Greater capillary-fiber interface per fiber mitochondrial volume in skeletal muscles of old rats.

The objective was to examine whether muscle structural capacity for O2 flux (i.e., capillary-to-fiber surface ratio) relative to fiber mitochondrial volume deteriorates with the muscle atrophy of aging in predominantly slow- (soleus, S) and fast-twitch (extensor digitorum longus, EDL) muscles of old (24 mo) and very old (35 mo) F344BN rats compared with adult (12 mo old). Wet muscle mass decreased 29% (196 +/- 4 to 139 +/- 5 mg) in S and 22% (192 +/- 3 to 150 +/- 3 mg) in EDL between 12 and 35 mo of age, without decline in body mass. Capillary density increased 65% (1,387 +/- 54 to 2,291 +/- 238 mm(-2)) in S and 130% (964 +/- 95 to 2,216 +/- 311 mm(-2)) in EDL, because of the muscle fiber atrophy, whereas capillary per fiber number remained unchanged. Altered capillary geometry, i.e., lesser contribution of tortuosity and branching to capillary length, was found in S at 35 compared with 12 and 24 mo, and not in EDL. Accounting for capillary geometry revealed 55% (1,776 +/- 78 to 2,750 +/- 271 mm(-2)) and 113% (1,194 +/- 112 to 2,540 +/- 343 mm(-2)) increases in capillary length-to-fiber volume ratio between 12 and 35 mo of age in S and EDL, respectively. Fiber mitochondrial volume density was unchanged over the same period, causing mitochondrial volume per micrometer fiber length to decrease in proportion to the fiber atrophy in both muscles. As a result of the smaller fiber mitochondrial volume in the face of the unchanged capillary-to-fiber number ratio, capillary-to-fiber surface ratio relative to fiber mitochondrial volume not only did not deteriorate, but in fact increased twofold in both muscles between 12 and 35 mo of age, independent of their different fiber type.

Comparative studies of high performance swimming in sharks I. Red muscle morphometrics, vascularization and ultrastructure.

Tunas (family Scombridae) and sharks in the family Lamnidae are highly convergent for features commonly related to efficient and high-performance (i.e. sustained, aerobic) swimming. High-performance swimming by fishes requires adaptations augmenting the delivery, transfer and utilization of O(2) by the red myotomal muscle (RM), which powers continuous swimming. Tuna swimming performance is enhanced by a unique anterior and centrally positioned RM (i.e. closer to the vertebral column) and by structural features (relatively small fiber diameter, high capillary density and greater myoglobin concentration) increasing O(2) flux from RM capillaries to the mitochondria. A study of the structural and biochemical features of the mako shark (Isurus oxyrinchus) RM was undertaken to enable performance-capacity comparisons of tuna and lamnid RM. Similar to tunas, mako RM is positioned centrally and more anterior in the body. Another lamnid, the salmon shark (Lamna ditropis), also has this RM distribution, as does the closely related common thresher shark (Alopias vulpinus; family Alopiidae). However, in both the leopard shark (Triakis semifasciata) and the blue shark (Prionace glauca), RM occupies the position where it is typically found in most fishes; more posterior and along the lateral edge of the body. Comparisons among sharks in this study revealed no differences in the total RM quantity (approximately 2-3% of body mass) and, irrespective of position within the body, RM scaling is isometric in all species. Sharks thus have less RM than do tunas (4-13% of body mass). Relative to published data on other shark species, mako RM appears to have a higher capillary density, a greater capillary-to-fiber ratio and a higher myoglobin concentration. However, mako RM fiber size does not differ from that reported for other shark species and the total volume of mitochondria in mako RM is similar to that reported for other sharks and for tunas. Lamnid RM properties thus suggest a higher O(2) flux capacity than in other sharks; however, lamnid RM aerobic capacity appears to be less than that of tuna RM.

Aerobic capacities in the skeletal muscles of Weddell seals: key to longer dive durations?

