Anatomical changes in chronically stimulated skeletal muscles.
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Animals↗
Biomedical subjects
O Hudlicka
Publications and source records attributed to O Hudlicka.
The role of blood flow and/or muscle hypoxia in capillary growth in chronically stimulated fast muscles.
Capillary supply, the proportion of oxidative fibres and blood flow were studied in fast rat muscles (tibialis anterior, TA, and extensor digitorum, EDL) made ischaemic by ligation of the common iliac artery, in chronically stimulated muscles and in ischaemic chronically stimulated muscles. Stimulation was carried out for 6 h/day at 10 Hz (three periods of 2 h with 90-120-min intervals between stimulations) for 10-12 days using electrodes implanted in the vicinity of the lateral popliteal nerve. Blood flow (measured by radioactive microspheres) was 3.62 +/- 0.52 ml.100 g-1.min-1 at rest and 78.4 +/- 14.6 ml.100 g-1.min-1 (mean +/- SEM) during isometric contractions at 4 Hz. Ischaemic muscles had significantly lower blood flow at rest as well as during contractions (72 +/- 14% and 25 +/- 4% of the values in contralateral muscles respectively). Stimulated muscles had significantly higher flow than contralateral control muscles during contractions; stimulated ischaemic muscles had normal blood flow at rest, but the increase in flow during contractions was limited to a similar extent to that in ischaemic muscles alone. Of all anatomically present capillaries (staining for alkaline phosphatase in frozen sections) the capillary/fibre ratio increased by 36% in stimulated tibialis anterior, but was not significantly different from control muscles in stimulated ischaemic TA and was even lower than in control muscles in stimulated ischaemic EDL. The proportion of fast oxidative fibres (estimated on the basis of histochemical staining for myosin ATPase and succinate dehydrogenase) increased from 53.2 +/- 3.2% in normal EDL to 82.0 +/- 2.3% in chronically stimulated EDL and to 100% in chronically stimulated ischaemic muscles.(ABSTRACT TRUNCATED AT 250 WORDS)
Adenosine Triphosphatases↗
The response of muscle to enhanced and reduced activity.
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Animals↗
Changes in the microcirculation in slow and fast skeletal muscles with long term limitations of blood supply.
STUDY OBJECTIVE: Long term limitation of blood flow leads to decreased muscle performance and to damage, particularly in predominantly oxidative muscles. This study was performed to assess to what extent such damage can be explained by changes in microcirculation in capillaries supplying glycolytic or oxidative muscle fibres. DESIGN: Velocity and intermittency of flow, capillary diameters and proportion of adhering leucocytes were measured in capillaries in the epi-illuminated superficial glycolytic layer of rat tibialis anterior and in oxidative soleus muscles five weeks after ligation of the common iliac artery. EXPERIMENTAL MATERIAL: Male Sprague Dawley rats (approximately 350 g) were used, under sodium pentobarbitone anaesthesia. MEASUREMENTS AND RESULTS: Pressure in the saphenous artery, distal to the ligation site, recovered to 61% of carotid mean pressure. In tibialis anterior with limited blood supply, capillary diameters were narrower, red blood cell velocity was greater and intermittency of flow was reduced, resulting in a calculated volume flow per capillary which was only marginally reduced compared with controls. In soleus with limited blood supply, the time spent stationary by red blood cells was increased and there was a greater adhesion of white blood cells to venular walls. In neither muscle did limitation of blood supply result in a change in capillarity. CONCLUSIONS: Long term limitation of blood supply does not diminish the capillary density or capillary to fibre ratio of anatomically present capillaries in oxidative or glycolytic muscles and does not affect blood flow in capillaries supplying glycolytic fibres appreciably; circulation in capillaries supplying oxidative fibres is impaired.
Animals↗
Effect of activity on performance and morphology in ischaemic rat slow muscles.
