Age-related changes in the microcirculation of skeletal muscle

The age-related reduction in exercise capacity is associated with a reduction in cardiac output and maximal oxygen consumption (VO2max). The loss of muscle mass explains a large portion of the age-related decline in VO2max. The capillary supply to a muscle fibre is primarily determined by its size, but also by its metabolic profile and the metabolic profile of surrounding fibres. Thus the age-related fibre atrophy and changes in the fibre type composition are expected to be accompanied by changes in the capillarisation. The exchange of oxygen, blood-borne energy sources, metabolites and heat between the blood and muscle tissue takes place in the microcirculation. Changes in the microcirculation may thus affect the functioning and viability of the muscle. The resting blood flow is minimally affected by age, but blood flow during or following exercise is generally reduced. This may in part be due to a reduced vasodilatory capacity and a decreased capillarisation. However, the coupling between capillary supply to a fibre and its metabolic profile or the profile of the surrounding fibres is maintained. There are some changes in ultrastructure of the endothelium. The age-related changes in the microcirculation are associated with a reduced VO2max and exercise capacity. The adaptability of the microcirculation is maintained at old age.

Gender- and age-related differences in the regulatory influence of thyroid hormone on the contractility and myosin composition of single rat soleus muscle fibres

The effects of 4 weeks of thyroid hormone (3,5,3'-triiodothyronine, T3) treatment on the myosin isoform composition and maximum velocity of unloaded shortening (V0) of single soleus muscle fibres of young (3-6 months) and old (20-24 months) female (149 fibres) and male (200 fibres) rats were studied. Gender-related differences in the up-regulation of fast myosin heavy chain (MyHC) and myosin light chain (MyLC) isoforms were observed. In the female hyperthyroid rats, pure type I fibres and fibres co-expressing type I and type IIA MyHC (type I/IIA fibres) predominated. Some fibres expressed an alpha cardiac-like MyHC isoform either purely (alpha cardiac-like fibre type) or in co-expression with IIA MyHC (alpha cardiac-like/IIA fibre type). In the male hyperthyroid rats, on the other hand, all fibres were either type I/IIA or type I/IIAX. The relative quantities of fast MyLC isoforms in type I/IIA and type I/IIAX fibres was higher in female than in male hyperthyroid rats. V0 was similar in male and female control rats, and decreased with age in both genders (P<0.001). After T3 treatment, the average V0 increased (P<0.001) in females with a concomitant up-regulation of fast MyHC and fast MyLC isoforms irrespective of age. The average V0 of the pooled fibres was higher (P<0.001) in female than in male hyperthyroid rats at both ages. In conclusion, gender- and age-related differences were observed in the regulatory influence of 4 weeks' T3 treatment on myosin isoform composition and V0 in soleus fibres. These differences are presumably related to an interaction of thyroid and sex hormones in the regulation of myosin gene expression.

Morphological and functional evidence, and clinical importance, of vascular anastomoses in the latissimus dorsi muscle of the sheep

Mobilisation of the latissimus dorsi muscle as a functional graft necessarily involves division of perforating arteries that enter the distal portion of the muscle, rendering it vulnerable to ischaemic damage when the muscle is stimulated electrically. Using a fluorescent microsphere technique we showed that the blood flow contributed by the thoracodorsal artery decreases in a proximal-to-distal direction, and that of the perforating arteries in a distal-to-proximal direction, but for neither does the flow decline to zero. This is consistent with earlier reports of anastomotic connections between the 2 arterial territories. We went on to use fluorescence microscopy to demonstrate the existence of these vascular anastomoses, the first such evidence obtained under physiological conditions of pressure and flow. In clinical applications, the existence of anastomotic connections offers the prospect of maintaining flow to the distal part of the grafted muscle without the delays inherent in neovascularisation procedures.

Intramuscular pressure, force and blood flow in rabbit tibialis anterior muscles during single and repetitive contractions

The elevated intramuscular pressure (IMP) associated with sustained muscle contraction can affect blood flow, and could influence the long-term viability of functional skeletal muscle grafts. We therefore examined the relationship between force, peak IMP and blood flow in the tibialis anterior muscle of the anaesthetized rabbit. During isometric contractions. IMP was related linearly to force, and only the slope of the relationship varied between animals. During isotonic contractions, however, the highest values of IMP were found at the lowest force levels, and IMP appeared to be related to the amount and speed of shortening. During repeated isometric contractions, the ratio of IMP to force varied with time, stimulation pattern and subject. Mean blood flow did not differ appreciably between repetitive isometric contractions at duty cycles of 10-40%, and was unrelated to integrated pressure, integrated force, or depth from the surface. We conclude: (1) that IMP is unlikely to affect mean blood flow during cyclic activity that has a duty cycle less than 40%; and (2) that the clinical use of IMP as a predictor of muscle force appears to be justified only for single isometric contractions, and needs to be interpreted cautiously when contractions involve shortening or fatigue.