In contrast to terrestrial animals that function under hypoxic conditions but display the typical exercise response of increasing ventilation and cardiac output, marine mammals exercise under a different form of hypoxic stress. They function for the duration of a dive under progressive asphyxia, which is the combination of increasing hypoxia, hypercapnia and acidosis. Our previous studies on short-duration, shallow divers found marked adaptations in their skeletal muscles, which culminated in enhanced aerobic capacities that are similar to those of athletic terrestrial mammals. The purpose of the present study was to assess the aerobic capacity of skeletal muscles from long-duration divers. Swimming and non-swimming muscles were collected from adult Weddell seals, Leptonychotes weddelli, and processed for morphometric analysis, enzymology, myoglobin concentrations and fiber-type distribution. The results showed that the skeletal muscles of Weddell seals do not have enhanced aerobic capacities compared with those of terrestrial mammals but are adapted to maintain low levels of an aerobic lipid-based metabolism, especially under the hypoxic conditions associated with diving. The lower aerobic capacity of Weddell seal muscle as compared with that of shorter-duration divers appears to reflect their energy-conserving modes of locomotion, which enable longer and deeper dives.

Fiber capillarization relative to mitochondrial volume in diaphragm of shrew.

The objective was to examine fiber capillarization in relation to fiber mitochondrial volume in the highly aerobic diaphragm of the shrew, the smallest mammal. The diaphragms of four common shrews [Sorex araneus; body mass, 8.2 +/- 1.3 (SE) g] and four lesser shrews (Sorex minutus, 2.6 +/- 0.1 g) were perfusion fixed in situ, processed for electron microscopy, and analyzed by morphometry. Capillary length per fiber volume was extremely high, at values of 8,008 +/- 1,054 and 12,332 +/- 625 mm(-2) in S. araneus and S. minutus, respectively (P = 0.012), with no difference in capillary geometry between the two species. Fiber mitochondrial volume density was 28.5 +/- 2.3% (S. araneus) and 36.5 +/- 1.4% (S. minutus; P = 0.025), yielding capillary length per milliliter mitochondria values (S. araneus, 27.8 +/- 1.5 km; S. minutus, 33.9 +/- 2.2 km; P = 0.06) as high as in the flight muscle of the hummingbird and small bats. The size of the capillary-fiber interface (i.e., capillary surface per fiber surface ratio) per fiber mitochondrial volume in shrew diaphragm was also as high as in bird and bat flight muscles, and it was about two times greater than in rat hindlimb muscle. Thus, whereas fiber capillary and mitochondrial volume densities decreased with increased body mass in S. araneus compared with S. minutus Soricinae shrews, fiber capillarization per milliliter mitochondria in both species was much higher than previously reported for shrew diaphragm, and it matched that of the intensely aerobic flight muscles of birds and mammals.

Estimating the size of the capillary-to-fiber interface in skeletal muscle: a comparison of methods.

Current evidence suggests that the size of the capillary-to-fiber (C/F) interface is a major determinant of O2 flux into muscle fibers, and methods have been developed for estimating the size of this region via the C/F perimeter ratio in perfusion-fixed material (Mathieu-Costello O, Ellis CG, Potter RF, MacDonald IC, and Groom AC. Am J Physiol Heart Circ Physiol 261: H1617-H1625, 1991) and the quotient of the individual, fiber-based C/F number ratio and fiber perimeter (C/F perimeter exchange index) in muscle biopsies (Hepple RT. Can J Appl Physiol 22: 11-22, 1997). The purpose of this study was to compare the two methods and examine how differences in muscle tissue preparation between perfusion fixation and frozen biopsy can influence the estimate of the size of the C/F interface. The left medial gastrocnemius muscle of nine purpose-bred dogs was perfusion fixed in situ, and a sample from the midportion of the midbelly was processed for microscopy. A corresponding sample from the right gastrocnemius muscle obtained by open biopsy in six of the nine animals was frozen for histochemistry. A significant correlation was found between the two estimates of the size of the C/F interface in the same sections of perfusion-fixed material (r = 0.75, P < 0.05). However, estimates of the size of the C/F interface were smaller in biopsies than perfusion-fixed material, and there was no significant relationship between the estimates in the two preparations. This was due to differences in fiber size (33% larger fiber cross-sectional area in biopsy material after normalization for sarcomere length; P < 0.05) and muscle sampling between the two tissue preparations.

Muscle adaptation to altitude: tissue capillarity and capacity for aerobic metabolism.