Muscle performance and structure was studied in rat soleus muscle with limited blood supply in combination with chronic muscle stimulation. Blood supply to the lower leg was restricted by ligation of the common iliac artery, electrodes were implanted in the vicinity of the sciatic nerve and ankle flexors were denervated. Three days later, soleus and gastrocnemius muscles were stimulated at 4 Hz four times a day for a period of 20 min with 2 h intervals between stimulations; this procedure was continued for 4 days. Muscle performance, histochemistry and ultrastructure were studied on the eighth day after operation in these muscles and in ischaemic unstimulated muscles with denervated ankle flexors. Both were compared with control animals. Muscles with limited blood supply developed less isometric twitch tension than control muscles (peak twitch tension in ischaemic muscle was 60.3 +/- 4.8 g g-1 muscle, mean +/- S.E.M., compared to 79.7 +/- 6.9 g g-1 in control muscle; tensions after 5 min contraction were 54.5 +/- 5.5 g g-1 in ischaemic muscle compared to 70.6 +/- 6 g g-1 in controls). Stimulated muscles with limited blood supply had higher peak (85 +/- 16.6 g g-1) and final (87 +/- 12 g g-1) tensions, and also fatigued less than muscles with limited blood supply but no stimulation. Histochemical estimation of capillary density (by staining for alkaline phosphatase) and slow (SO) and fast (FOG) fibres (by myosin ATPase staining) revealed similar capillary to fibre ratios (2.5) and a similar proportion of FOG fibres (around 18%) in all muscles. The proportion of glycogen-depleted fibres (estimated from the periodic acid Schiff reaction, PAS) in muscles removed from animals 10 min after a 5 min period of isometric twitches was significantly lower in ischaemic muscles (45.1 +/- 1.9%) than in control (80.5 +/- 1.5%) or chronically stimulated ischaemic muscles (67.3 +/- 4.0%). Electron microscopy showed disorganised myofibrils with Z-line streaming in 7.48 +/- 3.04% of fibres in muscles with limited blood supply. Swollen and degenerated mitochondria, dilated sarcoplasmic reticulum and areas of disrupted sarcolemma were also observed. Stimulated ligated muscles showed a significantly lower proportion of fibres with disorganised filaments (0.65 +/- 0.32%) and other signs of damage were much less frequent. The reduced damage and improved performance of chronically stimulated slow muscle may be the result of improved microcirculation, preventing accumulation of lactate.
Animals↗
Collateral circulation in skeletal muscles: effect of pentoxifylline and torbafylline.
Vascular casts were prepared using Batson Anatomical corrosion compound, from maximally dilated rat tibialis anterior (TA), extensor digitorum longus (EDL) and soleus (SOL) with normal and limited blood supply (unilateral ligation of the common iliac artery). The weight of casts was expressed as a percentage of muscle wet weight. This value was considered representative for the size of the vascular bed. The value was 2.9 +/- 0.12% (mean +/- SEM) in normal TA, 3.37 +/- 0.15% in EDL and 4.8 +/- 0.43% in SOL. The weight of vascular casts decreased in all muscles to 40% of the values found in the contralateral muscles one week after ligation. Five weeks after ligation, it was about 60% of the control values in TA and EDL, and 80% in SOL. Treatment with torbafylline for one week did not change the weight of the vascular casts. After five weeks the size of the vascular bed was back to control values in EDL and SOL, and only 20% lower than control muscles in TA. This could be attributed to improved development of collateral circulation. Treatment with pentoxifylline improved collateral circulation only in SOL.
Animals↗
The effect of long-term administration of alpha 1-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, alpha 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 microgram.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 less than 0.001) and from 1.56 +/- 0.04 in control TA to 2.16 +/- 0.04 (P less than 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 less than 0.025) and the oxidative core of the muscle (from 1.837 +/- 0.14 to 2.51 +/- 0.12, P less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Beneficial effect of chronic bradycardial pacing on capillary growth and heart performance in volume overload heart hypertrophy.
