Measurement of the spatial redistribution of water in rabbit Achilles tendon in response to static tensile loading

The redistribution of water in response to static tensile loading was investigated in rabbit Achilles tendons in vitro. The distribution of water was measured along a radially oriented line, using a one-dimensional proton-density map created from fits to diffusion-weighted magnetic resonance (MR) data. Water movements were measured during application of tensile loads of 5N (N=7) and 10N (N=6). Water distribution along the line was measured before loading and up to 42 min after load application. Static loading with either 5 or 10N loads caused a steady increase in proton density in the outside edge (rim) of the tendon. The 10N load lowered the proton density in the core of the tendon, but did so in a single step that was observed when the load was applied. The 5N load caused no change in proton density in the core region. The immediate redistribution from the core was statistically significant for the 10N load, but not the 5N load application. Statistically significant within-group proton-density increases were observed in the rim after 42 min postload for all tendons irrespective of load condition. The rate of proton-density postload increase at the rim region did not depend upon load. The rate for the 5N load case was 0.010 +/- 0.002 min(-1) and 0.007 +/- 0.002 min(-1) in the 10N case. Thus, while generally consistent with an extrusion model, the data show other features that argue for a more complex model.     

Scaling in tensile "skeletons": scale dependent length of the Achilles tendon in mammals

The Achilles tendon of a diverse group of mammals ranging from the mouse (12g) to the rhinoceros (1300kg) scales so that the tendon length varies as tendon diameter.^0-931±0.069 ( r =0.983). Tendon length scales as (body mass)^0-342±0.028, and tendon diameter scales as (body mass)^0-361±0.029. If tendon stress and strain are scale independent, the capacity of the tendon to store elastic strain energy remains proportion to body mass. If tendon stress and strain increase with body mass, energy storage may scale somewhat higher. The scaling of the Achilles tendon is consistent with its role in storing strain energy and different from that of a variety of other tensile skeletal elements which exhibit scale independent length dimensions.     

Regulation of lipopolysaccharide-induced increases in neutrophil glucose uptake

The pathogenesis of many lung diseases involves neutrophilic inflammation. Neutrophil functions, in turn, are critically dependent on glucose uptake and glycolysis to supply the necessary energy to meet these functions. In this study, we determined the effects of p38 mitogen-activated protein kinase and hypoxia-inducible factor (HIF)-1, as well as their potential interaction, on the expression of membrane glucose transporters and on glucose uptake in murine neutrophils. Neutrophils were harvested and purified from C57BL/6 mice and stimulated with lipopolysaccharide (LPS) in the presence or absence of specific p38 and HIF-1 inhibitors. Glucose uptake was measured as the rate of [3H]deoxyglucose (DG) uptake. We identified GLUT-1 in mouse neutrophils, but neither GLUT-3 nor GLUT-4 were detected using Western blot analysis, even after LPS stimulation. LPS stimulation did not increase GLUT-1 protein levels but did cause translocation of GLUT-1 from the cell interior to the cell surface, together with a dose-dependent increase in [3H]DG uptake, indicating that glucose uptake is regulated in these cells. LPS also activated both p38 and the HIF-1 pathway. Inhibitors of p38 and HIF-1 blocked GLUT-1 translocation and [3H]DG uptake. These data suggest that LPS-induced increases in neutrophil glucose uptake are mediated by GLUT-1 translocation to the cell surface in response to sequential activation of neutrophil p38 and HIF-1alpha in neutrophils. Given that neutrophil function and glucose metabolism are closely linked, control of the latter may represent a new target to ameliorate the deleterious effects of neutrophils on the lungs.     

Structural Achilles tendon properties in athletes subjected to different exercise modes and in Achilles tendon rupture patients.

The prevalence of Achilles tendon (AT) injury is high in various sports, and AT rupture patients have been reported to have a 200-fold risk of sustaining a contralateral rupture. Tendon adaptation to different exercise modes is not fully understood. The present study investigated the structural properties of the AT in male elite athletes that subject their AT to different exercise modes as well as in Achilles rupture patients. Magnetic resonance imaging of the foot and leg, anthropometric measurements, and maximal isometric plantar flexion force were obtained in 6 male AT rupture patients and 25 male elite athletes (kayak/control group n = 9, volleyball n = 8 and endurance running n = 8). AT cross-sectional area (CSA) was normalized to body mass. Runners had a larger normalized AT CSA along the entire length of the tendon compared with the control group (P < 0.05). The volleyball subjects had a larger normalized CSA compared with the control group (P < 0.05) in the area of thinnest tendon CSA. No structural differences of the AT were found in the rupture subjects compared with the control group. Rupture subjects did not subject their AT to greater force or stress during a maximal voluntary isometric plantar flexion than the other groups. The CSA of the triceps surae musculature was the strongest predictor of AT CSA (r(s) = 0.569, P < 0.001). This study is the first to show larger CSA in tendons that are subjected to intermittent high loads. AT rupture patients did not display differences in structural or loading properties of the tendons.     

