Changes in the capillarity of skeletal muscle in the growing rat

Capillary density (CD), capillary to fiber ratio (C/F), fiber cross sectional area (FCSA) and fiber composition were measured in the soleus and the gastrocnemius (medial head) muscles of rats weighing between 99 and 666 g. Muscle samples obtained from the anesthetized animal were rapidly frozen (–130°C) sliced transversely at 16–18 m, and treated histochemically by the ATPase method after preincubation at pH's of 4.0 and 4.4 to visualize capillaries and typify fibers. In both muscles the FCSA was positively related to body weight (BW) and muscle weight. At a given BW, the FCSA of the soleus was greater than that of the gastrocnemius. In both muscles CD decreased hyperbolically with FCSA (soleus: CD=1.0613 ×10⁶/FCSA+298.71; gastrocnemius: CD=1.0349 ×10⁶/FCSA+240.74). At the same time a positive linear correlation between C/F and FCSA was found (soleus: C/F=3.92×10^–4 FCSA+0.82; gastrocnemius: C/F=2.90×10^–4 FCSA+0.93). At a given FCSA, CD and C/F were greater in the soleus than in the gastrocnemius because of differences in fiber composition between the two muscles. The soleus had only oxidative fibers (STO and FTOG) whereas the gastrocnemius had 54% glycolytic fibers (FTG). The very large variability in CD and C/F values reported in the literature could, in part, be due to the differences in capillarity observed with maturation. A change in fiber composition with BW was observed in the soleus, but no systematic change occurred in the gastrocnemius.     

Adaptations in rat skeletal muscle following long-term resistance exercise training

The purpose of this investigation was to determine whether long-term, heavy resistance training would cause adaptations in rat skeletal muscle structure and function. Ten male Wistar rats (3 weeks old) were trained to climb a 40-cm vertical ladder (4 days/week) while carrying progressively heavier loads secured to their tails. After 26 weeks of training the rats were capable of lifting up to 800?g or 140% of their individual body mass for four sets of 12–15 repetitions per session. No difference in body mass was observed between the trained rats and age-matched sedentary control rats. Absolute and relative heart mass were greater in trained rats than control rats. When expressed relative to body mass, the mass of the extensor digitorum longus (EDL) and soleus muscles was greater in trained rats than control rats. No difference in absolute muscle mass or maximum force-producing capacity was evident in either the EDL or soleus muscles after training, although both muscles exhibited an increased resistance to fatigue. Individual fibre hypertrophy was evident in all four skeletal muscles investigated, i.e. EDL, soleus, plantaris and rectus femoris muscles of trained rats, but muscle fibre type proportions within each of the muscles tested remained unchanged. Despite an increased ability of the rats to lift progressively heavier loads, this heavy resistance training model did not induce gross muscle hypertrophy nor did it increase the force-producing capacity of the EDL or soleus muscles.     

Effects of hypoxia on capillary density and fiber composition in rat skeletal muscle

Summary Capillary density, fiber composition and myoglobin concentration were determined in skeletal muscle of Sprague-Dawley female rats. P_IO₂ for control (C) rats was 123 mm Hg; for hypoxic (H) rats the O₂ concentration in air was lowered to 12.6% (P_IO₂=73 mm Hg) over 4 days and then kept at this level for 39 days. Body weight (BW) in C rats went from 182 to 258 g in 6 weeks. H rats showed an initial 13 g drop in BW but later gained weight at the same rate. A difference of 20 g was maintained between C and H rats. Five groups of about 10 rats each (3 C and 2 H) were studied by histochemistry using frozen samples from the tibialis anterior, gastrocnemius (medial head) and soleus muscles. 20 m slices, cut transversely, were treated for alkaline phosphatase and ATPase after preincubation at pH's of 3.8 to 9.4 in order to visualize capillaries and typify fibers. In both C and H rats, capillary density decreased at the same rate with increasing BW, as a consequence of the increase in the size of the fibers resulting from growth. At the same BW capillary densities for a given muscle were not statistically different. No consistent differences in fiber composition were found in the soleus after exposure to hypoxia. In the gastrocnemius and the tibialis anterior, however, the percentage population of white fibers tended to be higher in the H rats than in the C rats, while the population of red fibers decreased. Myoglobin concentration was higher in the soleus, a muscle with a preponderant intermediate fiber population, than in the other two muscles. Hypoxia did not systematically affect myoglobin concentration.Supported by GRS 523 and HL 18145, N.I.H.Supported by a fellowship from the Rockefeller Fndn.     