We have previously reported that chronic bradycardial pacing increases both capillary density/mm2 (CD) and maximal work output in normal rabbit hearts. This technique has now been applied to rabbits with volume-overload hypertrophy due to lesion of the aortic valve. Four groups of animals were studied: controls (C), paced (P), valve-lesioned (VL), and paced valve-lesioned (PVL). The aortic valve was lesioned 8 weeks before the acute experiments; pacing was started 4 weeks before the acute experiments, and thus, the PVL group had developed hypertrophy before pacing was started. The degree of hypertrophy was similar in VL hearts whether paced or not: heart wt/body wt ratio increased by 33.5 +/- 8.9% (mean +/- SEM) in VL and 25.2 +/- 8.2% in PVL versus control animals of similar body weight (p less than 0.001). The hearts of the PVL animals showed a higher CD (2,277 +/- 107) than VL hearts (1,383 +/- 43), CD in C hearts of similar weights being 1,595 +/- 103, and in P hearts 2,350 +/- 194. Thus, CD was lower by 14% in VL and higher by 43% in PVL than in C hearts. Valve-lesioning had a significant effect in reducing maximal cardiac minute work (p less than 0.001), whereas pacing significantly improved maximal cardiac minute work (p less than 0.001) to 2.467 +/- 0.206 J/g x 10(-4) in the P group versus 1.609 +/- 0.105 in the C group.(ABSTRACT TRUNCATED AT 250 WORDS)
Analysis of Variance↗
The effect of long-term activity on the microvasculature of rat glycolytic skeletal muscle.
Long-term electrical stimulation of a fast skeletal muscle (rat tibialis anterior, TA), at 10 Hz, 8 hours/day for 7 days, caused an increase in capillarity, determined both as capillary density/mm2 and as capillary to muscle fibre ratio (C/F), and an increase in the weight of corrosion cast of the entire vascular bed. Chronic muscle activity caused a greater linear velocity of RBCs (Vrbc) in the capillaries supplying predominantly glycolytic fibres of the resting TA. During contractions imposed acutely, the absolute value attained was greater than in control TA, but the increase over resting Vrbc was not as great. Factors which did not change with long-term electrical stimulation were the individual capillary dimensions and the intermittency of flow; those remained similar to control values. It was calculated from capillary dimensions and density, and from the increase in corrosion cast/muscle wet weight ratios, that capillary neoformation alone could account for the growth of the microvasculature in the predominantly glycolytic surface layers of TA, but not necessarily in the oxidative core or in other mixed skeletal muscles.
Animals↗
Can growth of capillaries in the heart and skeletal muscle be explained by the presence of an angiogenic factor?
Capillary growth was induced in rabbit hearts by long-term bradycardial pacing, and in skeletal muscles by long-term electrical stimulation. In order to find out what factors may be responsible for it, samples of all tissues were analysed for angiogenic activity (AA). To estimate the possible role of mechanical factors, blood flow was measured at rest and during maximal dilatation. Rabbit hearts were paced at half the normal frequency for 24 h/day for 1-2 months by electrodes implanted in the right atrium. Gastrocnemius-plantaris muscles were stimulated at 10 Hz via implanted electrodes, 8 h/day for 14 days. Unpaced hearts and non-stimulated muscles served as controls. Capillary density (estimated in frozen cross-sections stained for alkaline phosphatase) was higher in paced than in control hearts (2235 +/- 86, s.e.m. cap/mm2, 1815 +/- 83, P less than 0.005); capillary/fibre ratio was 2.84 +/- 0.21 in stimulated and 1.243 +/- 0.06 in control gastrocnemius (P less than 0.001). The presence of AA was assayed on chicken chorioallantoic membrane (CAM). All paced and 50% of control hearts showed positive CAM results. Control gastrocnemii gave positive results in 25%, plantaris in 30%: stimulated muscles showed 30% and 37.5% positive responses. Coronary blood flow at rest was significantly lower in chronically paced than control hearts (2.172 g/ml/min, 3.025 +/- 0.187, P less than 0.05) and not significantly different during maximal dilatation (9.217 +/- 1.722 and 11.166 +/- 1.158 respectively). Blood flow per heart beat was significantly higher during acute bradycardia as well as in bradycardially paced hearts at rest. Blood flow in stimulated muscles was significantly higher than in controls both at rest (26.2 +/- 3.36 ml/100 g/min as compared to 8.5 +/- 2.15, P less than 0.001) and during muscle contractions (56.1 +/- 4.5 and 20.7 +/- 2.6). It can thus be concluded that growth of capillaries in skeletal muscles may be due to mechanical factors connected with the increased blood flow while in the heart AA may act in concert with blood flow changes.
Angiogenesis Inducing Agents↗
The effect of chronic bradycardial pacing on the oxidative capacity in rabbit hearts.