Achilles tendon loading during walking: application of a novel optic fiber technique

An optic fiber (? 0.5 mm) was utilized for the study of Achilles tendon forces (ATF) in eight volunteers who walked over a 10 m force platform at three speeds (1.1 ± 0.1 m × s⁻¹, 1.5 ± 0.1 m × s⁻¹ and 1.8 ± 0.2 m × s⁻¹). The presented ATF-time curves showed great intersubject variation in magnitudes of the sudden release of force after initial contact and in the peak ATF's (1430 ± 500 N). This intersubject variation in the peak force decreased only by 4% when cross-sectional area of the tendon was considered. Measured ground reaction forces and plantar pressures confirmed that the subjects walked quite normally during recordings. The peak ATF was found to be rather insensitive to speed in contrast to the rate of ATF development which increased 32% ( p < 0.5) from slow to fast walking speed. It is concluded that the optic fiber technique can be applied to study loading of the musculo-tendinous complex during normal locomotion such as walking. Accepted: 13 October 1997     

Influence of Achilles tendon vibration on the vertical posture during standing with asymmetrical leg loading

The shift of the common center of pressure (CCP) and the center of pressure (CP) of one leg was studied during the Achilles tendon vibration of one or both legs while the subject was standing with symmetrical load on the legs or with the load transferred to one leg. The CP shift of the standing subject during unilateral Achilles tendon vibration depended on both the side of application of vibration and on the distribution of the leg load. During standing with a asymmetrical load on the legs, the shift of the CCP was larger than when the vibration was applied to the loaded leg. The CP shift of one leg was greater if both vibration and the load were applied to it. Vibration of the unloaded leg caused a CP shift in the loaded contralateral leg. In this case, vibration of the left unloaded leg did not cause any noticeable CP shift of the left leg, while vibration of the unloaded right leg caused a CP shift of the right leg. Under the similar conditions of loading and vibration, the displacement of the CP of the right leg was larger than the displacement of the CP of the left leg. It may be suggested that postural asymmetry and unilateral vibration of the leg muscles change the internal representation of the position of the body axis in relation to the vertical, which affects the displacement of the CP of one leg in response to afferent stimulation of the leg muscles.     

Biomechanical and structural response of healing Achilles tendon to fatigue loading following acute injury

Achilles tendon injuries affect both athletes and the general population, and their incidence is rising. In particular, the Achilles tendon is subject to dynamic loading at or near failure loads during activity, and fatigue induced damage is likely a contributing factor to ultimate tendon failure. Unfortunately, little is known about how injured Achilles tendons respond mechanically and structurally to fatigue loading during healing. Knowledge of these properties remains critical to best evaluate tendon damage induction and the ability of the tendon to maintain mechanical properties with repeated loading. Thus, this study investigated the mechanical and structural changes in healing mouse Achilles tendons during fatigue loading. Twenty four mice received bilateral full thickness, partial width excisional injuries to their Achilles tendons (IACUC approved) and twelve tendons from six uninjured mice were used as controls. Tendons were fatigue loaded to assess mechanical and structural properties simultaneously after 0, 1, 3, and 6 weeks of healing using an integrated polarized light system. Results showed that the number of cycles to failure decreased dramatically (37-fold, p<0.005) due to injury, but increased throughout healing, ultimately recovering after 6 weeks. The tangent stiffness, hysteresis, and dynamic modulus did not improve with healing (p<0.005). Linear regression analysis was used to determine relationships between mechanical and structural properties. Of tendon structural properties, the apparent birefringence was able to best predict dynamic modulus (R²=0.88–0.92) throughout healing and fatigue life. This study reinforces the concept that fatigue loading is a sensitive metric to assess tendon healing and demonstrates potential structural metrics to predict mechanical properties.     