Effect of lipid additives on oxygen supply of skeletal muscle

R. E. Kavetskii Institute for Problems in Oncology, Academy of Sciences of the Ukrainian SSR, Kiev. Research Institute of Food Hygiene, Republican Scientific Hygienic Center, Ministry of Health of the Ukrainian SSSR, Kiev. (Presented by Academician of the Academy of Medical Sciences of the USSR G. N. Kryzhanovskii.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 112, No. 8, pp. 137–139, August, 1991.     

Effect of growth on muscle capillarity and fiber type composition in rat diaphragm

Summary The effect of growth on the capillarity and fiber type composition of the diaphragm, soleus and extensor digitorum longus (EDL) muscles of rats weighing between 55 and 330 g have been studied. Muscle samples obtained from the anesthetized rat were rapidly frozen and sliced transversely in a cryostat. The sections were stained histochemically by the SDH method and the myosin ATPase method after preincubation at pH 4.3 to typify fibers (FG, FOG and SO fibers). To visualize capillaries, the myosin ATPase method after preincubation at pH 4.0 was used. The percentage of FOG fibers decreased in all muscles with growth. While the FG and SO fibers increased in the diaphragm, SO fibers increased in the soleus, and FG fibers increased in the EDL. The capillary density showed a hyperbolic decrease with growth in all muscles, while the number of capillaries around each fiber increased in all muscles with growth. It is concluded that growth causes the changing properties of the motoneurons and the new capillary formation in the diaphragm muscle, as well as the soleus and EDL muscles.     

The effect of long-term administration of α1-blocker prazosin on capillary density in cardiac and skeletal muscle

The effect of prazosin on heart and muscle blood flow and capillary density was studied in rats. In acute experiments, ₁-blocker prazosin almost trebled blood flow in fast skeletal muscles [tibialis anterior (TA) and extensor digitorum longus (EDL)], but did not affect coronary flow when infused i.v. at a dose of 0.5 g · ml^–1 · min^–1 for 30 min. Prazosin in an equivalent dose was then given orally over a period of 5 weeks to investigate its effect on capillarisation in heart and skeletal muscle. Capillary density (CD, capillaries · mm^–2), estimated in frozen sections stained for alkaline phosphatase, was similar in the hearts of prazosin-treated and control rats. Capillary/fibre ratio in skeletal muscles increased from 1.52±0.019 in control EDL to 1.69±0.01 (P<0.001) and from 1.56±0.04 in control TA to 2.16±0.04 (P<0.001). In TA, the increase was greater than in EDL both in the glycolytic periphery (from 1.30±0.13 to 1.75±0.11, P<0.025) and the oxidative core of the muscle (from 1.837±0.14 to 2.51±0.12, P<0.005). Unilateral crush of the lateral peroneal nerve and subsequent reinnervation over the next 7 weeks resulted in redistribution of fibre types from a typical mosaic pattern into groups composed of fibres of similar oxidative capacity. Capillary density as well as capillary/fibre ratio in purely glycolytic areas was lower when compared to supply of glycolytic fibres in normal muscles. Oral administration of prazosin over the whole period of reinnervation not only maintained the original level of capillarity associated with fast glycolytic fibres in control muscles, but considerably increased it. Thus long-term prazosin administration not only causes an increase in capillary supply in normal muscles but also prevents loss of capillaries during reinnervation. The fact that it only increases capillarisation in tissues where it increases flow further supports the hypothesis that capillary growth can be initiated by mechanical factors connected with high blood flow.     

Regional changes in the masseter muscle of rats after reduction of blood supply

The masticatory musculature is an integral functional part of the stomatognathic system and influences craniofacial morphogenesis and morphology. This animal study aimed to investigate the morphological consequences of restricted regional blood supply to the m. masseter. A total of 20 adult male Wistar rats (Rattus norvegicus Berkenhout) were divided into an experimental group and a control group comprised of 10 animals respectively and kept under standardized conditions. The experimental group underwent a dextrolateral complete surgical ligation of the a. carotis communis and, after 5 weeks, specimens were taken from the masseters. The muscle samples were analyzed immunohistochemically for fiber distribution and capillary density. Analysis revealed a discrete increase in the proportion of type I fibers with a significant increase of capillary number per area. Although no agreement exists on the alterations occurring in chronically ischemic muscles, it may be assumed that chronic ischemia evokes histomorphological adaptation processes similar to endurance training effects.     

