Distribution of red blood cell velocity in capillary network, and endothelial ultrastructure, in aged rat skeletal muscle

The capillary fascicle in skeletal muscle: Structural and functional physiology of RBC distribution in capillary networks

KEY POINTS

The capillary module, consisting of parallel capillaries from arteriole to venule, is classically considered as the building block of complex capillary networks. In skeletal muscle, this structure fails to address how blood flow is regulated along the entire length of the synchronously contracting muscle fibres. Using intravital video microscopy of resting extensor digitorum longus muscle in rats, we demonstrated the capillary fascicle as a series of interconnected modules forming continuous columns that align naturally with the dimensions of the muscle fascicle. We observed structural heterogeneity for module topology, and functional heterogeneity in space and time for capillary-red blood cell (RBC) haemodynamics within a module and between modules. We found that module RBC haemodynamics were independent of module resistance, providing direct evidence for microvascular flow regulation at the level of the capillary module. The capillary fascicle is an updated paradigm for characterizing blood flow and RBC distribution in skeletal muscle capillary networks.

ABSTRACT

Capillary networks are the fundamental site of oxygen exchange in the microcirculation. The capillary module (CM), consisting of parallel capillaries from terminal arteriole (TA) to post-capillary venule (PCV), is classically considered as the building block of complex capillary networks. In skeletal muscle, this structure fails to address how blood flow is regulated along the entire length of the synchronously contracting muscle fibres, requiring co-ordination from numerous modules. It has previously been recognized that TAs and PCVs interact with multiple CMs, creating interconnected networks. Using label-free intravital video microscopy of resting extensor digitorum longus muscle in rats, we found that these networks form continuous columns of linked CMs spanning thousands of microns, herein denoted as the capillary fascicle (CF); this structure aligns naturally with the dimensions of the muscle fascicle. We measured capillary-red blood cell (RBC) haemodynamics and module topology (n = 9 networks, 327 modules, 1491 capillary segments). The average module had length 481 μm, width 157 μm and 9.51 parallel capillaries. We observed structural heterogeneity for CM topology, and functional heterogeneity in space and time for capillary-RBC haemodynamics within a module and between modules. There was no correlation between capillary RBC velocity and lineal density. A passive inverse relationship between module length and haemodynamics was remarkably absent, providing direct evidence for microvascular flow regulation at the level of the CM. In summary, the CF is an updated paradigm for characterizing RBC distribution in skeletal muscle, and strengthens the theory of capillary networks as major contributors to the signal that regulates capillary perfusion.

Systemic microvascular shunting through hyperdynamic capillaries after acute physiological disturbances following cardiopulmonary bypass

Previously we showed that cardiopulmonary bypass (CPB) during cardiac surgery is associated with reduced sublingual microcirculatory perfusion and oxygenation. It has been suggested that impaired microcirculatory perfusion may be paralleled by increased heterogeneity of flow in the microvascular bed, possibly leading to arteriovenous shunting. Here we investigated our hypothesis that acute hemodynamic disturbances during extracorporeal circulation indeed lead to microcirculatory heterogeneity with hyperdynamic capillary perfusion and reduced systemic oxygen extraction. In this single-center prospective observational study, patients undergoing cardiac surgery with (n = 18) or without (n = 13) CPB were included. Perioperative microcirculatory perfusion was assessed sublingually with sidestream darkfield imaging, and recordings were quantified for microcirculatory heterogeneity and hyperdynamic capillary perfusion. The relationship with hemodynamic and oxygenation parameters was analyzed. Microcirculatory heterogeneity index increased substantially after onset of CPB [0.5 (0.0-0.9) to 1.0 (0.3-1.3); P = 0.031] but not during off-pump surgery. Median capillary red blood cell (RBC) velocity increased intraoperatively in the CPB group only [1,600 (913-2,500 μm/s) vs. 380 (190-480 μm/s); P < 0.001], with 31% of capillaries supporting high RBC velocities (>2,000 μm/s). Hyperdynamic microcirculatory perfusion was associated with reduced arteriovenous oxygen difference and systemic oxygen consumption during and after CPB. The current study provides the first direct human evidence for a microvascular shunting phenomenon through hyperdynamic capillaries following acute physiological disturbances after onset of CPB. The hypothesis of impaired systemic oxygen offloading caused by hyperdynamic capillaries was supported by reduced blood arteriovenous oxygen difference and low systemic oxygen extraction associated with CPB.

Microvascular flow modeling using in vivo hemodynamic measurements in reconstructed 3D capillary networks

OBJECTIVE

We describe a systematic approach to modeling blood flow using reconstructed capillary networks and in vivo hemodynamic measurements. Our goal was to produce flow solutions that represent convective O(2) delivery in vivo.