Effects of age and gender on shortening velocity and myosin isoforms in single rat muscle fibres

The maximum velocity of unloaded shortening (V0) and the myosin heavy chain (MyHC) and light chain (MyLC) isoform composition were determined in single fibres from soleus and extensor digitorum longus (EDL) muscles of male and female rate 3-6 and 22-24 months old. In the soleus muscle, the beta/slow (type I MyHC) isoform predominated in both young and old animals, irrespective of gender. In the EDL, fibres expressing type IIX MyHC or a combination of IIX and IIB (IIXB) MyHC isoforms were predominant in old rats, while type IIB MyHC fibres predominated in young individuals of both genders. The V0 of soleus fibres expressing the type I MyHC isoform decreased (P < 0.001) by 40% with age in spite of an unchanged MyLC composition. In the EDL, the V0 of fibres expressing IIX, IIXB and IIB MyHC isoforms did not changes with age or differ between males and females. In conclusion, similar age-related changes in V0 and MyHC composition were observed in single muscle cells from both male and female rats. The present results demonstrate that the relationship between V0 and MyHC isoform composition at the single fibre level is similar in male and female rats, and that similar qualitative changes take place during ageing in both genders.

Intramural blood flow of skeletal muscle ventricles functioning as aortic counterpulsators

BACKGROUND

Skeletal muscle ventricles (SMVs) working as aortic counterpulsators have provided long-term left ventricular assistance under experimental conditions. However, gradual deterioration of SMV pump function and rupture have been observed, and this may be related to compromised intramural blood flow during synchronized counterpulsation under systemic working conditions.

METHODS

Transformed, double-layered SMVs in 6 sheep were stimulated for 3-minute periods (5 V, 30 Hz, burst duration and delay from QRS both 40% of the cardiac cycle) to work as diastolic counterpulsators in the systemic circulation at a 1:2 (SMV:heart) and 1:1 ratio, and on a mock circulation with low-pressure loading conditions at a 1:2 ratio. Thoracodorsal artery blood flow was monitored by ultrasonic flow probe, and intramural blood flow distribution was investigated by fluorescent microspheres. Thoracodorsal venous lactate concentrations were measured before and after each period of stimulation.

RESULTS

Thoracodorsal artery blood flow increased significantly (p < 0.001) after stimulation. The magnitude of augmentation (89%; 95% confidence interval, 36% to 163%) was similar for all working conditions studied. Reactive hyperemia was observed after most 1:1 regimens but was rare after 1:2 regimens. A significant (p < 0.05) 15% increase in serum lactate levels was present after 1:1 regimens only. All regimens of stimulation resulted in a significant increase (p < 0.01) in blood flow to sections in the outer wall of the SMV, but a significant increase (p < 0.05) in blood flow to sections in the inner wall was observed only under low loading conditions.

CONCLUSIONS

Skeletal muscle ventricles subjected to 1:1 systemic counterpulsation regimens work under partly anaerobic conditions. High loading conditions may compromise SMV inner wall blood flow.

The use of coloured dye-extraction microspheres to measure blood flow in rabbit skeletal muscle: a validation study with special emphasis on repeated measurements

We evaluated the use of coloured dye-extraction microspheres for measuring blood flow in the skeletal muscle of anaesthetized rabbits. Spheres were injected into the left atrium after sternotomy or lateral thoracotomy, or into the left ventricle via the left carotid artery. Both routes of administration produced adequate mixing of microspheres with the blood. In general, the values of flow obtained from spheres of different colour were well correlated. Resting flows were difficult to measure, owing to the low number of spheres and the correspondingly low absorption of the samples. Occlusion of the capillary bed by trapped spheres was less than 10% in both tibialis anterior and extensor digitorum longus muscles after injection of a total of 15 x 10(6) spheres. Injection via the left ventricle, a closed-chest procedure, gave a more stable preparation than injections via the left atrium. However, in both cases there was a progressive decline in hyperaemic muscle blood flow during a series of injections. These results indicate that it is possible to use this technique to measure skeletal muscle blood flow in the rabbit, but that it is difficult to exploit the advantages of sequential microsphere injection in this species.

