Skeletal Muscle Microvasculature Research Articles

Nitrite Attenuates Ischemia-Reperfusion–Induced Microcirculatory Alterations and Mitochondrial Dysfunction in the Microvasculature of Skeletal Muscle

Recently, nitrite has been rediscovered as a physiologically relevant storage reservoir of nitric oxide in blood and it can readily be converted to nitric oxide under hypoxic and acidic conditions. In this study, the authors evaluated the therapeutic efficacy of nitrite on reperfusion-induced microcirculatory alterations and mitochondrial dysfunction in the microvasculature of skeletal muscle. The authors used a vascular pedicle isolated rat cremaster model that underwent 4 hours of warm ischemia followed by 2 hours or 17 hours of reperfusion. At 5 minutes before reperfusion, normal saline, sodium nitrite (0.20 μM/minute/kg), or nitrite mixed with 2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (potassium salt) (0.2 mg/minute/kg) was infused into the microcirculation of ischemic cremaster by means of intraarterial infusion. Ischemia-reperfusion-induced microcirculatory alterations were measured after 2 hours of reperfusion. Microvasculature of the cremaster muscle including the vascular pedicle was harvested to determine the mitochondrial dysfunction. The blood concentration of methemoglobin was also measured to determine the toxicity of nitrite. The authors found that nitrite significantly attenuated ischemia-reperfusion-induced vasoconstriction, arteriole stagnation, and capillary no-reflow in the early phase of reperfusion and the depolarization of mitochondrial membrane potential and cytochrome c release in the late phase of reperfusion. Nitrite-induced protection was significantly blocked by a nitric oxide scavenger (potassium salt). The methemoglobin results showed that the doses of nitrite we used in the present study were safe. The supplementation of a low dose of nitrite, directly into the microcirculation of ischemic muscle through local intraarterial infusion, significantly attenuated ischemia-reperfusion-induced microcirculatory alterations in vivo and mitochondrial dysfunction in vitro in the microvasculature of skeletal muscle.

Hemodynamic Systems Analysis of Capillary Network Remodeling During the Progression of Type 2 Diabetes

Early alterations in the skeletal muscle microvasculature may contribute to the onset and progression of type 2 diabetes (DM2) by limiting insulin and glucose availability to skeletal muscle. Microvascular alterations reported with DM2 are numerous and include impaired endothelium-mediated vasodilation, increased arteriole wall stiffness, and decreased capillary density. Most previous analyses of skeletal muscle microvascular architecture have been limited to skeletal muscle cross sections and thus have not presented an integrated, quantitative analysis of the relative significance of observed alterations to elevated microvascular network resistance and decreased blood flow. In this work, we tested the hypothesis that the onset of diabetes would influence microvascular architecture in a manner that would significantly increase capillary network resistance and reduce blood flow. In whole-mount spinotrapezius muscle capillary networks from Zucker diabetic fatty (ZDF) rats before and after the onset of DM2, we found a significant 37% decrease in microvascular branching and a 19% decrease in microvessel length density associated with the onset of the disease. This was previously indiscernible in skeletal muscle cross-section data. Hemodynamic computational analysis revealed that the changes in DM2 capillary network connectivity result in a significant 44% decrease in computed capillary network flow compared to controls. A hemodynamic sensitivity analysis showed that DM2 networks were predicted to be less robust in their ability to maintain perfused network surface area in the event of upstream terminal arteriole constriction. This study illustrates that capillary network connectivity is altered by DM2 and this negatively impacts microvascular hemodynamics. This work can serve as a basis for a more quantitative approach to evaluating DM2 microvascular networks and their potential use as an early diagnostic aid and/or method for identifying therapeutic targets.

Hemodynamic responses to heat stress in the resting and exercising human leg: insight into the effect of temperature on skeletal muscle blood flow.

