Capillary recruitment and heterogeneity of microvascular flow in skeletal muscle before and after contraction

August Krogh's theory of muscle microvascular control and oxygen delivery: a paradigm shift based on new data

August Krogh twice won the prestigious international Steegen Prize, for nitrogen metabolism (1906) and overturning the concept of active transport of gases across the pulmonary epithelium (1910). Despite this, at the beginning of 1920, the consummate experimentalist was relatively unknown worldwide and even among his own University of Copenhagen faculty. But, in early 1919, he had submitted three papers to Dr Langley, then editor of The Journal of Physiology in England. These papers coalesced anatomical observations of skeletal muscle capillary numbers with O₂ diffusion theory to propose a novel active role for capillaries that explained the prodigious increase in blood-muscle O₂ flux from rest to exercise. Despite his own appraisal of the first two papers as "rather dull" to his friend, the eminent Cambridge respiratory physiologist, Joseph Barcroft, Krogh believed that the third one, dealing with O₂ supply and capillary regulation, was"interesting". These papers, which won Krogh an unopposed Nobel Prize for Physiology or Medicine in 1920, form the foundation for this review. They single-handedly transformed the role of capillaries from passive conduit and exchange vessels, functioning at the mercy of their upstream arterioles, into independent contractile units that were predominantly closed at rest and opened actively during muscle contractions in a process he termed 'capillary recruitment'. Herein we examine Krogh's findings and some of the experimental difficulties he faced. In particular, the boundary conditions selected for his model (e.g. heavily anaesthetized animals, negligible intramyocyte O₂ partial pressure, binary open-closed capillary function) have not withstood the test of time. Subsequently, we update the reader with intervening discoveries that underpin our current understanding of muscle microcirculatory control and place a retrospectroscope on Krogh's discoveries. The perspective is presented that the imprimatur of the Nobel Prize, in this instance, may have led scientists to discount compelling evidence. Much as he and Marie Krogh demonstrated that active transport of gases across the blood-gas barrier was unnecessary in the lung, capillaries in skeletal muscle do not open and close spontaneously or actively, nor is this necessary to account for the increase in blood-muscle O₂ flux during exercise. Thus, a contemporary model of capillary function features most muscle capillaries supporting blood flow at rest, and, rather than capillaries actively vasodilating from rest to exercise, increased blood-myocyte O₂ flux occurs predominantly via elevating red blood cell and plasma flux in already flowing capillaries. Krogh is lauded for his brilliance as an experimentalist and for raising scientific questions that led to fertile avenues of investigation, including the study of microvascular function.

Krogh’s capillary recruitment hypothesis, 100 years on: Is the opening of previously closed capillaries necessary to ensure muscle oxygenation during exercise?

In 1919, August Krogh published his seminal work on skeletal muscle oxygenation. Krogh’s observations indicated that muscle capillary diameter is actively regulated, rather than a passive result of arterial blood flow regulation. Indeed, combining a mathematical model with the number of ink-filled capillaries he observed in muscle cross sections taken at different workloads, Krogh was able to account for muscle tissue’s remarkably efficient oxygen extraction during exercise in terms of passive diffusion from nearby capillaries. Krogh was awarded the 1920 Nobel Prize for his account of muscle oxygenation. Today, his observations are engrained in the notion of capillary recruitment: the opening of previously closed capillaries. While the binary distinction between “closed” and “open” was key to Krogh’s model argument, he did in fact report a continuum of capillary diameters, degrees of erythrocyte deformation, and perfusion states. Indeed, modern observations question the presence of closed muscle capillaries. We therefore examined whether changes in capillary flow patterns and hematocrit among open capillaries can account for oxygen extraction in muscle across orders-of-magnitude changes in blood flow. Our four-compartment model of oxygen extraction in muscle confirms this notion and provides a framework for quantifying the impact of changes in microvascular function on muscle oxygenation in health and disease. Our results underscore the importance of capillary function for oxygen extraction in muscle tissue as first proposed by Krogh. While Krogh’s model calculations still hold, our model predictions support that capillary recruitment can be viewed in the context of continuous, rather than binary, erythrocyte distributions among capillaries.