Dynamics of oxygen uptake following exercise onset in rat skeletal muscle

Technical limitations have precluded measurement of the V(O(2)) profile within contracting muscle (mV(O(2))) and hence it is not known to what extent V(O(2)) dynamics measured across limbs in humans or muscles in the dog are influenced by transit delays between the muscle microvasculature and venous effluent. Measurements of capillary red blood cell flux and microvascular P(O(2)) (P(O(2)m)) were combined to resolve the time course of mV(O(2)) across the rest-stimulation transient (1 Hz, twitch contractions). mV(O(2)) began to rise at the onset of contractions in a close to monoexponential fashion (time constant, J = 23.2 +/- 1.0 sec) and reached it's steady-state value at 4.5-fold above baseline. Using computer simulation in healthy and disease conditions (diabetes and chronic heart failure), our findings suggest that: (1) mV(O(2)) increases essentially immediately (< 2 sec) following exercise onset; (2) within healthy muscle the J blood flow (thus O(2) delivery, J Q(O(2)m)) is faster than JmV(O(2)) such that oxygen delivery is not limiting, and 3) a faster P(O(2)m) fall to a P(O(2)m) value below steady-state values within muscle from diseased animals is consistent with a relatively sluggish Q(O(2)m) response compared to that of mV(O(2)).     

Effects of chronic hypoxia and endurance training on muscle capillarity in rats

Skeletal muscle capillarity expressed as capillary density (CD), and number of capillaries per fibre (C/F), as well as the mean fibre cross-sectional area (FCSA), were determined in the extensor digitorum longus (EDL), plantaris (PLA) and soleus (SOL) muscles of four groups of eight rodents trained on a swimming exercise programme (T) or maintained sedentary (S), at sea level (SL) or at simulated altitude (HA), barometric pressure 61.7 kPa (463 torr) for 12 weeks. It was shown that both HA exposure and endurance training decreased body and skeletal muscles weights (P<0.001). However, neither HA exposure nor endurance training induce any variation in relative importance in the skeletal muscle mass. Altitude exposure and endurance training had increasing effects on CD in all muscles studied (P<0.001). This study confirms the fact that altitude exposure has no direct effect on capillary development. On the other hand, the capillary supply of the several slow- and fast- twitch skeletal muscles studied is increased by endurance training. This real enhancement in capillary network is ascertained by an increase in the C/F ratio (+7%, +26%, +16%, in PLA, EDL, and SOL muscles, respectively at sea level, and +19.5%, +30%, and +14% respectively at HA). These results indicate that the effects of chronic exercise on skeletal muscle capillarity estimated by the C/F ratio, are greater in an hypobaric environment than in a SL environment.     

Heart rate changes caused by varying the oxygen supply to isolated hind legs of rats

Summary In a rat with an isolated hind leg circulation perfused with varying tyrode solutions, heart rate (HR) changes were studied in dependence of in the isolated hind leg and of , [K⁺], pH and lactic acid concentration ([Lac]) measured in the venous outflow of the isolated hind leg. In experimental series I the inflow was kept constantly high (either about 65 or 72 kPa). The perfusion pressure alternated between 16 and 24 kPa leading to flow rates in isolated hind legs ( ) from 30 to 50 ml · 100 g⁻¹ · min⁻¹. The depended on the momentary (flow-limited oxygen uptake). The [K⁺] and [Lac], the pH and the remained nearly constant while the was lower at small flow rates. The HR decreases some 4 min after initial enhancement of and . Series II comprised experiments with low flow rates and a medium oxygen supply ( =2.5−17.4 ml · 100 g⁻¹ · min⁻¹), =17.5−62.7 kPa). The ranged between 0.02 and 0.2 ml · 100 g⁻¹ · min⁻¹. The [K⁺] and [Lac], the and the HR increased while the pH decreased. The [Lac] in the outflow showed a strong dependence on oxygen uptake and — at a weak oxygen supply — on the time. Cross-correlation analyses between the parameters confirmed that the HR was best temporally correlated to the [Lac] in the outflow. In series III a 17 min perfusion of normoxic solution ( =65.3 kPa) was followed by perfusion with a hypoxic tyrode solution ( =8.7 kPa). was 30 ml · 100 g⁻¹ · min⁻¹. The [Lac], the and the HR increased accompanied by a decrease in pH. However a HR increase was observed only when the actual values of [Lac], and pH exceeded their normal ranges for a resting muscle. The results support the hypothesis that heart rate is additionally influenced by metabolic muscle receptors measuring lactic acid concentration in working muscle.     