Prolonged exposure to high altitude leads to reduced muscle mass and performance. The fall in muscle mass follows a reduction in fiber size, which at first was believed to be accompanied by increased fiber capillarization and aerobic enzymes. Subsequent studies showed that hypoxia alone does not alter capillary number and geometry in skeletal muscles of mammals at altitude. It was also found that alterations in fiber size and aerobic enzymes depend on a number of additional factors, including animal activity and the level of hypoxia (e.g., moderate vs. extreme altitude). With training at altitude, fiber capillary number and aerobic enzymes are increased, indicating that muscle potential for plasticity is conserved in hypoxia. Recent studies have also shown that capillary number and geometry are altered in muscles of several species of birds native or exposed to higher altitude; that is, that capillary growth can occur in skeletal muscle in response to chronic exposure to high altitude. In this mini review, we summarize these data and current knowledge on muscle capillary to fiber structural relationships and their implications for muscle aerobic function at altitude.

Skeletal muscle capillarity and angiogenic mRNA levels after exercise training in normoxia and chronic hypoxia.

Gene expression of vascular endothelial growth factor (VEGF), and to a lesser extent of transforming growth factor-beta(1) (TGF-beta(1)) and basic fibroblast growth factor (bFGF), has been found to increase in rat skeletal muscle after a single exercise bout. In addition, acute hypoxia augments the VEGF mRNA response to exercise, which suggests that, if VEGF is important in muscle angiogenesis, hypoxic training might produce greater capillary growth than normoxic training. Therefore, we examined the effects of exercise training (treadmill running at the same absolute intensity) in normoxia and hypoxia (inspired O(2) fraction = 0.12) on rat skeletal muscle capillarity and on resting and postexercise gene expression of VEGF, its major receptors (flt-1 and flk-1), TGF-beta(1), and bFGF. Normoxic training did not alter basal or exercise-induced VEGF mRNA levels but produced a modest twofold increase in bFGF mRNA (P < 0.05). Rats trained in hypoxia exhibited an attenuated VEGF mRNA response to exercise (1.8-fold compared 3.4-fold with normoxic training; P < 0.05), absent TGF-beta(1) and flt-1 mRNA responses to exercise, and an approximately threefold (P < 0.05) decrease in bFGF mRNA levels. flk-1 mRNA levels were not significantly altered by either normoxic or hypoxic training. An increase in skeletal muscle capillarity was observed only in hypoxically trained rats. These data show that, whereas training in hypoxia potentiates the adaptive angiogenic response of skeletal muscle to a given absolute intensity of exercise, this was not evident in the gene expression of VEGF or its receptors when assessed at the end of training.

Muscle capillary supply in harbor seals.

The purpose of this study was to examine muscle capillary supply in harbor seals. Locomotory and nonlocomotory muscles of four harbor seals (mass = 17.5-41 kg) were glutaraldehyde-perfusion fixed and samples processed for electron microscopy and analyzed by morphometry. Capillary-to-fiber number and surface ratios were 0.81 +/- 0.05 and 0.16 +/- 0.01, respectively. Capillary length and surface area per volume of muscle fiber were 1,495 +/- 83 mm/mm(3) and 22.4 +/- 1.6 mm(2)/mm(3), respectively. In the locomotory muscles, we measured capillary length and surface area per volume mitochondria (20.1 +/- 1.7 km/ml and 2,531 +/- 440 cm(2)/ml). All these values are 1.5-3 times lower than in muscles with similar or lower volume densities of mitochondria in dogs of comparable size. Compared with terrestrial mammals, the skeletal muscles of harbor seals do not match their increased aerobic enzyme capacities and mitochondrial volume densities with greater muscle capillary supply. They have a smaller capillary-to-fiber interface and capillary supply per fiber mitochondrial volume than terrestrial mammals of comparable size.

Chronic hypoxia attenuates resting and exercise-induced VEGF, flt-1, and flk-1 mRNA levels in skeletal muscle.

Vascular endothelial growth factor (VEGF) is a hypoxia-inducible angiogenic mitogen. However, chronic hypoxia is generally not found to increase mammalian skeletal muscle capillarity. We sought to determine the effect of chronic hypoxia (8 wk, inspired O2 fraction = 0.12) on skeletal muscle gene expression of VEGF, its receptors (flt-1 and flk-1), basic fibroblast growth factor, and transforming growth factor-beta1. Wistar rats were exposed to chronic hypoxia (n = 12) or room air (n = 12). After the exposure period, six animals from each group were subjected to a single 1-h treadmill exercise bout (18 m/min on a 10 degrees incline) in room air while the remaining six animals served as rest controls. Morphological analysis revealed that chronic hypoxia did not increase skeletal muscle capillarity. Northern blot analyses showed that chronic hypoxia decreased resting VEGF, flt-1, and flk-1 mRNA by 23, 68, and 42%, respectively (P < 0.05). The VEGF mRNA response to exercise was also decreased (4.1- and 2.7-fold increase in room air and chronic hypoxia, respectively, P < 0.05). In contrast, neither transforming growth factor-beta1 nor basic fibroblast growth factor mRNA was significantly altered by chronic hypoxia. In conclusion, prolonged exposure to hypoxia attenuated gene expression of VEGF and its receptors flt-1 and flk-1 in rat gastrocnemius muscle. These findings may provide an explanation for the lack of mammalian skeletal muscle angiogenesis that is observed after chronic hypoxia.