Interleukin-15 increases glucose uptake in skeletal muscle. An antidiabetogenic effect of the cytokine

Previous studies have demonstrated that interleukin-15 (IL-15) has important anabolic effects on muscle protein metabolism. In the present investigation we have analysed the effects of IL-15 on glucose metabolism in skeletal muscle. Administration of a single dose of the cytokine (100 microg/kg body weight) resulted in a 32% increase on glucose uptake (as measured by the uptake of 2-deoxyglucose) in skeletal muscle. The effects observed on glucose uptake were direct since in vitro incubations of rat EDL muscles in the presence of the cytokine resulted in a 30% increase in glucose uptake. Similarly, IL-15 increased glucose uptake in C2C12 cell cultures, this being related with an increase in both glucose oxidation to CO2 and the incorporation into muscle lipid. The effects of the cytokine were associated with an increase in GLUT-4 mRNA, suggesting a higher effect in insulin sensitivity. In conclusion, the data presented here indicate that IL-15 facilitates glucose metabolism in skeletal muscle and, therefore, a possible role of the cytokine as an antidiabetogenic drug merits future investigations.     

Gap junction permeability between tenocytes within tendon fascicles is suppressed by tensile loading

Gap junction communication is an essential component in the mechanosensitive response of tenocytes. However, little is known about direct mechanoregulation of gap junction turnover and permeability. The present study tests the hypothesis that mechanical loading alters gap junction communication between tenocyte within tendon fascicles. Viable tenocytes within rat tail tendon fasicles were labelled with calcein-AM and subjected to a fluorescent loss induced by photobleaching (FLIP) protocol. A designated target cell within a row of tenocytes was continuously photobleached at 100% laser power whilst recording the fluorescent intensity of neighbouring cells. A mathematical compartment model was developed to estimate the intercellular communication between tenocytes based upon the experimental FLIP data. This produced a permeability parameter, k, which quantifies the degree of functioning gap functions between cells as confirmed by the complete inhibition of FLIP by the inhibitor 18α-glycyrrhentic acid. The application of 1N static tensile load for 10?min had no effect on gap junction communication. However, when loading was increased to 1?h, there was a statistically significant reduction in gap junction permeability. This coincided with suppression of connexin 43 protein expression in loaded samples as determined by confocal immunofluorescence. However, there was an upregulation of connexin 43 mRNA. These findings demonstrate that tenocytes remodel their gap junctions in response to alterations in mechanical loading with a complex mechanosensitive mechanism of breakdown and remodelling. This is therefore the first study to show that tenocyte gap junctions are not only important in transmitting mechanically activated signals but that mechanical loading directly regulates gap junction permeability.     

Chronic hypoxia increases insulin-stimulated glucose uptake in mouse soleus muscle

People living at high altitude appear to have lower blood glucose levels and decreased incidence of diabetes. Faster glucose uptake and increased insulin sensitivity are likely explanations for these findings: skeletal muscle is the largest glucose sink in the body, and its adaptation to the hypoxia of altitude may influence glucose uptake and insulin sensitivity. This study tested the hypothesis that chronic normobaric hypoxia increases insulin-stimulated glucose uptake in soleus muscles and decreases plasma glucose levels. Adult male C57BL/6J mice were kept in normoxia [fraction of inspired O? = 21% (Control)] or normobaric hypoxia [fraction of inspired O? = 10% (Hypoxia)] for 4 wk. Then blood glucose and insulin levels, in vitro muscle glucose uptake, and indexes of insulin signaling were measured. Chronic hypoxia lowered blood glucose and plasma insulin [glucose: 14.3 ± 0.65 mM in Control vs. 9.9 ± 0.83 mM in Hypoxia (P < 0.001); insulin: 1.2 ± 0.2 ng/ml in Control vs. 0.7 ± 0.1 ng/ml in Hypoxia (P < 0.05)] and increased insulin sensitivity determined by homeostatic model assessment 2 [21.5 ± 3.8 in Control vs. 39.3 ± 5.7 in Hypoxia (P < 0.03)]. There was no significant difference in basal glucose uptake in vitro in soleus muscle (1.59 ± 0.24 and 1.71 ± 0.15 μmol·g?1·h?1 in Control and Hypoxia, respectively). However, insulin-stimulated glucose uptake was 30% higher in the soleus after 4 wk of hypoxia than Control (6.24 ± 0.23 vs. 4.87 ± 0.37 μmol·g?1·h?1, P < 0.02). Muscle glycogen content was not significantly different between the two groups. Levels of glucose transporters 4 and 1, phosphoinositide 3-kinase, glycogen synthase kinase 3, protein kinase B/Akt, and AMP-activated protein kinase were not affected by chronic hypoxia. Akt phosphorylation following insulin stimulation in soleus muscle was significantly (25%) higher in Hypoxia than Control (P < 0.05). Neither glycogen synthase kinase 3 nor AMP-activated protein kinase phosphorylation changed after 4 wk of hypoxia. These results demonstrate that the adaptation of skeletal muscles to chronic hypoxia includes increased insulin-stimulated glucose uptake.     