NEW & NOTEWORTHY Oxygen extraction in working muscle is extremely efficient in view of single capillaries properties. The underlying mechanisms have been widely debated. Here, we develop a four-compartment model to quantify the influence of each of the hypothesized mechanisms on muscle oxygenation. Our results show that changes in capillary flow pattern and hematocrit can account for the high oxygen extraction observed in working muscle, while capillary recruitment is not required to account for these extraction properties.

Edward F. Adolph Distinguished Lecture. Contemporary model of muscle microcirculation: gateway to function and dysfunction

This review strikes at the very heart of how the microcirculation functions to facilitate blood-tissue oxygen, substrate, and metabolite fluxes in skeletal muscle. Contemporary evidence, marshalled from animals and humans using the latest techniques, challenges iconic perspectives that have changed little over the past century. Those perspectives include the following: the presence of contractile or collapsible capillaries in muscle, unitary control by precapillary sphincters, capillary recruitment at the onset of contractions, and the notion of capillary-to-mitochondrial diffusion distances as limiting O₂ delivery. Today a wealth of physiological, morphological, and intravital microscopy evidence presents a completely different picture of microcirculatory control. Specifically, capillary red blood cell (RBC) and plasma flux is controlled primarily at the arteriolar level with most capillaries, in healthy muscle, supporting at least some flow at rest. In healthy skeletal muscle, this permits substrate access (whether carried in RBCs or plasma) to a prodigious total capillary surface area. Pathologies such as heart failure or diabetes decrease access to that exchange surface by reducing the proportion of flowing capillaries at rest and during exercise. Capillary morphology and function vary disparately among tissues. The contemporary model of capillary function explains how, following the onset of exercise, muscle O₂ uptake kinetics can be extremely fast in health but slowed in heart failure and diabetes impairing contractile function and exercise tolerance. It is argued that adoption of this model is fundamental for understanding microvascular function and dysfunction and, as such, to the design and evaluation of effective therapeutic strategies to improve exercise tolerance and decrease morbidity and mortality in disease.

Muscle oxygenation dynamics in response to electrical stimulation as measured with near-infrared spectroscopy: A pilot study

Neuromuscular electrical stimulation (NMES) is used for preventing muscle atrophy and improving muscle strength in patients and healthy people. However, the current intensity of NMES is usually set at a level that causes the stimulated muscles to contract. This typically causes pain. Quantifying the instantaneous changes in muscle microcirculation and metabolism during NMES before muscle contraction occurs is crucial, because it enables the current intensity to be optimally tuned, thereby reducing the NMES-induced muscle pain and fatigue. We applied near-infrared spectroscopy (NIRS) to measure instantaneous tissue oxygenation and deoxygenation changes in 43 healthy young adults during NMES at 10, 15, 20, 25, 30, and 35 mA. Having been stabilized at the NIRS signal baseline, the tissue oxygenation and total hemoglobin concentration increased immediately after stimulation in a dose-dependent manner (P < 0.05) until stimulation was stopped at the level causing muscle contraction without pain. Tissue deoxygenation appeared relatively unchanged during NMES. We conclude that NIRS can be used to determine the optimal NMES current intensity by monitoring oxygenation changes.

Exercise-induced calf muscle hyperemia: quantitative mapping with low-dose dynamic contrast enhanced magnetic resonance imaging