Effect of high transcapillary pressures on capillary ultrastructure and permeability coefficients in dog lung.

To determine the correlation between ultrastructural and physiological changes in blood-gas barrier function in lungs transiently exposed to very high vascular pressures, we increased capillary transmural pressure (Ptm) of 6 canine isolated perfused left lower lung lobe preparations (high-pressure group) to 80.3 Torr for 3.8 min and then determined the capillary filtration (K(fc)) and osmotic reflection (sigma(d)) coefficients at a Ptm of 19.1 Torr in the ventilated lung lobes. This was followed by perfusion fixation of the lobes at a Ptm of 20.5 Torr for ultrastructural analysis. These data were compared with those obtained in six lobes in which Ptm was not transiently elevated before K(fc), sigma(d), and ultrastructural evaluation. K(fc) was higher [0.249 +/- 0.042 (SE) vs. 0.054 +/- 0.009 g. min(-1). Torr(-1). 100 g(-1); P < 0.01] and sigma(d) was lower (0.52 +/- 0.07 vs. 0.85 +/- 0.08; P < 0.01) in the high-pressure group. In contrast, although endothelial and epithelial breaks were occasionally observed in some experiments, their incidence was not increased in the high-pressure group. These data suggest that the increased transvascular water and protein flux occurred through pathways of a size not resolvable by electron microscopy after vascular perfusion-fixation at a Ptm of 20.5 Torr.

Effect of acute exercise on citrate synthase activity in untrained and trained human skeletal muscle.

Maximal citrate synthase activity (CS) is routinely used as a marker of aerobic capacity and mitochondrial density in skeletal muscle. However, reported CS has been notoriously variable, even with similar experimental protocols and sampling from the same muscles. Exercise training has resulted in increases in CS ranging from 0 to 100%. Previously, it has been reported that acute exercise may significantly affect CS. To investigate the hypothesis that the large variation in CS that occurs with training is influenced by alterations during the exercise itself, we studied CS in human vastus lateralis both in the rested and acutely exercised state while trained and untrained (n = 6). Tissues obtained from four biopsies (untrained rested, untrained acutely exercised, trained rested, and trained acutely exercised) were analyzed spectrophotometrically for maximal CS. Exercise training measured in a rested state resulted in an 18.2% increase in CS (12.3 +/- 0.3 to 14.5 +/- 0.3 micromol x min(-1) x g tissue(-1), P < or = 0.05). However, even greater increases were recorded 1 h after acute exercise: 49.4% in the untrained state (12.3 +/- 0.3 to 18.3 +/- 0.5 micromol x min(-1) x g tissue(-1), P < or = 0.05) and 50.8% in the trained state (14.5 +/- 0.3 to 21.8 +/- 0.4 micromol x min(-1) x g tissue(-1), P < or = 0.05). Ultrastructural analysis, by electron microscopy, supported an effect of acute exercise with the finding of numerous swollen mitochondria 1 h after exercise that may result in greater access to the CS itself in the CS assay. In conclusion, although unexplained, the increased CS with acute exercise can clearly confound training responses and artificially elevate CS values. Therefore, the timing of muscle sampling relative to the last exercise session is critical when measuring CS and offers an explanation for the large variation in CS previously reported.

Structural and functional changes in the microvasculature of disused skeletal muscle.