Expression of angiogenic regulators and skeletal muscle capillarity in selectively bred high aerobic capacity mice

Selective breeding for high voluntary wheel running in untrained mice has resulted in a 'mini muscle' (MM) phenotype, which has increased skeletal muscle capillarity compared with muscles from non-selected control lines. Vascular endothelial growth factor (VEGF) and thrombospondin-1 (TSP-1) are essential mediators of skeletal muscle angiogenesis; thus, we hypothesized that untrained MM mice with elevated muscle capillarity would have higher basal VEGF expression and lower basal TSP-1 expression, and potentially an exaggerated VEGF response to acute exercise. We examined skeletal muscle morphology and skeletal muscle protein expression of VEGF and TSP-1 in male mice from two (untrained) mouse lines selectively bred for high exercise capacity (MM and Non-MM), as well as one non-selected control mouse line (normal aerobic capacity). In the MM mice, gastrocnemius (GA) and plantaris (PLT) muscle capillarity (i.e. capillary-to-fibre ratio and capillary density) were greater compared with control mice (P < 0.05). In Non-MM mice, only muscle capillarity in PLT was greater than in control mice (P < 0.001). The soleus (SOL) showed no statistical differences in muscle capillarity among groups. In the GA, MM mice had 58% greater basal VEGF (P < 0.05), with no statistical difference in basal TSP-1 when compared with control mice. In the PLT, MM mice had a 79% increase in basal VEGF (P < 0.05) and a 39% lower basal TSP-1 (P < 0.05) compared with the control animals. Non-MM mice showed no difference in basal VEGF in either the GA or the PLT compared with control mice. In contrast, basal TSP-1 was elevated in the PLT, but not in the GA, of Non-MM mice compared with control mice. Neither VEGF nor TSP-1 was significantly different in SOL muscle among the three mouse lines. In response to acute exercise, MM mice displayed a 41 and 28% increase (P < 0.05) in VEGF in the GA and PLT, respectively, whereas neither control nor Non-MM mice showed a significant VEGF response to acute exercise. In contrast, TSP-1 levels were decreased by 90% in GA (P < 0.05) but increased by 50% in PLT (P < 0.05) in response to acute exercise in MM mice. The SOL showed no response to exercise for either VEGF or TSP-1 for any of the mouse lines. These data, with the exception of the Non-MM plantaris muscle, suggest that elevated capillarity is associated with altered balance between positive and negative angiogenic regulators (i.e. VEGF versus TSP-1, respectively). Based on the greater capillarity and significant VEGF response to exercise in MM mice, these data suggest that VEGF expression may, at least in part, be genetically determined.     

Global deletion of thrombospondin-1 increases cardiac and skeletal muscle capillarity and exercise capacity in mice

Thrombospondin-1 (TSP-1) is a known inhibitor of angiogenesis; however, a skeletal muscle phenotype of TSP-1 null mice has not been investigated. The purposes of this study were to compare and contrast TSP-1 null and wild-type mice by examining the following: (1) capillarity in the skeletal and cardiac muscles; (2) fibre type composition and oxidative enzyme activity in the hindlimb; and (3) the consequences of TSP-1 gene deletion for exercise capacity. In TSP-1 null mice, maximal running speed was 11% greater and time to exhaustion during submaximal endurance running was 67% greater compared with wild-type mice. Morphometric analyses revealed that TSP-1 null mice had higher ( P < 0.05) capillarity in the heart and skeletal muscle than wild-type mice, whereas no differences for fibre type composition or oxidative enzyme activity were present between the two groups. Cardiac function, as measured by transthoracic echocardiography, revealed no difference in myocardial contractility but greater left ventricular end-diastolic and systolic dimensions, corresponding to an elevated heart mass in the TSP-1 null mice. The results of this study indicate that TSP-1 is an important endogenous negative regulator of angiogenesis that prevents excessive capillarization in the heart and skeletal muscles. The increased capillarity alone was sufficient to increase ( P < 0.05) exercise capacity. These data demonstrate that the capillary-to-muscle interface is a critical factor that limits oxygen transport during exercise.     

Capillary supply and lipoprotein-lipase activity in skeletal muscle in man

Circulating plasma triglycerides cannot be taken up by different tissues until they have been hydrolysed by lipoprotein lipase (Robinson 1970). The enzyme is located on the intraluminal surface of the capillaries (Scow et al. 1976). It is synthesised in adipocyte (Scow et al. 1976) and (heart) muscle cells (Chohan & Cryer 1978, Bagby et al. 1977) and transferred to the surface of the capillary endothelial cells, migrating over this surface to reach the intraluminal area (Olivecrona et al. 1976). Hydrolysis by lipoprotein lipase is a rate-limiting step in the removal process of plasma triglycerides and is therefore a determining factor for the plasma triglyceride concentration (Robinson 1970).     