Peripheral artery disease (PAD) in the lower extremities often leads to intermittent claudication. In the present study, we proposed a low-dose DCE MRI protocol for quantifying calf muscle perfusion stimulated with plantar flexion and multiple new metrics for interpreting perfusion maps, including the ratio of gastrocnemius over soleus perfusion (G/S; for assessing the vascular redistribution between the two muscles) and muscle perfusion normalized by whole body perfusion (for quantifying the muscle's active hyperemia). Twenty-eight human subjects participated in this Institutional Review Board-approved study, with 10 healthy subjects ( group A) for assessing interday reproducibility and 8 healthy subjects ( group B) for exploring the relationship between plantar-flexion load and induced muscle perfusion. In a pilot group of five elderly healthy subjects and five patients with PAD ( group C), we proposed a protocol that measured perfusion for a low-intensity exercise and for an exhaustion exercise in a single MRI session. In group A, perfusion estimates for calf muscles were highly reproducible, with correlation coefficients of 0.90-0.93. In group B, gastrocnemius perfusion increased linearly with the exercise workload ( P < 0.05). With the low-intensity exercise, patients with PAD in group C showed substantially lower gastrocnemius perfusion compared with elderly healthy subjects [43.4 (SD 23.5) vs. 106.7 (SD 73.2) ml·min^-1·100 g^-1]. With exhaustion exercise, G/S [1.0 (SD 0.4)] for patients with PAD was lower than both its low-intensity level [1.9 (SD 1.3)] and the level in elderly healthy subjects [2.7 (SD 2.1)]. In conclusion, the proposed MRI protocol and the new metrics are feasible for quantifying exercise-induced muscle hyperemia, a promising functional test of PAD. NEW & NOTEWORTHY To quantitatively map exercise-induced hyperemia in calf muscles, we proposed a high-resolution MRI method shown to be highly reproducible and sensitive to exercise load. With the use of low contrast, it is feasible to measure calf muscle hyperemia for both low-intensity and exhaustion exercises in a single MRI session. The newly proposed metrics for interpreting perfusion maps are promising for quantifying intermuscle vascular redistribution or a muscle's active hyperemia.

Blood transit time heterogeneity is associated to oxygen extraction in exercising human skeletal muscle

Capillary transit time and its heterogeneity have a marked impact on oxygen extraction in different tissues. Animal studies have shown that exercise shortens capillary transit time but the effects on capillary transit time heterogeneity have been controversial. We investigated whether exercise changes muscle blood transit time heterogeneity in humans in vivo and whether this heterogeneity correlates to muscle oxygen extraction. Muscle blood flow, blood volume, and oxygen uptake were measured during rest and low-intensity exercise in 12 healthy men using positron emission tomography (PET). Blood transit time was calculated from parametric PET images voxel by voxel by dividing blood volume with blood flow. Oxygen extraction was calculated by nonlinear fitting from dynamic 15O-O2 data. Relative dispersion (=SD/mean) was calculated as an index of heterogeneity of blood volume and blood transit time. As expected, exercise significantly shortened blood transit time and increased oxygen extraction. Furthermore, exercise decreased transit time heterogeneity (from 47 +/- 9% to 39 +/- 10%, P=0.07). Transit time heterogeneity correlated inversely to oxygen extraction in the exercising (r=-0.76, P=0.004) but not in the resting muscle (r=0.04, P=0.89). These results show that even low-intensity exercise shortens blood transit time markedly and decreases its heterogeneity in human skeletal muscle in vivo. Findings in correlation analyses suggest that less heterogeneous blood transit time associates to better muscle oxygen extraction during exercise. This may have effects on muscle oxygenation during exercise.

Partitioning the Energetics of Walking and Running: Swinging the Limbs Is Expensive

Explaining the energetics of walking and running has been difficult because the distribution of energy use among individual muscles has not been known. We estimated energy use by measuring blood flow to the hindlimb muscles in guinea fowl. Blood flow to skeletal muscles is controlled locally and varies directly with metabolic rate. We estimate that the swing-phase muscles consume 26% of the energy used by the limbs and the stance-phase muscles consume the remaining 74%, independent of speed. Thus, contrary to some previous suggestions, swinging the limbs requires an appreciable fraction of the energy used during terrestrial legged locomotion. Models integrating the energetics and mechanics of running will benefit from more detailed information on the distribution of energy use by the muscles.

Oxygen tension distribution in postcapillary venules in resting skeletal muscle

We tested the hypothesis that blood flow is distributed among capillary networks in resting skeletal muscle in such a manner as to maintain uniform end-capillary PO2. Oxygen tension in venules draining two to five capillaries was obtained by using the phosphorescence decay methodology in rat spinotrapezius muscle. For 64 postcapillary venules among 18 networks in 10 animals, the mean PO2 was 30.1 Torr (range, 9.7-43.5 Torr) with a coefficient of variation (CV; standard deviation/mean) of 0.26. Oxygen levels of postcapillary venules within a single network or single animal, however, displayed a much smaller CV (0.064 and 0.094, respectively). By comparison, the CV of blood flow in 57 postcapillary venules of 17 networks in 9 animals was 1.27 with a mean flow of 0.011 +/- 0.014 nl/s and a range of 3.7 x 10(-4) to 6.5 x 10(-2) nl/s. Blood flow of postcapillary venules within single networks displayed a lower CV (mean, 0.51), whereas that in individual animals was 0.78. Results indicate that among venular networks, heterogeneity of oxygen tension is less than that of blood flow and within venular networks the heterogeneity of oxygen tension is much less than that of blood flow. In addition, postcapillary PO2 was independent of flow among venules in which both were measured. Results of this study may be attributable to three factors: 1) O2 diffusion between adjacent capillaries and venules, 2) structural remodeling in regions of lower PO2, and 3) O2-dependent local control mechanisms.