Skeletal muscle and its microvasculature can exhibit remarkable plasticity in response to decreased functional demand (i.e., muscle disuse). Since the microvascular adaptation to disuse does seem to not depend solely on reduced demand, this review examines the various factors that may be responsible for the observed regression of microvascular structure and function during disuse. There are several animal models of muscle disuse; their common feature is that they are associated with a variety of confounding effects that make the interpretation of the "pure" disuse effect challenging. As well, in clinical studies, the effect of disuse can be difficult to separate from that of various pathologies. Regardless of methodological difficulties, degeneration of the capillary wall, capillary loss, arteriolar remodelling, reduced resting state blood flow, and reduced arteriolar responsiveness to acute vasodilative and vasoconstrictive stimuli have all been observed in disused muscles. The level, and presence/absence of these changes may depend on many factors including the duration of disuse, degree of muscle atrophy, residual muscle activity, microvascular blood flow, release of vasoactive agents from the degenerating muscle, muscle type, and the particular pathology associated with the muscle withering in humans. It is the present challenge to discover the presence/absence of key agents (possibly originating at the interface between the blood stream and the vascular wall, within the extracellular matrix, or the muscle fibres themselves) that alter the intra- and/or inter-cellular signalling to explain the mechanism of adaptation of the microvasculature to skeletal muscle disuse.

Functional lung unit in the pig.

To study the size of the vessels supplying the functional lung unit, polystyrene beads of uniform diameter were injected intravenously in anaesthetised pigs and subsequent gas exchange abnormalities were studied using the multiple inert gas elimination technique. Beads of different sizes, ranging from 63 to 262 microm, were used, each pig receiving beads of only one size. Successive 0.25 g boli of beads (cumulative dose 1.0-1.5 g) increased shunt (from 3% baseline to 20% of cardiac output) and pulmonary artery mean pressure (from 26 to 45 mmHg) and decreased arterial P(O(2)) (from 96 to 43 mmHg) and cardiac output from 2.8 to 2.2 L min(-1) with no differences according to bead sizes. The dispersion of the ventilation dist ribution (log SDV), normal at 0.39 before beads, increased progressively with bead size from 0.48 (63 microm to 0.91 (262 microm). The 63 microm beads were lodged in vessels associated with respiratory bronchioles and smaller airways, whereas larger beads were positioned in vessels associated with non-respiratory airways. A linear correlation analysis between log SDV and bead size showed that 59 microm beads produce a log SDV that is 2 SEM above mean baseline log SDV. These findings suggest that the functional lung unit in this species (with no collateral ventilation) is smaller than in a species of the same size and with collateral ventilation (dog) in whom occlusion of 124 microm or larger diameter vessels is required to increase log SDV.

Mitochondrial function in flying honeybees (Apis mellifera): respiratory chain enzymes and electron flow from complex III to oxygen.

The biochemical bases for the high mass-specific metabolic rates of flying insects remain poorly understood. To gain insights into mitochondrial function during flight, metabolic rates of individual flying honeybees were measured using respirometry, and their thoracic muscles were fixed for electron microscopy. Mitochondrial volume densities and cristae surface densities, combined with biochemical data concerning cytochrome content per unit mass, were used to estimate respiratory chain enzyme densities per unit cristae surface area. Despite the high content of respiratory enzymes per unit muscle mass, these are accommodated by abundant mitochondria and high cristae surface densities such that enzyme densities per unit cristae surface area are similar to those found in mammalian muscle and liver. These results support the idea that a unit area of mitochondrial inner membrane constitutes an invariant structural unit. Rates of O(2) consumption per unit cristae surface area are much higher than those estimated in mammals as a consequence of higher enzyme turnover rates (electron transfer rates per enzyme molecule) during flight. Cytochrome c oxidase, in particular, operates close to its maximum catalytic capacity (k(cat)). Thus, high flux rates are achieved via (i) high respiratory enzyme content per unit muscle mass and (ii) the operation of these enzymes at high fractional velocities.

Structural basis of muscle O(2) diffusing capacity: evidence from muscle function in situ.