Heat stress increases limb blood flow and cardiac output (Q) in humans, presumably in sole response to an augmented thermoregulatory demand of the skin circulation. Here we tested the hypothesis that local hyperthermia also increases skeletal muscle blood flow at rest and during exercise. Hemodynamics, blood and tissue oxygenation, and muscle, skin, and core temperatures were measured at rest and during exercise in 11 males across four conditions of progressive whole body heat stress and at rest during isolated leg heat stress. During whole body heat stress, leg blood flow (LBF), Q, and leg (LVC) and systemic vascular conductance increased gradually with elevations in muscle temperature both at rest and during exercise (r(2) = 0.86-0.99; P < 0.05). Enhanced LBF and LVC were accompanied by reductions in leg arteriovenous oxygen (a-vO(2)) difference and increases in deep femoral venous O(2) content and quadriceps tissue oxygenation, reflecting elevations in muscle and skin perfusion. The increase in LVC occurred despite an augmented plasma norepinephrine (P < 0.05) and was associated with elevations in muscle temperature (r(2) = 0.85; P = 0.001) and arterial plasma ATP (r(2) = 0.87; P < 0.001). Isolated leg heat stress accounted for one-half of the increase in LBF with severe whole body heat stress. Our findings suggest that local hyperthermia also induces vasodilatation of the skeletal muscle microvasculature, thereby contributing to heat stress and exercise hyperemia. The increased limb muscle vasodilatation in these conditions of elevated muscle sympathetic vasoconstrictor activity is closely related to the rise in arterial plasma ATP and local tissue temperature.

Free Fatty Acids Induce Insulin Resistance in Both Cardiac and Skeletal Muscle Microvasculature in Humans

Insulin recruits microvasculature in both cardiac and skeletal muscle, which increases the endothelial exchange surface area. Plasma concentrations of free fatty acids (FFAs) are elevated in patients with diabetes, which impairs insulin-mediated skeletal muscle microvascular recruitment. The objective of the study was to examine whether elevated FFAs likewise cause insulin resistance in cardiac muscle microvasculature. The study was conducted at the General Clinical Research Center at the University of Virginia. Twenty-two healthy, young adults were studied twice in random order after an overnight fast. Each subject received a 5-h systemic infusion of either saline or Intralipid/heparin with a 1 mU/min · kg euglycemic insulin clamp superimposed for the last 2 h. Cardiac and forearm skeletal muscle microvascular blood volume (MBV) and flow velocity were measured and microvascular blood flow (MBF) calculated before and at the end of the insulin infusion. Insulin significantly increased MBV and MBF in both cardiac (P < 0.0001 for both) and skeletal (P = 0.008 and < 0.03, respectively) muscle. Microvascular flow velocity increased slightly but significantly in the skeletal (P = 0.04) but not in cardiac muscle. Lipid infusion lowered insulin-stimulated whole-body glucose disposal and abolished insulin-mediated increases in MBV and MBF in both cardiac and skeletal muscle. Whole-body insulin sensitivity predicted skeletal but not cardiac muscle microvascular responses to insulin. Insulin even decreased skeletal muscle MBV during lipid infusion in subjects who were moderately sensitive to insulin metabolically. In conclusion, high plasma concentrations of FFAs cause insulin resistance in cardiac as well as skeletal muscle microvasculature in healthy humans. This may contribute to the association of cardiac complications with metabolic insulin resistance in diabetes.

Erythrocytes as controllers of perfusion distribution in the microvasculature of skeletal muscle

In 1929, August Krogh identified the matching of oxygen (O(2)) supply with demand in skeletal muscle as a fundamental physiological process. In the intervening decades, much research has been focused on elucidating the mechanisms by which this important process occurs. For any control system to be effective, there must be a means by which the need is determined and a mechanism by which that information is coupled to an appropriate response. The focus of this review was to highlight current research in support of the hypothesis that the mobile erythrocyte, when exposed to reduced O(2) tension, releases ATP in a controlled manner. This ATP interacts with purinergic receptors on the endothelium producing both local and conducted vasodilation enabling the erythrocyte to distribute perfusion to precisely match O(2) delivery with need in skeletal muscle. If this is an important mechanism for normal physiological control of microvascular perfusion, defects in this process would be anticipated to have pathophysiological consequences. Individuals with either type 2 diabetes (DM2) or pre-diabetes have microvascular dysfunction that contributes to morbidity and mortality. DM2 erythrocytes and erythrocytes incubated with insulin at levels similar to those seen in pre-diabetes fail to release ATP in response to reduced O(2) tension. Knowledge of the components of the signal transduction pathway for low O(2) -induced ATP release suggest novel therapeutic approaches to ameliorating this defect. Although the erythrocyte may be but one component of the complex O(2) delivery process, it appears to play an important role in distributing oxygen within the microvasculature.