A finite element model predicts the mechanotransduction response of tendon cells to cyclic tensile loading

The importance of fluid-flow-induced shear stress and matrix-induced cell deformation in transmitting the global tendon load into a cellular mechanotransduction response is yet to be determined. A multiscale computational tendon model composed of both matrix and fluid phases was created to examine how global tendon loading may affect fluid-flow-induced shear stresses and membrane strains at the cellular level. The model was then used to develop a quantitative experiment to help understand the roles of membrane strains and fluid-induced shear stresses on the biological response of individual cells. The model was able to predict the global response of tendon to applied strain (stress, fluid exudation), as well as the associated cellular response of increased fluid-flow-induced shear stress with strain rate and matrix-induced cell deformation with strain amplitude. The model analysis, combined with the experimental results, demonstrated that both strain rate and strain amplitude are able to independently alter rat interstitial collagenase gene expression through increases in fluid-flow-induced shear stress and matrix-induced cell deformation, respectively.     

Hepatic glycogen formation by direct uptake of glucose following oral glucose loading in man

The extent of direct uptake of glucose into hepatic glycogen following oral glucose loading in man is determined by mobilizing newly formed glycogen with a glucagon infusion in the immediately postabsorptive period. The amount of glucose flushed from liver glycogen is measured by using "out-of-steady-state" tracer turnover techniques. Following a 93 +/- 1 g load of glucose, at most 7.7 +/- 1 g of ingested glucose is recovered from glycogen. If the unlabelled glucose pool is taken into account, at most 10 g of available glucose can be said to be taken up directly into hepatic glycogen during the absorption of the glucose load.     

Free-flap coverage of the exposed Achilles tendon

Posterior skin loss of the distal lower leg enhances the risk of exposure of the Achilles tendon. Most commonly, these wounds are a sequela to peripheral vascular insufficiency or else posttraumatic in origin. As a consequence, local flaps or skin grafts frequently are inadequate options for achieving coverage. Free-tissue transfers have proven to be a reasonable alternative in these situations for preservation of tendon function or even limb salvage. In this series of 12 patients, small defects were best covered with fasciocutaneous flaps, whereas the larger and usually chronic, concomitantly suppurating wounds required muscle flaps. Eighty-three percent (10 of 12) of patients remained ambulatory with healed wounds, obviating the need for extremity amputation.     

The 3D structure of crimps in the rat Achilles tendon.

The ultrastructure of crimps of the Achilles tendon of rat, excised and processed in a slack condition, was investigated by atomic force microscopy in air, in fluid and by scanning electron microscopy and stereo reconstruction. The tendon was made of distinct fascicles, each comprising a succession of straight segments connected by sharp angles. The length of the segments and the interposed angles varied widely. In particular, the angles ranged from almost zero to over 135 degrees . We did not observe a unique structure for the hinge regions, but rather a variety of gradations of buckling and/or torsion with no evident correlation with other features of tendon. A constant hallmark was the local loss of regular molecular packing, as revealed by the disappearance of the D-banding. Our results do not support recent reports of a helical structure or smooth sinusoidal waves in tendons. Such structures may nonetheless exist in other non-tensile structures whose collagen fibrils exhibit a helical inner architecture and are able to follow a highly convoluted course without buckling or crimping.     

Contractile activity increases glucose uptake by muscle in severely diabetic rats

Muscle contractile activity is associated with an acceleration of glucose transport into muscle. It has been reported that the acceleration of glucose uptake by contractile activity in perfused rat muscles requires the presence of insulin in the perfusate. This claim was investigated using the perfused rat hindlimb preparation in the present study. Rats were made diabetic by injection of 125 mg/kg of streptozotocin and either studied 72 h later or maintained on insulin for 2 wk and then studied 3 days after cessation of insulin therapy. Only rats with plasma insulin levels too low to measure were used. The hindlimbs were washed out with 630 ml of medium over 75 min using a single flow-through washout before muscle stimulation. Despite the absence of insulin in the perfusion medium, stimulation of muscle contraction resulted in large increases in glucose uptake in both the diabetic and control rats. These findings do not support the claim that the stimulatory effect of muscle contraction on glucose uptake by perfused rat muscles requires the presence of insulin.