Although evidence for muscle O(2) diffusion limitation of maximal O(2) uptake has been found in the intact organism and isolated muscle, its relationship to diffusion distance has not been examined. Thus we studied six sets of three purpose-bred littermate dogs (aged 10-12 mo), with 1 dog per litter allocated to each of three groups: control (C), exercise trained for 8 wk (T), or left leg immobilized for 3 wk (I). The left gastrocnemius muscle from each animal was surgically isolated, pump-perfused, and electrically stimulated to peak O(2) uptake at three randomly applied levels of arterial oxygenation [normoxia, arterial PO(2) (Pa(O(2))) 77 +/- 2 (SE) Torr; moderate hypoxia, Pa(O(2)): 33 +/- 1 Torr; and severe hypoxia, Pa(O(2)): 22 +/- 1 Torr]. O(2) delivery (ml. min(-1). 100 g(-1)) was kept constant among groups for each level of oxygenation, with O(2) delivery decreasing with decreasing Pa(O(2)). O(2) extraction (%) was lower in I than T or C for each condition, but calculated muscle O(2) diffusing capacity (Dmus(O(2))) per 100 grams of muscle was not different among groups. After the experiment, the muscle was perfusion fixed in situ, and a sample from the midbelly was processed for microscopy. Immobilized muscle showed a 45% reduction of muscle fiber cross-sectional area (P < 0.05), and a resulting 59% increase in capillary density (P < 0.05) but minimal reduction in capillary-to-fiber ratio (not significant). In contrast, capillarity was not significantly different in T vs. C muscle. The results show that a dramatically increased capillary density (and reduced diffusion distance) after short-term immobilization does not improve Dmus(O(2)) in heavily working skeletal muscle.

Quantitative electron microscopic study of the hypoxic fetal sheep heart.

In order to determine the effects of chronic, high-altitude hypoxia on the ovine fetal heart, we exposed pregnant ewes to 3,820 m beginning at 30 days gestation. We previously showed that following approximately 110 days of hypoxia the fetal heart showed significant reduction in cardiac output (76% of control) and contractility, and elevated levels of citrate synthase and lactate dehydrogenase. To investigate ultrastructural influences on these observed physiologic changes at altitude, we hypothesized that the volume densities of myofibrils and mitochondria, and glycogen content would be reduced in the ovine fetal heart and that this may contribute to contraction and cardiac output deficits in hypoxia. Mitochondria and myofibril volume density were determined by standard point-counting techniques and glycogen content was determined by biochemical analysis. The glycogen content from the hypoxic right ventricle (4.8 +/- 0.3%) was significantly lower than in control right ventricle (6.8 +/- 0.5%) and both left ventricles (hypoxia, 7.2 +/- 0.5; control, 7.8 +/- 0. 4%). Total mitochondrial volume density was also significantly reduced following hypoxia (15.5 +/- 0.7%) compared to controls (16.9 +/- 0.4%). As is common in the ovine fetal heart, the myofibril volume density of the right ventricle from both groups was significantly higher than the left ventricle (RV, 58.6 +/- 1.6; LV 54.3 +/- 0.9%). However, it was not different between control and high altitude. In support of our hypothesis, we may speculate that deficits in the quantity of myocyte glycogen and mitochondria contribute to the observed reduction in cardiac output and contractility, despite the upregulation of citrate synthase and lactate dehydrogenase. In contrast, myofibril volume density was unchanged.

Remodeling of lung interstitium but not resistance vessels in canine pacing-induced heart failure.

We previously showed that pacing-induced heart failure in dogs results in an enhancement of pulmonary vascular reactivity. In the present study we hypothesized that enhanced matrix deposition and structural remodeling of lung resistance microvessels would underlie these functional changes. Using biochemical measures, we found no difference in the normalized lung content of hyaluronan, uronic acid, and collagen between control dogs and dogs paced for 1 mo, although lung dry weight and noncollagen protein content increased significantly in the paced group (P < 0.05). From separate Formalin-fixed lung lobes, 5-microm frozen sections were prepared and stained with Masson's trichrome, and vascular structure was evaluated using standard morphometric techniques. When perivascular fluid cuffs were excluded from the measure of wall thickness, collagen and media volume fractions in any size range did not differ between paced and control groups. Similarly, in the paced group, medial thickness in <400-microm arterial or venular microvessels did not vary significantly from that in the controls. In contrast, the relationship of interstitial fluid pressure to lung water was significantly shifted to the right in the paced group, such that normal tissue pressures were observed, despite the increased water content. We conclude that although 1 mo of pacing-induced heart failure results in altered interstitial function, the attendant pulmonary hypertension and/or hormonal responses are insufficient to induce medial hypertrophy or other remodeling of the extra-alveolar microvasculature.

Differential microvascular response to disuse in rat hindlimb skeletal muscles.