Intravoxel partially coherent motion technique: Characterization of the anisotropy of skeletal muscle microvasculature

To propose a reformulation of the intravoxel incoherent motion (IVIM) technique exploiting the low b-value diffusion-weighted imaging regime that can characterize microcirculation of tissues perfused with partially coherent blood flow. The new methodology, termed intravoxel partially coherent motion (IVPCM) technique, is suitable for probing microcirculation in tissues with ordered microvasculature, such as skeletal muscle. We employ a subvoxel model utilizing a randomly oriented bundle of straight vessels whose orientation statistics are characterized by a Fisher axial distribution with concentration parameter K quantifying the anisotropy of the distribution (K = 0 indicates isotropic capillary orientation). The methodology is first validated with a proof-of-principle phantom experiment and is then applied to analyze the microvasculature of human calf muscle at rest. The microcirculatory part of the diffusion-weighted signal at b < 200 s/mm(2) is anisotropic. The variation of the diffusion-weighted signal with b-value exhibits stronger deviation from the expected monoexponential decay when the diffusion encoding gradient is applied parallel to the mean myofiber direction in the calf muscle of three healthy volunteers. The application of the model to data from the medial gastrocnemius and the soleus of the three volunteers gives results within the expected range for the mean microvascular volume fraction, the mean microflow velocity, and the parameter K. The proposed methodology has the capability of characterizing the anisotropy of the capillary network in vivo in a manner analogous to the capability of high b-value diffusion to characterize the anisotropy of muscle fibers.

Remodeling of skeletal muscle microvasculature in sickle cell trait and α-thalassemia

The influence of sickle cell trait and/or alpha-thalassemia on skeletal muscle microvascular network characteristics was assessed and compared with control subjects [hemoglobin (Hb) AA] in 30 Cameroonian residents [10 HbAA, 5 HbAA alpha-thalassemia (alpha-t), 6 HbAS, and 9 HbASalpha-t] matched for maximal work capacity and daily energy expenditure. Subjects performed an incremental exercise to exhaustion and underwent a muscle biopsy. Muscle fiber type and surface area were not different among groups. However, sickle cell trait (SCT) was associated with lower capillary density (P < 0.05), lower capillary tortuosity (P < 0.001), and enlarged microvessels (P < 0.01). SCT carriers had reduced counts of microvessels <5-microm diameter, but a higher percentage of broader microvessels, i.e., diameter >10 microm (P < 0.05). alpha-Thalassemia seemed to be characterized by a higher capillary tortuosity and unchanged capillary density and diameter. Thus, while SCT is a priori clinically benign, we demonstrate for the first time that significant remodeling of the microvasculature occurs in SCT carriers. These modifications may possibly reflect protective adaptations against hemorheological and microcirculatory dysfunction induced by the presence of HbS. The remodeling of the microvascular network occurs to a lesser extent in alpha-thalassemia. In alpha-thalassemic subjects, increased capillary tortuosity would promote oxygen supply to muscle tissues and might compensate for the lower Hb content often reported in those subjects.

TIMP‐1 protein but not secretion is increased by shear stress in the skeletal muscle microvasculature

Luminal splitting is a form of angiogenesis caused by increased shear stress. A feature of luminal splitting is a lack of extracellular matrix proteolysis correlated with decreased production of matrix metalloproteinase (MMP)‐2. We hypothesized that an MMP inhibitor, tissue inhibitor of matrix metalloproteinase (TIMP)‐1, is upregulated by shear stress. TIMP‐1 mRNA increased in the EDL of prasozin‐treated rats (p&lt;0.05, n=4) as measured by RT‐PCR. Rat skeletal muscle microvascular endothelial cells were exposed to shear stress (12 dyne/cm2). TIMP‐1 mRNA increased in cells after 2 and 24 hours of shear (p&lt;0.05, n=3), and TIMP‐1 protein increased at 2 and 24 but not 4 hours (p&lt;0.05, n=3) as assessed by Western. TIMP‐1 secretion decreased at 2 hours of shear and returned to control levels at 24 hours (p&lt;0.01, n=3) as measured by reverse zymography of concentrated media. Similarly, MMP‐2 activity increased in media at 2 but not 24 hours of shear (p&lt;0.05, n=7) as assessed by an in vitro MMP‐2 activity assay (Anaspec). 2 hours of shear caused no change in TIMP‐1 surface staining (ns, n=3) as assessed by confocal microscopy, but did cause an increase in intracellular TIMP‐1 staining (p&lt;0.05, n=3). Our results show that intracellular TIMP‐1 protein is increased by shear stress, due to increased production and decreased secretion. The function of intracellular TIMP‐1 is unknown.Funded by the Heart and Stroke Foundation of Canada.Grant Funding SourceHeart and Stroke Foundation of Canada