Determination of oxygen consumption in muscle during exercise using near infrared spectroscopy

The aim of this study was to determine oxygen consumption (VO2) during isometric exercise in human muscles using near infrared spectroscopy (NIRS). The technique was used to study the relationship between VO2 in the soleus muscle and the level of isometric exercise expressed as percentages of the maximum voluntary contraction (MVC). For the study 11 healthy male volunteers were recruited. Reproducibility was studied in 6 subjects. The subjects were seated in a chair with the knee joint at an angle of 90 degrees. The optodes of the NIRS instrument were attached to the lateral aspect of the soleus muscle. A horizontal bar above the knee was connected to a dynamometer. Subjects applied isometric force to the bar by producing a torque at the ankle joint. Firstly the MVC was determined. Secondly the VO2 at rest and at 5 levels of isometric exercise, ranging from 5% to 25% of MVC and increasing by 5% each stage, was measured. In all cases the VO2 at rest or during isometric contraction was determined from the decrease of the oxyhaemoglobin (O2Hb) signal immediately after arterial occlusion of the thigh. Repeated measurements showed no significant difference between trials, indicating that the measurements were reproducible. At rest a VO2 of 6.7 +/- 1.1 microMO2Hb.min-1 (mean +/- S.E.M.) was found, a result comparable with other studies. In all subjects a linear relationship was found between the VO2 and the level of exercise. The average slope of the regression lines of all individuals was 0.85 +/- 0.22 microMO2Hb.min-1.%MVC-1 (mean +/- S.E.M.). Inter-individual variation of the slopes was high and ranged from 0.28 to 2.29 microMO.Hb.min-1.%MVC-1, which can be explained by differences in fat percentage and in the measuring volume of the NIRS instrument. NIRS appeared to be a reproducible and reliable method for the non-invasive measurement of VO2 in human muscles. The method could be used to investigate regional differences as well as changes in time between muscle groups as a function of training.

The development of compensatory hypertrophy in the plantaris muscle of the rat

The aim of this investigation was to study the time course of compensatory hypertrophy (CH) over a seven week period after its surgical induction in the lower limb of the rat. CH of the left plantaris muscle of the rat was induced by denervation of the ipsilateral gastrocnemius and soleus muscles. Muscle fibres were classified as type I, Ic, IIa and IIb. Hypertrophy of the muscle was first observed about ten days after induction of CH. All fibre types appeared to contribute to this hypertrophy. During the period between four and twenty eight days there was a marked increase in the percentage of type I fibres, mainly at the expense of type IIa, as compared with control muscles. During this CH period so called 'intermediate' Ic fibres were found, indicating fibre type transition taking place. The isometric twitch time to peak tension (TPT) of the plantaris muscle was studied in situ. The TPT of CH muscles remained the same during the experimental period of seven weeks. This might be explained by the effect of the increase in type I (slow) fibres being masked by the far larger number of fast fibres, which still accounted for approximately 79% of the total number of fibres after CH.

Specific force of the rat plantaris muscle changes with age, but not with overload

Age and overload effects on the specific force of the rat plantaris muscle were investigated. The specific force was affected by age, but not by overload, inducing a 30% hypertrophy, at any age. The relative amount of non-contractile tissue only minimally affected the conclusions.

Changes in oxidative capacity and fatigue resistance in skeletal muscle

In conclusion, it appears that in general an increase in the fatigue resistance of a muscle is accompanied by an increase in its oxidative capacity. Fatigue resistance of a muscle seems to be partly determined by its oxidative capacity. On the single motor unit (Burke et al, 1973; Hamm et al, 1988; Kugelberg and Lindegren 1979; Larsson et al, 1991) and single fibre level (Nemeth et al, 1981) the relation between fatigue resistance and oxidative capacity seems to be valid. However, this does not appear necessarily to be the case on the level of the whole muscle. Kugelberg and Lindegren (1979) suggested, that the endurance of each link in the chain of events leading to contraction is under aerobic conditions matched to the contractile capacity of the fibre expressed by its oxidative enzyme activity. Therefore, it might be that several tests for endurance capacity are more strenuous than the aerobic capacity of the muscle. Indeed, several studies suggest that the Burke test (Burke et al, 1973) or other fatiguing protocols might primarily test for other endurance-related properties as the excitation-contraction coupling (Kernell et al, 1987; Mayne et al, 1991b). Another explanation for the discrepancy in changes in oxidative capacity and fatigue resistance might be, that the mechanical responses of the motor units (which have different biochemical and contractile properties) during the fatigue test do not summate linearly during whole muscle contraction as was found by Gardiner and Olha (1987).(ABSTRACT TRUNCATED AT 250 WORDS)

Capillarisation and fibre types in hypertrophied m. plantaris in rats of various ages

Influences of age, overload obtained through denervation of synergists, and training on the capillarisation of the m. plantaris were compared in 5-, 13- and 25-month-old rats in relation to different fibre types. Overload resulted in about 30% hypertrophy in each age group. Age effects were significant only in the deep (more oxidative) region of the muscle. From 5 to 13 months, the percentage of FOG fibres increased at the expense of FG fibres, while the fibre cross-sectional areas (FCSA) of each fibre type increased. From 13 to 25 months, the FCSA of FG fibres decreased, as did the local capillary-to-fibre ratio (LCFR) of each fibre type, indicating capillary loss and a declined capillary density for each fibre type (CFD). Overload effects were identical for both the superficial (more glycolytic) and the deep region for each age group. With overload, FCSA and LCFR of each fibre type increased, while CFD decreased, indicating that capillary proliferation occurred with overload, even at old age, although lagging behind increases in FCSAs. Training showed minor effects.