Blood flow and oxygen consumption were estimated in isolated hearts from control rabbits or animals with chronic bradycardia achieved by transvenous atrial pacing for 4 weeks; these parameters were related to ultrastructural changes. Chronic reduction of heart frequency to about 50% of control values resulted in an increased capillary/fibre ratio (1.43 +/- 0.07 in paced, 1.26 +/- 0.07 in control hearts), increased capillary density (2339 +/- 148 vs 1897 +/- 95) and increased volume density of mitochondria (36.36 +/- 1.28% vs 31.38 +/- 1.7%), in paced and control hearts, respectively, with no signs of heart hypertrophy (fibre diameters 28.5 micron in both groups). Maximal blood flow (3 ml.g-1.min-1) was similar in paced and control hearts. Maximal oxygen consumption (achieved by a gradual increase in preload and increase in force of contraction by noradrenalin infusion) in paced hearts was significantly higher than in control hearts and was achieved by a higher oxygen extraction. This could be due both to the increased volume density of mitochondria and to a more homogeneous distribution of flow through an enlarged capillary bed. The increased oxidative capacity induced by chronic bradycardial pacing can explain improved maximal cardiac work found under these conditions previously in vivo.
Animals↗
Relationship between the size of the capillary bed and oxidative capacity in various cat skeletal muscles.
The size of the capillary bed, assessed by capillary density (CD), capillary per muscle fibre ratio (C/F), total capillary length, surface area and volume was related to the oxidative capacity, assessed by the volume density of mitochondria and VO2max in cat muscles with a different composition of glycolytic and oxidative fibres: predominantly glycolytic gracilis, purely oxidative soleus and gracilis transformed towards oxidative by chronic low frequency (10 Hz) electrical stimulation. Maximal blood flow and lactate output were measured in the muscles during isometric contractions. When capillary supply was estimated by C/F ratio, there was a close correlation between various parameters only in stimulated gracilis. The combined data of all muscles showed a significant correlation between the total volume of mitochondria, VO2max and total capillary surface area. Capillary volume showed a tight correlation with maximal blood flow in both control and stimulated gracilis, but not in soleus. Maximal blood flow was correlated with VO2max in oxidative muscles (stimulated gracilis and soleus) but not in control glycolytic gracilis. Moreover normal gracilis did not show any relationship between the volume density of mitochondria and the size of the capillary bed. The latter was inversely correlated with the output of lactate which was greater in muscles with a lower C/P ratio. The data on gracilis indicates that the capillary bed can adapt to the increased demand for oxygen and a greater oxidative capacity induced by long-term activity imposed on a glycolytic muscle, while it may be more important for the removal of lactate in the glycolytic muscles under their normal activity.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
A comparison of the microcirculation in rat fast glycolytic and slow oxidative muscles at rest and during contractions.
We compared the microcirculation of the predominantly glycolytic (cortex of tibialis anterior, TA) and purely oxidative (soleus) muscles of the rat. The TA has wider (3.4 +/- 0.1 microns diameter compared to 2.7 +/- 0.05 microns), longer (405 +/- 29 and 205 +/- 17 microns), and straighter capillaries. Velocity of RBCs at rest is higher in TA (0.30 +/- 0.02 mm/sec) and reaches a higher value during contractions at 1 Hz (0.38 +/- 0.04 mm/sec) more quickly than in soleus (0.21 +/- 0.02-0.28 +/- 0.03 mm/sec). The number of continuously perfused capillaries in TA increased during contractions, but there was little change in soleus. A computer program was devised to estimate the proportion of time spent stationary by RBCs in the capillaries. This was greater in soleus than in TA at rest and was reduced in TA only during contractions. The transit time (TT) through capillaries was much reduced in TA during contractions (from 1.69 +/- 0.17 to 0.78 +/- 0.13 sec) but remained unchanged in soleus (1.17 +/- 0.21 and 0.97 +/- 0.13 sec). The lack of functional hyperemia in soleus may be a direct consequence of this invariability in the TT.
Animals↗
Relationship between mitochondria and oxygen consumption in isolated cat muscles.