Characterizing the Response of Skeletal Muscle Microvasculature to Imposed Oxygen Variations

Computational modeling of the oxygen regulatory system has shown how signals conducted upstream from capillaries change the flow pattern in the arteriole tree. The simulation showed that the hemodynamic response to higher frequency oscillations (period 30 sec) is attenuated relative to lower frequency oscillations (period 90 sec). To test this prediction in vivo, a custom O2 gas chamber is used to supply O2 at the surface of exposed skeletal muscle and a sequence of 3 oscillations (1st and 3rd 30 sec period, 2nd 90 sec period, see figure) were generated. The hemodynamic response in capillaries to this stimulus was quantified using intravital video microscopy and a functional image analysis system (RBC velocity and hematocrit to yield RBC supply rate, SR). The RBC supply rate response supported the results from the computational model for the 1st and 2nd sine oscillation. The SR response to the higher frequency oscillation was attenuated to ~60‐70% of the lower frequency stimulus. However, the 3rd sinusoidal oscillation showed no response to the O2 signal suggesting that other factors need to be included in the model. Overall, the model and experimental results support the hypothesis that regulation of O2 supply involves a conducted signal along the arteriolar tree controlling both flow resistance and distribution of RBCs at bifurcations. (NIH R33 HL089094).

Calcium-activated potassium channels contribute to human skeletal muscle microvascular endothelial dysfunction related to cardiopulmonary bypass

We investigated the role of calcium-activated potassium (K(Ca)) channel activity in human skeletal muscle microvascular function in the setting of cardiopulmonary bypass (CPB). Human skeletal muscle arterioles (80- to 180 microm in diameter) were dissected from tissue harvested before and after CPB. In vitro relaxation responses of precontracted arterioles in a pressurized no-flow state were examined in the presence of K(Ca) channel activators/blockers and several other vasodilators. Post-CPB responses to the activator of intermediate (IK(Ca)) and small conductance (SK(Ca)) K(Ca) channels, NS309, to the endothelium-dependent vasodilator adenosine 5'-diphosphate (ADP), and to substance P were reduced compared with pre-CPB responses (P < .05), respectively, whereas responses to the activator of large conductance (BK(Ca)) K(Ca) channels, NS1619, and to the endothelium-independent vasodilator, sodium nitroprusside (SNP) were unchanged. Endothelial denudation decreased NS309-induced relaxation and abolished that induced by ADP or substance P (P < .05), but had no effect on relaxation induced by either NS1619 or SNP. Polypeptide levels of BK(Ca), IK(Ca), and SK3(Ca) were not altered post-CPB. IK/SK-mediated relaxation is predominantly endothelium dependent, whereas BK-mediated relaxation seems to be largely independent of endothelial function in human skeletal muscle microvasculature. CPB-associated microvascular dysfunction likely arises in part from impaired function of endothelial SK and IK channels in the peripheral microvasculature.