Metabolic capacity, fibre type area and capillarization of rat plantaris muscle. Effects of age, overload and training and relationship with fatigue resistance

1. The influences of age (5, 13 and 25-month-old rats), overload as obtained by denervation of synergists, and training on the metabolic capacity, relative muscle cross-sectional area occupied by each fibre type, capillarization and fatigue resistance of the rat m. plantaris were investigated. 2. Creatine kinase, phosphorylase and citrate synthase activities were lower in muscles of 25 than in those of 13-month-old rats (P < 0.001). 3. Overload resulted in an increased relative area of type I and IIa fibres at all ages (P = 0.001). 4. Capillary density decreased with overload and increasing age (P < 0.001). 5. Fatigue resistance was higher in muscles of 13 than in those of 5-month-old rats (P < 0.05), and increased with overload (P < 0.05) at all ages. 6. Fatigue resistance of the whole muscle was not closely related to its oxidative capacity in contrast to what is generally found for single fibres or motor units.

Compensatory hypertrophy and training effects on the functioning of aging rat m. plantaris

The influence of age, compensatory hypertrophy and training on isometric contraction characteristics of rat m. plantaris were investigated in 5-, 13- and 25-month-old rats. Each age group was subdivided into Control Not Trained, Control Trained, Operated Not Trained and Operated Trained groups. Run training was applied. The Operation i.c. denervation of synergists induced a 30% compensatory hypertrophy (P < 0.001). Age and training did not significantly affect muscle weight. The tetanic force was highest in 13-month-old and lowest in 25-month-old muscles as was the tetanic force/muscle weight (P < 0.001). Trained and hypertrophied muscles had increased tetanic force (P < 0.01), but tetanic force/muscle weight was not significantly affected. Twitch contraction time was longer in hypertrophied muscles than in controls (P < 0.001). Half relaxation time decreased with training (P < 0.05). Resistance to fatigue was increased in hypertrophied muscles as compared to controls (P < 0.04) and was lower in 5-month-old than in 13-month-old muscles, but their fatigue resistance did not differ from the 25-month-old muscles. Each age group showed comparable effects of training and hypertrophy on the contraction characteristics. These results indicate that the force generating capacity of the M. plantaris was optimal in the 13-month-old animals compared to the other age groups. Furthermore, our data show that aged muscles react in a similar way to increased functional demand as muscles of younger age.

The relationship between capillarisation and fibre types during compensatory hypertrophy of the plantaris muscle in the rat

Compensatory hypertrophy of the plantaris muscle was obtained by denervation of its synergists. This hypertrophy is characterised by a 32% increase in muscle mass. The muscle consists of type I and IIa (oxidative), and IIb (glycolytic) fibres. Fibres of all types were enlarged in hypertrophied muscles and the proportion of type I fibres was increased. We investigated the capillarisation after hypertrophy as related to fibre type. In order to obtain this information a new technique was used, capable of estimating not only the traditional overall capillary density (CD) but also an index of heterogeneity in capillary spacing (LogSD), the 'local capillary to fibre ratio' (LCFR), obtained separately for each muscle fibre type, and finally a capillary density for each respective fibre type, the 'capillary fibre density' (CFD). It was found in both control and hypertrophied muscles that CD was higher in the deep (few IIb fibres) than in the superficial part of the muscle (considerable number of IIb fibres). The LogSD was lower, indicating less heterogeneity, in the deep than in the superficial part of the muscle. The LCFR and CFD of each fibre type was greater in the deep than in the superficial region of both control and hypertrophied muscles. Furthermore the CFD and LCFR were larger in type I and IIa fibres than in IIb fibres in each region of control and hypertrophied muscles. In hypertrophied muscles the CD was not significantly different from that of control muscles. However, LCFR of all fibre types was increased significantly in hypertrophied muscles as compared with controls, demonstrating capillary proliferation. The decreased CFD of type I and IIa fibres in the deep region of hypertrophied muscles as compared with controls suggests that here the capillary proliferation lags behind the increase in muscle mass. Endurance training had no significant effects for any region in any of the indices that were used.