Time course of capillary structure changes in rat skeletal muscle following strenuous eccentric exercise

AIM: We examined the time course of capillary structure changes in rat skeletal muscle at 1, 3 and 7 days after strenuous eccentric exercise. METHODS: The right gastrocnemius muscles of anaesthetized male Wistar rats were subjected to 300 controlled eccentric contractions using electrical stimulation. The contralateral gastrocnemius muscle was used as control. All morphometric parameters were determined in in situ perfused gastrocnemius muscles in red (Gr, predominantly slow-twitch fibre) and white (Gw, predominantly fast-twitch fibre) portions. RESULTS: Muscle fibre damage was evident on days 1, 3 and 7 in Gr (29.3-53.9% damaged fibres) and Gw (58.9-86.8% damaged fibres) of exercised legs. Electron micrographs of transverse sections did not display collapsed or obstructed capillaries in exercised legs, and capillary endothelial cells retained their normal structures. However, capillary luminal shapes and area were altered in exercised legs on days 1 and 3. The ratio between minimal and maximal capillary diameter in a transverse section (i.e. luminal ellipticity) significantly differed when comparing control (Gr, 0.75 +/- 0.02; Gw, 0.79 +/- 0.03) and exercised legs (Gr, 0.65 +/- 0.03; Gw, 0.66 +/- 0.04) at 1 day after exercise. The mean capillary luminal area was significantly increased in exercised legs after 1 day (Gw, +24.3%) and 3 days (Gr, +31.9%; Gw, +62.2%) compared with control. CONCLUSION: We conclude that (1) capillary endothelial cell structure was maintained in damaged muscles, (2) changes in capillary lumen shapes and distensibility occur in the degenerated muscle up to 3 days after the eccentric contraction period.     

Enzymatic changes in hypertrophied fast-twitch skeletal muscle

Summary Fast-twitch plantaris muscles of female rats were subjected to unilateral compensatory overload, induced by partial excision of synergistic muscles. One group of rats remained sedentary whereas another was subjected to a supplemental program of treadmill exercise consisting of walking 3 m/min, 65% grade, 2 h/day, 5 days/week. Groups of rats were sacrificed after 1, 2, 4, and 8 weeks and their muscles were weighed and analyzed for protein, citrate synthase, phosphofructokinase (PFK) and myofibril ATPase. Absolute and relative (muscle weight/body weight) muscle weights were much greater in both overloaded groups as compared to contralateral controls. However, treadmill exercise also increased the absolute and relative muscle mass of control plantaris muscles in the exercising group as compared to normal sedentary contralateral controls. Citrate synthase activity was decreased in overloaded, sedentary muscles as compared to contralateral controls, but after 8 weeks of exercise, it returned to normal levels. PFK was decreased in both sedentary and exercised overloaded muscles througout the 8 week period. Myofibril ATPase showed a tendency to be reduced in sedentary, overloaded muscles, and was significantly reduced in overloaded, exercising muscles. These results collectively suggest that certain fibers of overloaded fast-patterns take on similar in certain aspects to that normally seen in differentiated slow-twitch muscle fibers.     

Role of erythrocyte‐released ATP in the regulation of microvascular oxygen supply in skeletal muscle

In a 1914 book entitled The Respiratory Function of the Blood, Joseph Barcroft stated that ‘the cell takes what it needs and leaves the rest’. He postulated that there must be both a ‘call for oxygen’ and a ‘mechanism by which the call elicits a response…’ In the past century, intensive investigation has provided significant insights into the haemodynamic and biophysical mechanisms involved in supplying oxygen to skeletal muscle. However, the identification of the mechanism by which tissue oxygen needs are sensed and the affector responsible for altering the upstream vasculature to enable the need to be appropriately met has been a challenge. In 1995, Ellsworth et al. proposed that the oxygen‐carrying erythrocyte, by virtue of its capacity to release the vasoactive mediator ATP in response to a decrease in oxygen saturation, could serve both roles. Several in vitro and in situ studies have established that exposure of erythrocytes to reduced oxygen tension induces the release of ATP which does result in a conducted arteriolar vasodilation with a sufficiently rapid time course to make the mechanism physiologically relevant. The components of the signalling pathway for the controlled release of ATP from erythrocytes in response to exposure to low oxygen tension have been determined. In addition, the implications of defective ATP release on human pathological conditions have been explored. This review provides a perspective on oxygen supply and the role that such a mechanism plays in meeting the oxygen needs of skeletal muscle.