Shear stress‐induced Ets‐1 modulates protease inhibitor expression in microvascular endothelial cells

Elevated shear stress within the skeletal muscle microvasculature is implicated in the induction of a longitudinal splitting form of angiogenesis, which is characterized by the lack of basement membrane breakage. We investigated whether the transcriptional regulator, Ets-1, is responsive to changes in hemodynamic forces and if so, whether Ets-1 controls microvascular endothelial cell integrity by inducing the expression of inhibitors of matrix degrading proteases. Rats were treated with prazosin for 2, 4, and 7 days to increase in microvascular shear stress in hindlimb skeletal muscles. In complimentary in vitro experiments, rat microvascular skeletal muscle endothelial cells were exposed to laminar shear stress (15 dyne/cm(2)) for 0.5, 2, and 24 h. TaqMan PCR analysis of laser microdissected capillaries isolated from EDL muscles demonstrated transient (after 2 days) induction of Ets-1 gene expression. In cultured cells, a transient up-regulation of Ets-1 mRNA was observed after 2 h shear stimulation, accompanied by increased phosphorylation of Ets-1 and enhanced Ets-1 DNA binding activity. This response was modulated by ERK1/2 and p38 MAP kinases, but was not dependent on NOS or COX-2 activity. PAI-1, TIMP-1 and TIMP-3 mRNA were elevated significantly in prazosin treated EDL, and in response to shear stimulation in vitro. In cultured endothelial cells, Ets-1 RNA interference abolished the shear-induced increases in Ets-1, PAI-1, TIMP-1, and TIMP-3 mRNA expression. These results suggest that enhanced laminar shear stress may act to preserve the integrity of microvascular walls in part through Ets-1-dependent induction of protease inhibitors.

New insights into orthostatic hypotension

we are approaching the 50th anniversary of manned spaceflight, which began in 1961 when Yuri Gagarin and Alan Shepard flew into space within a month of each other; much has been learned about the health risks associated with microgravity. One of the first indications that space travel could have

TIMP‐1 is increased by shear stress in the skeletal muscle microvasculature

Luminal splitting via internal division of capillaries is a form of angiogenesis caused by increased shear stress. A feature of luminal splitting is a lack of extracellular matrix proteolysis, which correlates with a decreased production of matrix metalloproteinase‐2 (MMP‐2). We hypothesized that the MMP inhibitor, tissue inhibitor of matrix metalloproteinase‐1 (TIMP‐1), is upregulated in response to shear stress. TIMP‐1 mRNA expression increased 3.8‐fold in the EDL of prasozin‐treated rats (p&lt;0.05, n=4). Skeletal muscle endothelial cells were exposed to 2, 4, or 24 hours of shear stress (12 dyne/cm2). TIMP‐1 mRNA expression increased 7.0 and 9.0‐fold in cells after 2 and 24 hours of exposure to shear stress respectively (p&lt;0.05, n=3), and TIMP‐1 protein increased 1.5 and 2.2‐fold after 2 and 24 hours of shear stress (p&lt;0.05, n=3). TIMP‐1‐mediated inhibition of MMP‐2 also was increased 1.5 and 1.3‐fold after 2 and 24 hours of shear stress (n=2 and p&lt;0.05, n=3 respectively). Our results show that TIMP‐1 is upregulated by shear stress, and suggest a mechanism for the absence of extracellular matrix proteolysis observed during luminal splitting.Funded by the Heart and Stroke Foundation of Canada.

Pathways used to transmit conducted dilations are dependent on the stimulus used to initiate the conducted response

We hypothesized that the pathways utilized for conducted dilations along skeletal muscle microvasculature are dependent on the dilator used to initiate the conducted response. We initiated conducted responses by micropipette application of either 10−4M acetylcholine (ACh) or 10−5M pinacidil (PIN, a KATP channel opener) to the capillaries or the transverse arteriole (TA, located three branch orders proximal to the capillaries) in the anaesthetized hamster cremaster muscle model in situ. When the capillaries were stimulated we observed changes in diameter of the arteriole directly supplying the capillaries and when the TA was stimulated we observed the TA approx. 1000um upstream (US) and downstream (DS) from the site of dilator application. We reapplied ACh and PIN after a 30 min micropipette application of 40uM 18‐B‐glycyrrhetinic acid (BGA), a gap junction uncoupler, between sites of initiation and observation. Significant conducted dilations were produced in proximal arterioles when either ACh (3.6+/−0.9um) or PIN (3.1+/−0.5um) was applied to the capillaries. BGA significantly attenuated dilation to ACh (0.6+/−0.2um) but not dilations to PIN (3.4+/−0.8um). Significant conducted dilations were observed on the TA in response to both ACh (US: 9.6+/−2.0um, DS: 7.8+/−2.4um) and PIN (US: 5.2+/−1.4um, DS: 7.0+/−1.7um). BGA attenuated the conducted responses to ACh (US: 4.4+/−1.8um, DS: 2.6+/−1.6um), but not to PIN (US: 5.1+/−1.4um, DS: 10.9+/−4.4um). Therefore, conducted responses may use different transmission pathways that are dependent on the stimulus initiating the conducted response. NSERC and PREA.