Skeletal muscle capillary hemodynamics from rest to contractions: implications for oxygen transfer

Muscle contractions evoke an immediate rise in blood flow. Distribution of this hyperemia within the capillary bed may be deterministic for muscle O(2) diffusing capacity and remains unresolved. We developed the exteriorized rat (n = 4) spinotrapezius muscle for evaluation of capillary hemodynamics before (rest), during, and immediately after (post) a bout of twitch contractions to resolve (second-by-second) alterations in red blood cell velocity (V(RBC)) and flux (f(RBC)). Contractions increased (all P < 0.05) capillary V(RBC) (rest: 270 +/- 62 microm/s; post: 428 +/- 47 microm/s), f(RBC) (rest: 22.4 +/- 5.5 cells/s; post: 44.3 +/- 5.5 cells/s), and hematocrit but not the percentage of capillaries supporting continuous RBC flow (rest: 84.0 +/- 0.7%; post: 89.5+/-1.4%; P > 0.05). V(RBC) peaked within the first one or two contractions, whereas f(RBC) increased to an initial short plateau (first 12-20 s) followed by a secondary rise to steady state. Hemodynamic temporal profiles were such that capillary hematocrit tended to decrease rather than increase over the first approximately 15 s of contractions. We conclude that contraction-induced alterations in capillary RBC flux and distribution augment both convective and diffusive mechanisms for blood-myocyte O(2) transfer. However, across the first 10-15 s of contractions, the immediate and precipitous rise in V(RBC) compared with the biphasic and prolonged increase of f(RBC) may act to lower O(2) diffusing capacity by not only reducing capillary transit time but by delaying the increase in the instantaneous RBC-to-capillary surface contact thought crucial for blood-myocyte O(2) flux.

Transcutaneous neuromuscular electrical stimulation effect on the degree of microvascular perfusion in autonomically denervated rat skeletal muscle

OBJECTIVE

To determine the effect of transcutaneous neuromuscular electrical stimulation (TNMES) on the degree of microvascular perfusion in autonomically denervated skeletal muscle.

DESIGN

A completely randomized experimental design was used to compare the effects of TNMES on the degree of microvascular perfusion in the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles from autonomically denervated rats (Ch-TES) to the degree of microvascular perfusion in the same muscles of untreated controls, rats receiving only TNMES (TES), and rats receiving only autonomic denervation (shams).

INTERVENTION

All electrical stimulation treatments were delivered via carbon silicone surface electrodes, and evoked sustained tetanic contraction of the TA and EDL muscles. Autonomic denervation was achieved by the application of chlorisondamine.

MAIN OUTCOME MEASURES

The degree of microvascular perfusion was determined for the deep (DTA) and superficial (STA) region of the TA muscle and the EDL muscle by calculating their perfused microvessel/muscle fiber (PV/F) ratio.

RESULTS

The PV/F ratio in the DTA from Ch-TES animals was greater (p < or = .05) than that in the same muscle from control and sham animals. The PV/F ratios in the STA and EDL from Ch-TES animals were not significantly (p > .05) different from the PV/F ratio in the respective muscles of shams.

CONCLUSIONS

The response of the microvasculature in autonomically denervated skeletal muscle to TNMES that evokes muscle contraction is variable, and (2) mechanisms other than autonomic regulation may be involved in this hyperemic response.