METHODS

Two capillary networks, I and II (84 × 168 × 342 and 70 × 157 × 268 μm(3)), were mapped using custom software. Total network red blood cell supply rate (SR) was calculated from in vivo data and used as a target metric for the flow model. To obtain inlet hematocrits, mass balances were applied recursively from downstream vessels. Pressure differences across the networks were adjusted to achieve target SR. Baseline flow solutions were used as inputs to existing O(2) transport models. To test the impact of flow redistribution, asymmetric flow solutions (Asym) were generated by applying a ± 20% pressure change to network outlets.

RESULTS

Asym solutions produced a mean absolute difference in SR per capillary of 27.6 ± 33.3% in network I and 33.2 ± 40.1% in network II vs. baseline. The O(2) transport model calculated mean tissue PO(2) of 28.2 ± 4.8 and 28.1 ± 3.5 mmHg for baseline and 27.6 ± 5.2 and 27.7 ± 3.7 mmHg for Asym.

CONCLUSIONS

This outcome illustrates that moderate changes in flow distribution within a capillary network have little impact on tissue PO(2) provided that total SR remains unchanged.

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.

Mapping of the functional microcirculation in vital organs using contrast-enhanced in vivo video microscopy

A functional microcirculation is vital to the survival of mammalian tissues. In vivo video microscopy is often used in animal models to assess microvascular function, providing real-time observation of blood flow in normal and diseased tissues. To extend the capabilities of in vivo video microscopy, we have developed a contrast-enhanced system with postprocessing video analysis tools that permit quantitative assessment of microvascular geometry and function in vital organs and tissues. FITC-labeled dextran (250 kDa) was injected intravenously into anesthetized mice to provide intravascular fluorescence contrast with darker red blood cell (RBC) motion. Digitized video images of microcirculation in a variety of internal organs (e.g., lung, liver, ovary, and kidney) were processed using computer-based motion correction to remove background respiratory and cardiac movement. Stabilized videos were analyzed to generate a series of functional images revealing microhemodynamic parameters, such as plasma perfusion, RBC perfusion, and RBC supply rate. Fluorescence contrast revealed characteristic microvascular arrangements within different organs, and images generated from video sequences of liver metastases showed a marked reduction in the proportion of tumor vessels that were functional. Analysis of processed video sequences showed large reductions in vessel volume, length, and branch-point density, with a near doubling in vessel segment length. This study demonstrates that postprocessing of fluorescence contrast video sequences of the microcirculation can provide quantitative images useful for studies in a wide range of model systems.

Altered regional blood flow responses to submaximal exercise in older rats

Maximal aerobic capacity and the ability to sustain submaximal exercise (Ex) declines with advancing age. Whether altered muscle blood flow (BF) plays a mechanistic role in these effects remains to be resolved. The present investigation determined the effects of aging on the hemodynamic and regional BF response to submaximal Ex in rats. Heart rate (HR), mean arterial pressure (MAP), and BF to different organs (kidneys, splanchnic organs, and 28 hindlimb muscles) were determined at rest and during submaximal treadmill Ex (20 m/min, 5% grade) with radiolabeled microspheres in young (Y; 6-8 mo old, 339 +/- 8 g, n = 9) and old (O; 27-29 mo old, 504 +/- 18 g, n = 7) Fischer 344 x Brown Norway rats. Results demonstrated that HR, MAP, and BF to the pancreas, small and large intestine, and total hindlimb musculature were similar between Y and O rats at rest. BF to the kidneys, spleen, and stomach were 33, 60, and 43% lower, respectively, in O compared with Y rats. BF to the total hindlimb musculature increased (P < 0.05) during Ex and was similar for both Y and O rats (Y: 16 +/- 3 to 124 +/- 7 vs. O: 20 +/- 3 to 137 +/- 12 ml.min-1.100 g-1). However, in O vs. Y rats, BF was reduced in 6 (highly oxidative) and elevated in 8 (highly glycolytic) of the 28 individual hindquarter muscles or muscle parts examined (P < 0.05). During Ex, BF to the spleen and stomach decreased (P < 0.05) from rest in Y rats, whereas BF decreased in the kidneys, pancreas, spleen, stomach, as well as the small and large intestines of O rats. In conclusion, these data demonstrate that, despite similar increases in total hindlimb BF in Y and O rats during submaximal Ex, there is a profound BF redistribution from highly oxidative to highly glycolytic muscles.