Counterpoint: Sickle cell trait should not be considered asymptomatic and as a benign condition during physical activity

Counterpoint: Sickle cell trait should not be considered asymptomatic and as a benign condition during physical activity

Reply to Kreier and Buijs: no sympathy for the claim of parasympathetic innervation of white adipose tissue

to the editor: Kreier and Buijs ([16][1]) have questioned our work ([14][2]) demonstrating the lack of parasympathetic nervous system (PSNS) innervation of white adipose tissue (WAT) in laboratory rats and mice, as well as in Siberian hamsters. We appreciate the opportunity to clarify and extend our

Exercise TVaining-Induced Changes in Skeletal Muscle Microvasculature Assessed by Contrast Enhanced Ultrasound

Contrast-enhanced ultrasound (CEU) can non-invasively quantify microvascular volume, blood velocity, and flow in tissue accessible to ultrasound. Changes in microvascular properties in skeletal muscle that occur longitudinally with a given intervention have not been documented with this technique. PURPOSE: To examine the effects of intensity of exercise training on skeletal muscle microvasculature in abdominally obese individuals with the metabolic syndrome. METHODS: We studied 27 abdominally obese adults with metabolic syndrome [20 F, 7 M, mean (SD) age 46.4 (9.8) yr, ht 169 (9.7) cm, wt 100.5 (19.2) kg] by CEU before and after a controlled, randomized exercise training intervention. Subjects' right foot flexors (gastrocnemius and soleus) were imaged to quantify microvascular properties at rest and during pedal ergometry. Subjects were randomized to one of three groups for 16 weeks: control (C), low-intensity aerobic training (LI), or high-intensity aerobic training (HI). Baseline comparisons between rest and exercise were made using pooled data by paired t-tests. Training responses using difference scores were analyzed by ANOVA. RESULTS: At baseline, acute pedal ergometry at 0.2 Hz resulted in significant increases from rest in: muscle microvascular volume (0.31 +/−0.2vs. 2.6+/−1.7 arbitrary units (a.u.); p < 0.001), erythrocyte velocity (0.09+/−0.04 vs. 0.11 +/−0.04 a.u.;p = 0.03), and microvascular flow (0.028+/−0.02 vs. 0.265 +/−0.16 a.u.; p < 0.001). Exercise training increased microvascular volume and flow both at rest (volume change: C = −0.008, LI = +0.23, HI = +0.28; flow change: C = −+0.0003, LI = +0.01, HI = +0.03) and during pedal exercise (volume change: C = +1.1, LI =+0.9, HI = +2.4; flow change: C = +0.0006, LI = +0.1, HI = +0.33), with the increase in flow during pedal exercise in the high-intensity training group significantly greater than control (p for ANOVA = 0.04). The degree of individual changes in microvascular properties did not correlate strongly with changes in abdominal visceral fat, maximal aerobic capacity, or insulin sensitivity assessed by QUICKI. CONCLUSION: We conclude that exercise training increases skeletal muscle microvascular volume and flow in an intensity dependent manner, and that these changes are independent of other variables associated with metabolic syndrome.

