studied at 20 wk of age. One-half of the ZDF rats were treated with quinapril (ZDF-Q) for 15 wk prior to study. CBV and capillary flow in hindlimb skeletal muscle were measured by contrast-enhanced ultrasound (CEU) at baseline and at 30 and 120 min after initiation of a euglycemic hyperinsulinemic clamp (3 mU.min(-1).kg(-1)). At baseline, ZDF and ZDF-Q rats were hyperglycemic and hyperinsulinemic vs. controls. Glucose utilization in ZDF rats was 60-70% lower (P < 0.05) than in controls after 30 and 120 min of hyperinsulinemia. In ZDF-Q rats, glucose utilization was impaired at 30 min but similar to controls at 120 min. Basal CBV was lower in ZDF and ZDF-Q rats compared with controls (13 +/- 4, 7 +/- 3, and 9 +/- 2 U, respectively). With hyperinsulinemia, CBV increased by about twofold in control animals at 30 and 120 min, did not change in ZDF animals, and increased in ZDF-Q animals only at 120 min to a level similar to controls. Anatomic capillary density on immunohistology was not different between groups. We conclude that insulin-mediated capillary recruitment in skeletal muscle, which participates in glucose utilization, is impaired in animals with DM and can be partially reversed by chronic ACE-I therapy.

Inflammation, Obesity, and the Metabolic Syndrome

Adipose tissue expresses cytokines which inhibit insulin signalling pathways. Obesity also results in impairment of endothelium-dependent vasodilatation to insulin. We have previously suggested that adipocytokines might contribute to the coexistence of insulin resistance and endothelial dysfunction. However, the adipocytokine best characterised as causing insulin resistance is tumour necrosis factor-alpha (TNF-alpha), a molecule which under normal circumstances circulates in low concentrations. We propose a vasoregulatory role for local deposits of fat around blood vessels, which may contribute both to insulin action and to vascular endothelial dysfunction. In particular, we propose that the localised fat depot around the origin of skeletal muscle arterioles may play a physiological role in blood flow distribution. Isolated rat arterioles are under dual regulation by insulin, which activates both endothelin-1 mediated vasoconstriction and nitric oxide mediated vasodilatation. In obese rat arterioles, insulin-stimulated nitric oxide synthesis is impaired, resulting in unopposed vasoconstriction. We propose this to be the consequence of production of TNF-alpha from the fat surrounding the vessel origin - a depot to which we ascribe a specialist vasoregulatory role. We suggest that this cytokine accesses the nutritive vascular tree to inhibit insulin-mediated capillary recruitment - a mechanism we term 'vasocrine' signalling. We also suggest a homology between periarteriolar fat and both periarterial and visceral fat, which may, through outside-to-inside signalling, play a direct role in producing the inflammatory changes found in atherosclerotic plaques, so explaining relationships between visceral fat, insulin resistance, and vascular disease.

Obesity, Insulin Resistance, and Capillary Recruitment

Insulin has vascular actions within the skeletal muscle microcirculation (capillary recruitment) that enhance its own access and that of glucose to the muscle cells. Obesity and insulin resistance are associated with dysregulated vascular function within muscle and a loss of insulin-mediated capillary recruitment. Furthermore, agents that impair insulin's vascular actions to recruit capillaries lead to acute insulin resistance in terms of muscle glucose uptake. Together these data suggest a strong connection between the loss of insulin-mediated capillary recruitment and the development of insulin resistance. This review examines the mechanisms involved in insulin-mediated capillary recruitment and the vascular defects associated with obesity and insulin resistance that may impair the capillary recruiting process. Understanding the mechanisms of insulin-mediated capillary recruitment and its impairment may lead to new treatment avenues to prevent the progression of obesity to diabetes.

Muscle metabolism and control of capillary blood flow: insulin and exercise

The evidence that muscle metabolism is determined by available capillary surface area is examined. From newly developed methods it is clear that exercise and insulin mediate capillary recruitment as part of their actions in vivo. In all insulin-resistant states examined thus far, insulin-mediated capillary recruitment is impaired with little or no change to the exercise response. Control mechanisms for capillary recruitment for exercise and insulin are considered, and the failure of the microvasculature to respond to insulin is examined for possible mechanisms that might account for impaired vascular responses to insulin in insulin resistance.

“Vasocrine” signalling from perivascular fat: a mechanism linking insulin resistance to vascular disease

Adipose tissue expresses cytokines that inhibit insulin signalling pathways in liver and muscle. Obesity also results in impairment of endothelium-dependent vasodilatation in response to insulin. We propose a vasoregulatory role for local deposits of fat around the origin of arterioles supplying skeletal muscle. Isolated first-order arterioles from rat cremaster muscle are under dual regulation by insulin, which activates both endothelin-1 mediated vasoconstriction and nitric-oxide-mediated vasodilatation. In obese rat arterioles, insulin-stimulated NO synthesis is impaired, resulting in unopposed vasoconstriction. We propose that this vasoconstriction is the consequence of production of the adipocytokine tumour necrosis factor alpha from the cuff of fat seen surrounding the origin of the arteriole in obese rats--a depot to which we ascribe a specialist vasoregulatory role. We suggest that this cytokine accesses the nutritive vascular tree to inhibit insulin-mediated capillary recruitment--a mechanism we term "vasocrine" signalling. We also suggest a homology between this vasoactive periarteriolar fat and both periarterial and visceral fat, which may explain relations between visceral fat, insulin resistance, and vascular disease.