Heterogeneity of red blood cell perfusion in capillary networks supplied by a single arteriole in resting skeletal muscle

Flow heterogeneity within capillary beds may have two sources: (1) unequal distribution of red blood cell (RBC) supply among arterioles and (2) unique properties of RBC flow in branching networks of capillaries. Our aim was to investigate the capillary network as a source of both spatial and temporal heterogeneity of RBC flow. Five networks, each supplied by a single arteriole, were studied in frog sartorius muscle (one network per frog) by intravital video microscopy. Simultaneous data on RBC velocity (millimeters per second), lineal density (RBCs per millimeter), and supply rate (RBCs per second) were measured continuously (10 samples per second) from video recordings in 5 to 10 capillary segments per network for 10 minutes by use of automated computer analysis. To quantify heterogeneity, mean values from successive 10-second intervals were tabulated for each flow parameter in each capillary segment (ie, portion of capillary between successive bifurcations), and percent coefficient of variation (SD/mean.100%) was calculated for (1) spatial heterogeneity among vessels (CVs) every 10 seconds and for the entire 10-minute sample and (2) temporal heterogeneity within vessels for every capillary segment and for the mean flow parameter. Analysis of these data indicates that (1) capillary networks are a significant source of both spatial and temporal flow heterogeneity, and (2) continuous redistributions of flow occur within networks, resulting in substantial temporal changes in CVs, although a persistent spatial heterogeneity of perfusion still exists on a 10-minute basis. In most networks, CVs decreased as supply rate within the network increased, thus indicating that rheology plays a significant role in determining the perfusion heterogeneity.

Capillary perfusion of the rat brain cortex. An in vivo confocal microscopy study

Confocal laser-scanning microscopy was used to visualize subsurface cerebral microvessels labeled with intravascular fluorescein in a closed cranial window model of the anesthetized rat. In noninvasive optical sections up to 250 microns beneath the brain surface, plasma perfusion and blood cell perfusion of individual capillaries were studied. Under resting conditions, in all cerebral capillaries the presence of plasma flow as demonstrated by the appearance of an intravenously injected fluorescent tracer within 20 seconds after injection. Plasma flow was verified even in capillaries that contained stationary erythrocytes or leukocytes; 91.1% of the capillaries contained flowing blood cells, 5.2% contained stationary blood cells, and no blood cells were seen in 3.6%. Mean blood cell velocity was 498.3 +/- 443.9 microns/s, and the mean blood cell supply rate was 35.75 +/- 28.01 cells per second. When capillaries were continuously observed for 1 minute, "on" and "off" periods of blood cell flow were noted. During hypercapnia (increase of PCO2 from 33.25 to 50.26 mm Hg), mean blood cell flux increased from 38.6 +/- 17.2 to 55.5 +/- 12.2 per second (P < .005, paired t test of mean values in six animals), and blood cell velocity increased from 519.5 +/- 254.8 to 828.5 +/- 460.8 microns/s (P = .074, paired t test of mean values in six animals). Homogeneity of blood cell flux increased as indicated by the coefficient of variation decreasing from 44.6% to 22.0%, and the portion of poorly perfused capillaries (blood cell flux, < 40 per second) decreased from 59.2% to 22.4%.(ABSTRACT TRUNCATED AT 250 WORDS)

An analysis of microcirculatory flow heterogeneity using measurements of transit time

Heterogeneity of blood flow distribution was measured in capillary networks in cremaster muscles of anesthetized Golden hamsters (nembutal, 70 mg/kg, ip). The relative dispersion of Q/PS, where Q is blood flow, P is permeability, and S is exchange surface area, was estimated; in microvascular terms (and assuming uniform permeability) this ratio reduces to vr/l, where v is plasma velocity, r is vessel radius, and l is vessel length, and where v/l = 1/T, where T is transit time. Distributions of 1/T across complete capillary networks significantly increased in relative dispersion from 68.2 to 97.8% during hyperemia, suggesting an increase in flow heterogeneity with increased inflow. In contrast, relative dispersion of 1/T did not change significantly from rest (72.0%) to hyperemia (66.1%) in capillary segments sampled randomly across the tissue. Other microvascular indices of flow (velocity, cell content) did not reflect the changes in relative flow dispersion shown by the changes in 1/T. The analysis demonstrates that estimates of flow heterogeneity are sensitive both to the selection of an appropriate flow variable and to the manner in which this variable is sampled in the capillary bed.