The aim of the study was to address discrepant findings in the literature regarding coupling between decreased functional demand during disuse and reduced capillarity. We previously reported [K. Tyml, O. Mathieu-Costello, and E. Noble. Microvasc. Res. 49: 17-32, 1995] that severe disuse of rat extensor digitorum longus (EDL) muscle caused by a 2-wk application of tetrodotoxin (TTX) on the sciatic nerve is not accompanied by capillary loss. Using the same animal model, the present study examined whether this absence of coupling could be explained in terms of 1) too short a duration of disuse and 2) muscle-specific response to disuse. Fischer 344 rats were exposed to either no treatment (control) or to 2- or 8-wk TTX applications. Fiber size, capillary density per fiber cross-sectional area, and capillary-to-fiber (C/F) ratio were determined by morphometry in the EDL muscle (control, 2- and 8-wk groups) and in the superficial portion of medial gastrocnemius (Gas) muscle (control, 2 wk). In both muscles, microvascular blood flow was evaluated by intravital microscopy [red blood cell velocity in capillaries (V(RBC))] and by laser Doppler flowmetry (LDF). Regardless of duration of TTX application or muscle type, TTX-induced disuse resulted in a significant reduction of fiber area (44-71%). However, capillary density increased in EDL muscle (both at 2 and 8 wk) but not in Gas muscle. C/F ratio decreased in EDL muscle at 8 wk (18%) and in Gas muscle (39%). This indicates that the effect on capillarity depended on duration of disuse and on muscle type. V(RBC) and LDF signal were significantly larger in EDL than in Gas muscle. Analysis of change in capillarity vs. V(RBC) suggested that the outcome of disuse may be modulated by blood flow. We conclude that the duration of skeletal muscle disuse per se does not dictate capillary loss, and we hypothesize that discrepant findings of coupling between functional demand and capillarity could be due to the presence/absence of flow-related angiogenesis superimposed on the capillary removal process during disuse.

Effects of long-term, high-altitude hypoxia on the capillarity of the ovine fetal heart.

To determine the effect of chronic hypoxia on myocardial capillarity, we exposed pregnant ewes to an altitude of 3,820 m from day 30 to day 139 of gestation and compared the fetus to low-altitude (approximately 300 m) controls. We hypothesized that capillarity would increase in the hypoxic myocardium to optimize oxygen and metabolite flux to hypoxic tissues. Fetal hearts were fixed by retrograde aortic perfusion and processed for microscopy and stereological evaluation. Fiber cross-sectional area and capillary density were measured and standardized to sarcomere length. Capillary volume density and capillary diameter were measured, capillary-to-fiber ratio and capillary length density were calculated, and the capillary anisotropy coefficient was obtained from a table of known values. Capillary-to-fiber ratio, capillary volume density, and the capillary anisotropy coefficient were not different between hypoxia and control groups. Capillary diameter was significantly larger in the right compared with the left ventricle of hypoxic but not control hearts; fiber cross-sectional area tended to be larger in the right ventricle of both groups, but this was not significant. As a result of larger fiber size, capillary density and capillary length density were significantly smaller in the right ventricle of hypoxic but not control fetal hearts. Contrary to our hypothesis, the ovine fetus does not show morphological adaptation in the myocardium after approximately 109 days of high-altitude hypoxic stress.

Structure, strength, failure, and remodeling of the pulmonary blood-gas barrier.

The pulmonary blood-gas barrier needs to satisfy two conflicting requirements. It must be extremely thin for efficient gas exchange, but also immensely strong to withstand the extremely high stresses in the capillary wall when capillary pressure rises during exercise. The strength of the blood-gas barrier on the thin side is attributable to the type IV collagen in the basement membranes. However, when the wall stresses rise to very high levels, ultrastructural changes in the barrier occur, a condition known as stress failure. Physiological conditions that alter the properties of the barrier include intense exercise in elite human athletes. Some animals, such as Thoroughbred racehorses, consistently break their alveolar capillaries during galloping, causing hemorrhage. Pathophysiological conditions causing stress failure include neurogenic pulmonary edema, high-altitude pulmonary edema, left heart failure, and overinflation of the lung. Remodeling of the capillary wall occurs in response to increased wall stress, a good example being the thickening of the capillary basement membrane in diseases such as mitral stenosis. The blood-gas barrier is able to maintain its extreme thinness with sufficient strength only through continual regulation of its wall structure. Recent experimental work suggests that rapid changes in gene expression for extracellular matrix proteins and growth factors occur in response to increases in capillary wall stress. How the blood-gas barrier is regulated to be extremely thin but sufficiently strong is a central issue in lung biology.