Effects of aging on capillary geometry and hemodynamics in rat spinotrapezius muscle

The effects of aging on muscle microvascular structure and function may play a key role in performance deficits and impairment of O2 exchange within skeletal muscle of senescent individuals. To determine the effects of aging on capillary geometry, red blood cell (RBC) hemodynamics, and hematocrit in a muscle of mixed fiber type, spinotrapezius muscles from Fischer 344 x Brown Norway hybrid rats aged 6-8 mo [young (Y); body mass 421 +/- 10 g, n = 6] and 26-28 mo [old (O); 561 +/- 12 g, n = 6] were observed by high-resolution transmission light microscopy under resting conditions. The percentage of RBC-perfused capillaries (Y: 78 +/- 3%; O: 75 +/- 2%) and degree of tortuosity and branching (Y: 13 +/- 2%; O: 13 +/- 2%, additional capillary length) were not different in O vs. Y muscles. Lineal density of RBC-perfused capillaries in O was significantly reduced (Y: 30.7 +/- 1.8, O: 22.8 +/- 3.1 capillaries/mm; P < 0.05). However, RBC-perfused capillaries from O rats (n = 78) exhibited increased RBC velocity (VRBC) (Y: 219 +/- 12, O: 310 +/- 14 microm/s; P < 0.05) and RBC flux (FRBC) (Y: 27 +/- 2, O: 41 +/- 2 RBC/s; P < 0.05) vs. Y rats (n = 66). Thus O2 delivery per unit of muscle was not different between groups (Y: 894 +/- 111, O: 887 +/- 118 RBC. s-1. mm muscle-1). Capillary hematocrit was not different in Y vs. O rats (Y: 26 +/- 1%, O: 28 +/- 1%: P > 0.05). These data indicate that in resting spinotrapezius muscle, aging decreases the lineal density of RBC-perfused capillaries while increasing mean VRBC and FRBC within those capillaries. Whereas muscle conductive O2 delivery and capillary hematocrit were unchanged, elevated VRBC reduces capillary RBC transit time and may impair the diffusive transport of O2 from blood to myocyte particularly under exercise conditions.

Expression of matrix metalloproteinases, inhibitor, and acid phosphatase in muscles of immobilized hindlimbs of rats

External fixation procedures of limb immobilization provide excellent experimental models to study mechanisms involved in muscle disuse atrophy and recovery. Female Wistar rats (7-8 months old) had their right hindlimbs immobilized by an external fixation procedure for 5, 10, 21, and 30 days. Muscle mass of the gastrocnemius and quadriceps muscles was reduced by 41-46% in comparison with contralateral nonimmobilized legs. Acid phosphatase activities were significantly increased after 21 and 30 days of hindlimb immobilization. Histochemical staining for acid phosphatase activities increased in myofibers after the external fixation and also in macrophages in the adjacent extracellular matrix. Matrix metalloproteinase (MMP-2 and MMP-9) activities assessed by gel zymography and also a tissue inhibitor of metalloproteinases (TIMP-1) assessed by Western blot were elevated in the immobilized hindlimb muscles. Our study demonstrated that metalloproteinases are expressed relatively late after limb immobilization and appear to be responsible to a large degree for degradation of the extracellular matrix in experimental disuse atrophy.

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.

Microvascular perfusion is impaired in a rat model of normotensive sepsis

We hypothesized that normotensive sepsis affects the ability of the microcirculation to appropriately regulate microregional red blood cell (RBC) flux. An extensor digitorum longus muscle preparation for intravital study was used to compare the distribution of RBC flux and the functional hyperemic response in SHAM rats and rats made septic by cecal ligation and perforation (CLP). Using intravital microscopy, we found that sepsis was associated with a 36% reduction in perfused capillary density (from 35.3 +/- 1.5 to 22.5 +/- 1.0 capillaries/mm of test line) and a 265% increase in stopped-flow capillaries (from 0.9 +/- 0.2 to 3.3 +/- 0.4 capillaries/mm); the spatial distribution of perfused capillaries was also 72% more heterogeneous. Mean intercapillary distance (ICD) increased 30% (from 25.7 +/- 0.8 to 33.5 +/- 1.6 microns), and the proportion of capillary pairs with intercapillary distances > 33.8 microns (the 75th percentile of ICDSHAM) was greater with sepsis. Mean capillary RBC velocity increased 17% in CLP rats (391 vs 333 microns/s). Laser Doppler flowmetry was used to assess the functional hyperemic response of the extensor digitorum longus muscle before and after a period of maximal twitch contraction designed to increase oxygen demand. RBC flux was 36% lower in the CLP rats at rest. After contraction, RBC flux increased in both SHAM and CLP rats; however, the relative increase was less in the CLP group. We concluded that sepsis affects the ability of the skeletal muscle microcirculation to appropriately distribute RBC flux and to respond to increases in oxygen need.