Microvascular response to ischemia, and tissue structure, in normal and atrophied skeletal muscle

The objective of this study was to explain why the normally observed reactive hyperemia in frog sartorius muscle following ischemia is absent when this muscle atrophies. Two possibilities were addressed: (1) absence is due to lowered O2 consumption, making the muscle more tolerant to ischemia, and (2) absence is linked to impaired vascular function in atrophy. We used 10 frogs after 2-3 months and 8 frogs after 7-14 months of laboratory captivity. Animals in the latter group had a significantly lower sartorius muscle weight, i.e., 85 +/- 33 vs 24 +/- 11 SD mg. Using intravital video microscopy, we measured red cell velocity in capillaries at the muscle surface, and densities of capillaries with moving (NCPER) and stationary red cells (NCSTAT) before and after 30 min ischemia. Ischemia induced a significant temporary increase in overall velocity (from 0.10 to 0.27 mm/sec) in normal muscles, but no increase in atrophied muscles. It resulted in no difference in NCPER between the two groups (preischemic levels in both groups: 15.0 cap/mm of test line), but in a significant difference in NCSTAT (3.8 vs 11.5 cap/mm in atrophy). Using light and electron microscopy, we also measured structural and ultrastructural parameters in both groups. In atrophied muscles the mean fiber cross-sectional area was lower (568 vs 1935 microns 2) and anatomical capillary density higher (892 vs 282 cap/mm2) than in normal muscles. Mitochondrial volume density was not statistically different from the 1.5% level in the normal muscle, while the lipid droplet volume density was larger (2.33 vs 0.58%). The percentage of capillaries with damaged endothelium was larger (33.5 vs 12.6%). Using histology, the white cell volume density per capillary volume was also found to be larger in atrophy (1.96 vs 0.83%). From the discrepancy between the lack of intergroup difference in preischemic NCPER and the 3.2-fold difference in anatomical capillary density we estimate that about 60% of capillaries were perfused with red cells in atrophied muscles. Although the preischemic rate of perfusion in these capillaries was comparable between the two groups, the postischemic response was not: reactive hyperemia was absent in atrophy. Our mitochondrial and lipid volume density data do not support the possibility that this absence was due to lowered O2 consumption, as these densities did not decrease with atrophy.(ABSTRACT TRUNCATED AT 400 WORDS)

Blood flow and blood volume in a transplanted rat fibrosarcoma: comparison with various normal tissues

Blood flow measurements following i.v. infusion of iodo-antipyrine labelled with 14C(14C-IAP) and blood volume measurements following i.v. injection of 125I human serum albumin and 51Cr-labelled red blood cells were made in a transplanted rat fibrosarcoma for comparison with various normal tissues. The tumour-blood partition co-efficient for 14C-IAP was found to be 0.79 +/- 0.07 which is similar to most of the normal tissues studied. The solubility of 14C-IAP in plasma was found to be higher than that in whole blood. Blood flow to tumours less than 1000 mm3 was found to be 17.9 +/- 4.0 ml blood 100 g tissue-1.min-1. These values were considered to be primarily measurements of nutritive flow. Blood in the tumours was found to occupy around 1% of the tissue space which was similar to that found for normal muscle and skin. There was no direct correlation between % blood volume and blood flow for the different tissues studied. The haematocrit of blood contained in tumour tissue was calculated to be significantly lower than that of blood contained in the normal tissues. It was suspected that permeability of tumour blood vessel walls to 125I-HSA could have accounted for this difference.

Localized heterogeneity of red cell velocity in skeletal muscle at rest and after contraction

Using intravital video microscopy, the present study focussed on a detailed analysis of Vrbc heterogeneity in a 2.4 x 1.8 x 0.15 surface volume of a frog sartorius muscle, before and after supramaximal contraction. Heterogeneity of Vrbc was evaluated (1) for an entire population of capillaries seen in this volume, (2) for a series of optical cross-sections, (3) along a series of longitudinal muscle strips, and (4) in terms of an asymmetry ratio for pairs of concurrent capillaries surrounding a muscle fibre. All four types of analysis showed an increased Vrbc homogeneity after contraction. Velocities became more homogeneous along rather than across muscle fibres. The mean asymmetry ratio became significantly larger during post-contraction hyperemia suggesting that each fibre receives a more uniform blood supply that will contribute to an improved exchange of materials across the capillary wall. The analysis of localized Vrbc heterogeneity provides new means of pinpointing the sources of perfusion heterogeneity. It enables, therefore, a specific experimental intervention that is aimed at an improved perfusion under both normal and abnormal conditions.