1. Oxygen consumption, mitochondrial content and composition, intracellular lipid stores and fibre size were studied in isolated cat muscles: predominantly glycolytic gracilis, purely oxidative soleus and gracilis transformed into an oxidative muscle by chronic low-frequency (10 Hz) electrical stimulation. 2. Oxygen consumption in control gracilis at rest (0.303 +/- 0.050 ml O2 min-1 100 g-1; mean +/- S.E. of mean) was three to five times lower than in either stimulated gracilis (1.16 +/- 0.40) or soleus (1.57 +/- 0.56); it was about two times lower during maximal contractions in control gracilis (5.15 +/- 0.24) than in either stimulated gracilis (11.6 +/- 2.0) or soleus (9.34 +/- 0.78). 3. The volume density of mitochondria in control gracilis (2.75 +/- 0.12%) was half that of soleus (6.23 +/- 0.76) and only one-third that of stimulated gracilis (8.35 +/- 0.71). Subsarcolemmal mitochondria represented a significantly smaller fraction of the total mitochondrial volume in control gracilis than in either soleus or stimulated gracilis. 4. The surface area of inner and outer mitochondrial membranes per unit volume of mitochondria ranged from 23.4 to 26.1 and from 14.0 to 16.5 m2 cm-3, respectively. Mean values of these variables were not significantly different among experimental groups. 5. The volume density of the intracellular lipid stores in control gracilis (0.232 +/- 0.041%) was one-fourth of that in stimulated gracilis (0.860 +/- 0.12) and one-fifth of that in soleus (1.17 +/- 0.27). 6. The fibre cross-sectional area was 1670 +/- 260 micron 2 in control gracilis, 2250 +/- 280 in stimulated gracilis and 2390 +/- 110 in soleus. The difference was statistically significant only between control gracilis and soleus. 7. There was a significant correlation between the volume density of mitochondria and maximal oxygen consumption for all three muscles combined. 8. It was found that mitochondrial structure was similar in muscles with different oxidative capacities and that equal amounts of mitochondria consumed equal amounts of oxygen under limiting conditions of maximal in vivo respiration.
Animals↗
Phosphorylation in vivo of the P light chain of myosin in rabbit fast and slow skeletal muscles.
The P light chain of myosin is partially phosphorylated in resting slow and fast twitch skeletal muscles of the rabbit in vivo. The extent of P light-chain phosphorylation increases in both muscles on stimulation. Rabbit slow-twitch muscles contain two forms of the P light chain that migrate with the same electrophoretic mobilities as the two forms of P light chain in rabbit ventricular muscle. The rate of phosphorylation of the P light chain in slow-twitch muscle is slower than its rate of phosphorylation in fast-twitch muscles during tetanus. The rate of P light-chain dephosphorylation is slow after tetanic contraction of fast-twitch muscles in vivo. The time course of dephosphorylation does not correlate with the decline of post-tetanic potentiation of peak twitch tension in rabbit fast-twitch muscles. The frequency of stimulation is an important factor in determining the extent of P light-chain phosphorylation in fast- and slow-twitch muscles.
Animals↗
Effects of different patterns of long-term stimulation on blood flow, fuel uptake and enzyme activities in rabbit fast skeletal muscles.
Long-term electrical stimulation (14-28 days) of rabbit fast muscles (tibialis anterior, TA and extensor digitorum longus, EDL) using intermittent high frequency (3 trains per min of 5 s duration at 40 Hz, for 8 h per day) produced changes in enzyme activities similar to those found with continuous stimulation at a frequency occurring in nerves to slow muscles (10 Hz). The activity of citrate synthetase, 3-hydroxyacyl-CoA dehydrogenase and succinate dehydrogenase increased two to 3-fold within 28 days. There was a 4-fold increase in hexokinase whereas phosphofructokinase, pyruvate kinase, lactate dehydrogenase and fructose-1,6-diphosphatase decreased to about 60% of the activity levels in the contralateral unstimulated muscles. Blood flow and oxygen consumption at rest were not changed even after 28 days of stimulation, but were increased during contractions in muscles stimulated at either frequency, the level being twice as high as in control muscles. Glucose uptake was similar to that in control muscles both at rest and during contractions and the output of lactate was similar to that found in control muscles in muscles stimulated at 40 Hz. Muscles stimulated at 10 Hz had smaller lactate output. Thus intermittent stimulation at high frequency (40 Hz) and continuous low frequency (10 Hz) produced similar changes in aerobic metabolism and fuel uptake provided that the total number of stimuli was comparable and that the stimulation was carried out for sufficiently long period.