TNF‐α induced variability of ICAM‐1 expression in skeletal muscle microvasculature

The observation that leukocyte-endothelial cell (EC) interactions are localized to specific regions on the vessel wall suggests that adhesion molecule distribution on the endothelium is not uniform. We used intravital confocal microscopy to compare the expression of ICAM-1 in arterioles vs venules in control and TNF-α (500 ng, intrascrotally) induced inflammation. ICAM-1 expression levels were indicated by normalized intensity of fluorescently labeled surface ICAM-1 in blood perfused cremaster muscle microvessels of anesthetized mice (Nembutal, 74 mg/ml) and were found to be not uniform. Under normal conditions venular ICAM-1 expression was approximately two fold higher than the expression in arterioles. Following TNF-α treatment ICAM-1 expression was significantly increased, 2.8±0.2 fold in arterioles and 1.7±0.2 in venules (n=4, p-value <0.05). However, ICAM-1 expression on activated arteriolar ECs only reached the level of venular ICAM-1 expression observed under control conditions. Additionally, arteriolar but not venular ECs underwent redistribution of ICAM-1 among cells, where some cells increased and some decreased the ICAM-1 expression, magnifying the variability of ICAM-1. Also, TNF-α treatment resulted in longer bright regions per unit length of the vessel (42% in control, 70% in TNF-α) along the arteriolar wall, but no significant change was observed in venules. The fact that arteriolar ECs respond to TNF-α by upregulation of ICAM-1 expression, although in a different way compared to venules, suggests an explicit role for arterioles in inflammation. (Supported by NIH HL18208 and HL75186)

Localization of the Renin–Angiotensin System Components to the Skeletal Muscle Microcirculation

Local renin-angiotensin systems have been found in various organs and tissues throughout the body. The studies described here tested the hypothesis that a fully functional renin-angiotensin system exists in the skeletal muscle microvasculature. The purpose of this study was to localize the components and products of the system to the skeletal muscle microvasculature. The presence of mRNA and protein for renin and angiotensinogen was shown by reverse transcriptase polymerase chain reaction analysis and immunohistochemistry, respectively. Angiotensin II concentration in isolated microvessels was measured by high-performance liquid chromatography separation and radioimmunoassay. Renin and angiotensinogen mRNA was detected from isolated cremaster microvessels. Specific staining for renin and angiotensinogen protein was found throughout the walls of microvessels. The concentration of the angiotensin II in the microvessels (104.3 +/- 18.1 fmol/g) was found to be higher than the concentration of angiotensin II in the plasma (8.7 +/- 1.2 fmol/mL) of the same animals. Also, it was shown that the microvessel angiotensin II concentrations in spontaneously hypertensive rats were higher than in Sprague-Dawley rats. The current study demonstrates that within the skeletal muscle microvessels, the components and products necessary for a local renin-angiotensin system are present, but does not rule out the possibility of interaction between circulating and tissue renin-angiotensin systems.

ACTIVE ROLE FOR THE VASCULATURE IN THE DELIVERY OF INSULIN TO SKELETAL MUSCLE

1. In the 80+ years since insulin's discovery, an enormous amount of literature has accumulated relating to its actions on body fat, glucose and protein metabolism. In particular, skeletal muscle has been extensively studied because of its major role as a site of insulin-mediated glucose disposal. Liver and adipose tissue are two other extensively studied sites of insulin action. Much less investigation has been directed towards delineating insulin's actions on cells other than myocytes, adipocytes and hepatocytes. 2. Over the past 5-10 years it has become increasingly evident that insulin exerts important actions on vascular cells. Here, we review evidence that insulin's action within muscle may be very much regulated by its ability to transit the vasculature to access the interstitial fluid (and hence the myocyte insulin receptor). Surprisingly little is known regarding the regulation of vascular events that first bring insulin to the capillary endothelium within muscle, whence presumably it transits from the vascular to the interstitial space. Recent studies suggest that insulin can increase blood flow and also influence the distribution of blood flow within skeletal muscle, potentially therefore regulating its own delivery to the capillary endothelium. Beyond insulin's ability to access the vascular lumen within skeletal muscle microvasculature lies the issue of its passing the endothelial barrier. Even less is known about the processes involved in insulin's actual transit across the endothelium. Available data do not clearly indicate whether this is a saturable, receptor-mediated process or a passive-diffusion pathway. Also, whether insulin in any manner regulates its own transit across the endothelium or its clearance via the lymphatic system is entirely unknown. 3. The aim of the present review is to identify areas where knowledge is deficient and highlight hypotheses which may lead to a better understanding of the coordinated relationship between insulin's vascular actions within muscle and its metabolic actions in that tissue. Even so, there is now sufficient evidence to indicate that insulin's vascular action within skeletal muscle is a major regulatory locus for its insulin mediated glucose disposal.