Variation in axial velocity profile of red cells passing through a single capillary

We have used an analysis of the velocity of individual red cells as the cells pass through a capillary in order to estimate the variability in cross-sectional area of the capillary lumen available for flow along the length of the vessel. The purpose of the study was to determine if there were irregularities of sufficient magnitude and frequency to support Secomb's hypothesis that local constrictions in the capillary lumen could hinder blood flow at low driving pressure, due to the energy required to deform red cells as they pass through the constriction. All capillary segments analyzed to date, in both rat and frog, have shown regions where the velocity of individual cells is consistently faster or slower than that of the mean velocity of all other cells in the same segment. There are approximately two constrictions per 100 microns in the rat and one per 100 microns in the frog. On average these constrictions appear to reduce the cross-sectional area by 30% in the rat and 16% in the frog. These results provide evidence in support of Secomb's hypothesis. In addition, our results from one bifurcation indicate that the capillary lumen increases in cross-sectional area as one moves from the parent vessel to the region of the bifurcation. Downstream of the bifurcation the lumen rapidly decreases in area by 45 to 54%. Thus a red cell must undergo even greater deformation as it passes through a capillary bifurcation than it will in most other sections of the capillary network.(ABSTRACT TRUNCATED AT 250 WORDS)

Capillary lengths and anastomoses in rat hindlimb muscles, studied by Aquablak perfusion during rest versus exercise

This investigation shows that provided an adequate perfusion time of the capillary network is allowed following injection of Aquablak, the presence of arterioles and capillaries having zero or near-zero flow rates can be demonstrated in resting muscle. During hyperemia, "flow recruitment" occurs in these vessels, as indicated by their perfusion with Aquablak. Our observations of Aquablak perfusion in hyperemic muscles show that in medial gastrocnemius, gracilis, and soleus the mean arteriolar-to-venular distances, and also the mean capillary pathlengths, were not dramatically different. What was striking, however, was the fact that capillary pathlengths in soleus were divided into twice the number of segments found in gastrocnemius and gracilis. This suggests the possibility that in oxidative muscles the capillary network may exhibit a much higher degree of branching than in glycolytic muscles. This would increase the area for diffusional exchange between blood and tissue in oxidative compared to glycolytic muscle.

Griffonia simplicifolia I: fluorescent tracer for microcirculatory vessels in nonperfused thin muscles and sectioned muscle

Previous studies on mice have revealed that the Griffonia simplicifolia I (GSI) lectin selectively binds to capillaries in a number of microvascular beds. These observations suggest that the lectin might be a suitable microvascular marker for physiological studies of skeletal muscle, particularly when fluorescent visualization of vessels is desired independently of their perfusion status. Since species and strain heterogeneity has been demonstrated for certain lectins associated with the microcirculatory vessels, lectin binding was studied in a number of muscles taken from the major species of mammals used for experimental purposes. Staining of cryostat sections confirmed the utility of GSI as a marker for capillaries from muscle of mice, rats, hamsters, rabbits, dogs, and monkeys. Differential staining of arterioles and veins was revealed by double labeling with GSI and antisera to Factor VIII-related antigen. Double labeling for GSI binding and alkaline phosphatase activity revealed that the GSI method detects many more capillaries and terminal arterioles than does the alkaline phosphatase method. GSI binding to unfixed whole mounts of thin skeletal muscles (hamster cheek pouch, mouse diaphragm, and rat cremaster) was studied to determine whether the GSI lectin would be a suitable marker for intravital studies. An extensive microvascular bed, including terminal arterioles, venules, and capillaries, was revealed which could be visualized in the complete absence of perfusion with fluorescent markers. These observations suggest that the GSI lectin may be extremely useful as a probe for the microcirculation of skeletal muscle in many types of physiological experiments.