Animals↗
The effect of long-term vasodilatation on capillary growth and performance in rabbit heart and skeletal muscle.
The long-term effect of adenosine and of a new xanthine derivative, 1-(5-oxohexyl)-3-methyl-7-propylxanthine (HWA 285) on capillary density was studied in rabbits. Doses of both agents were established in acute experiments such that they would produce a prolonged increase in coronary and skeletal muscle blood flows without significantly affecting blood pressure or cardiac output. These doses were then chronically administered (3 to 5 weeks) by continuous intravenous infusion from portable infusion pumps carried by the rabbits. Control animals were infused with saline. Long-term administration of adenosine and HWA 285 was well tolerated by the animals. In the acute experiments, adenosine (42 mumol.h-1 iv) reduced the heart rate and produced an increase in coronary blood flow (studied using 15 micrometers radioactive microspheres) and conductance of 38% and 65% respectively, with increases in skeletal muscle of 65% and 92%. Blood pressure, cardiac output and cardiac minute work were not affected. HWA 285 (57 mumol.h-1) slightly but significantly increased blood pressure, but did not affect heart rate, cardiac output or minute work. Coronary and skeletal muscle blood flow were increased by 41% and 72%, with conductance increases of 33% and 62% respectively. The number of all capillaries present was studied in the heart and skeletal muscle using histochemical staining for alkaline phosphatase. Myocardial capillary density (capillaries per mm2, means +/- SE) was 3092 +/- 97 in the adenosine infused group and 2870 +/- 153 in the HWA 285 infused group compared with 2426 +/- 93 in the controls, ie an increase of 27% (p less than 0.001) and 18% (p less than 0.02) respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
Adenosine↗
The effect of long-term high-frequency stimulation on capillary density and fibre types in rabbit fast muscles.
Rabbit fast muscles (tibialis anterior, t.a.; extensor digitorum longus, e.d.l.; and peroneal muscles) were stimulated for up to 28 days by electrodes implanted in the vicinity of the lateral popliteal (peroneal) nerve for 8 h/day, using either intermittent high-frequency (three trains at 40 Hz/min, each 5 s duration), or continuous stimulation at 10 Hz. This did not result in muscle hypertrophy even after 28 days. Capillary density (number of capillaries/mm2) was increased in e.d.l. from 251 +/- 3 to 366 +/- 6 after 14 days of stimulation and from 251 +/- 3 to 514 +/- 13 after 28 days of stimulation at 40 Hz. In t.a., capillary density increased from 373 +/- 5 to 583 +/- 10 after 14 days of stimulation at 40 Hz. The capillary/fibre ratio increased in e.d.l. from 1.25 +/- 0.02 to 1.86 +/- 0.04 at 14 days and to 2.07 +/- 0.06 at 28 days. In t.a., capillary/fibre ratio increased from 1.40 +/- 0.03 to 1.83 +/- 0.05 at 14 days. All these changes were significant (P less than 0.0005). Analysis of capillary density, capillary/fibre ratio, fibre areas and proportion of different fibre types in muscles stimulated for shorter periods showed no changes in capillary density, capillary/fibre ratio or fibre areas in e.d.l. or t.a. stimulated for 4 days; there was a decrease in the proportion of fast glycolytic fibres from 42 to 32% (P less than 0.0025) and increase in fast oxidative from 37.6 to 41.2% in e.d.l. Muscles stimulated for 7 days showed increases in capillary density and capillary/fibre ratio in fast predominantly glycolytic fibres in e.d.l., and a decrease in capillary density in fast and slow oxidative fibres in t.a. This was partly due to the increase in fibre areas in these groups (capillary/fibre ratio in t.a. was not significantly changed). No changes were observed in fibre areas in e.d.l. Stimulation at 10 Hz produced increase in capillary/fibre ratio in the vicinity of glycolytic fibres after only 4 days. High-frequency intermittent stimulation leads to a massive capillary growth which starts first in the muscle with a higher proportion of glycolytic fibres (e.d.l.), has a later onset than continuous low-frequency stimulation, and may be due to a combination of high blood flow and metabolic factors.
Animals↗