Blood flow distribution and its temporal variability in stimulated dog gastrocnemius muscle

The distribution of blood flow in skeletal muscle stimulated to rhythmic isotonic contractions was studied by injections of radioactive microspheres into the arterial supply in 8 gastrocnemius muscles (mean weight 84 g) of 6 anesthetized dogs (20-25 kg body weight). The distribution of 10 micron microspheres in regions of about 0.5 g was very similar to that of the standard 15 micron microspheres, whereas that of 25 micron microspheres was more uneven. The coefficient of variation (CV = SD/mean) of the ratio of simultaneously injected 10 micron and 15 micron microspheres, 0.12, was taken as the inherent scatter of the method. The average spatial distribution inequality of 10-15 micron microspheres corresponded to a CV of 0.45 and the specific local blood flow inhomogeneity to a CV = 0.43 ( = square root 0.45(2) - 0.12(2], but there were marked differences between muscles. At equal blood flow levels, the inhomogeneity during reactive hyperemia was similar to that observed during stimulation. The temporal variability of blood flow in individual muscle pieces was obtained from the comparison of fractional trapping of 4 to 5 differently labeled microspheres injected at intervals of 2 min into steadily stimulated muscles. The mean CV for the variations in time was 0.23 and that corrected for methodological scatter, 0.19, but the differences in the extent of temporal blood flow changes among muscle pieces within a muscle and between different muscles were large. The presence of considerable spatial and temporal variations of blood flow in exercising muscle during apparent steady state may be important in limiting and/or modulating tissue O2 supply.

Arteriovenous distribution of transit times in cremaster muscle of the rat

Video digitization was applied to determine the arteriovenous transit time (TT) of blood flow between functionally paired arterioles and venules in cremaster muscle (rat). After a bolus injection of cell-free dye (FITC-Dextran) into a contralateral femoral artery, intensity-time curves (ITCs) of the fluorescent emissions were recorded in successive network divisions. TTs were calculated by cross-correlation of ITCs acquired from the video images digitized at rates of 5-10 frames/sec for up to 10 arteriole-venule (AV) pairs per video field whose position in the network was characterized in terms of centrifugal order of branching. Mean TT for first-order AV pairs averaged 3.74 +/- 1.62 (SD) sec and decreased significantly to 0.76 +/- 0.95 sec in the fifth-order pairs, with 75% of this reduction occurring between the first- and third-order pairs. A near threefold increase in coefficient of variation from first to fifth orders evidenced a marked increase in the spatial heterogeneity of TT at the level of the true capillaries. Comparison of TT with distance along the arterial tree from the tissue hilus revealed consistently stronger correlations than with AV order. These trends suggest that TT may be a superior indicator of the functional deployment of microvessels compared to branching order.

Evidence for increased perfusion heterogeneity in skeletal muscle during reduced flow

Recently, it has been demonstrated (D. Cousineau, C. P. Rose, D. Lamoureux, and C. A. Goresky, 1983, Cir. Res., 53, 719-730; K. Tyml, 1986, Microvasc. Res., 32, 84-98) that heterogeneity of microvascular flow depends on tissue metabolism. The objective of this study was to examine the possibility that, independent of metabolism, heterogeneity is also a function of flow. Using an intravital video-microscopic approach, we evaluated heterogeneity in the frog sartorius muscle at different flow rates while maintaining the muscle in the same exercised state. The flow was altered via partial aortal clamping. Exercised state was achieved by direct electrical stimulation. Heterogeneity of flow was evaluated in terms of the coefficient of variation (CV = SD/mean) computed from simultaneous measurements of red cell velocities in a capillary network. In addition to velocity analysis, the number of perfused capillaries crossing a 1-mm test line on the video monitor was counted. Among 10 networks from eight muscles, the overall preocclusion hyperemic velocity and CV were 0.42 +/- 0.15 SD mm/sec and 40 +/- 12%, respectively. The overall capillary count was 16.4 +/- 3.0 cap/mm. In all networks, increasing clamping reduced the mean hyperemic velocity and increased CV. For reductions to less than 50% of the preocclusion velocity, CV increased significantly (P less than 0.05), up to 88%. In 6 networks only, increasing clamping reduced capillary count, down to 10.2 cap/mm. This reduction was due to flow stoppages in capillaries situated randomly throughout the network. The data demonstrate for the first time that, for the same exercised state, heterogeneity of velocity depends on the mean velocity.(ABSTRACT TRUNCATED AT 250 WORDS)