Functional capillary density in skeletal muscle during vasodilation induced by isoprenaline and muscular exercise

Mathematical Models for Peritoneal Transport Characteristics

Four mathematical models and for the description of peritoneal transport of fluid solutes are reviewed. The membrane model is usually applied for (1) separation of transport components, (2) formulation of the relationship between flow components and their driving forces, and (3) estimation of transport parameters. The three-pore model provides correct relationships between various transport parameters and demonstrates that the peritoneal membrane should be considered heteroporous. The extended threepore model discriminates between heteroporous capillary wall and tissue layer, which are assumed to be arranged in series; the model improves and modifies the results of the three-pore model. The distributed model includes all parameters involved in peritoneal transport and takes into account the real structure of the tissue with capillaries distributed at various distances from the surface of the tissue.

How the distributed model may be applied for the evaluation of the possible impact of perfusion rate on peritoneal transport, as recently discussed for clinical and experimental studies, is demonstrated. The distributed model should provide theoretical bases for the application of other models as approximate and simplified descriptions of peritoneal transport. However, an unsolved problem is the theoretical description of bi-directional fluid transport, which includes ultrafiltration to the peritoneal cavity owing to the osmotic pressure of dialysis fluid and absorption out of the peritoneal cavity owing to hydrostatic pressure.

Exercise-stimulated arterial transit time in calf muscles measured by dynamic contrast-enhanced magnetic resonance imaging

The primary goal of this study was to evaluate arterial transit time (ATT) in exercise-stimulated calf muscles as a promising indicator of muscle function. Following plantar flexion, ATT was measured by dynamic contrast-enhanced (DCE) MRI in young and elderly healthy subjects and patients with peripheral artery disease (PAD). In the young healthy subjects, gastrocnemius ATT decreased significantly (P < 0.01) from 4.3 ± 1.5 to 2.4 ± 0.4 sec when exercise load increased from 4 lbs to 16 lbs. For the same load of 4 lbs, gastrocnemius ATT was lower in the elderly healthy subjects (3.2 ± 1.1 sec; P = 0.08) and in the PAD patients (2.4 ± 1.2 sec; P = 0.02) than in the young healthy subjects. While the sensitivity of the exercise-stimulated ATT is diagnostically useful, it poses a challenge for arterial spin labeling (ASL), a noncontrast MRI method for measuring muscle perfusion. As a secondary goal of this study, we assessed the impact of ATT on ASL-measured perfusion with ASL data of multiple post labeling delays (PLDs) acquired from a healthy subject. Perfusion varied substantially with PLD in the activated gastrocnemius, which can be attributed to the ATT variability as verified by a simulation. In conclusion, muscle ATT is sensitive to exercise intensity, and it potentially reflects the functional impact of aging and PAD on calf muscles. For precise measurement of exercise-stimulated muscle perfusion, it is recommended that ATT be considered when quantifying muscle ASL data.

Microvascular circulation at cool, normal and warm temperatures in rat leg muscles examined by histochemistry using Lycopersicon esculentum lectin

Local cooling and/or warming of the body are widely used for therapy. For safer and more effective therapy, microvascular hemodynamics needs to be clarified. To examine blood circulation in rat leg muscles at 20, 30, 37 and 40°C, fluorescein isothiocyanate (FITC)-labeled Lycopersicon esculentum lectin was injected into the cardiac ventricle. Endothelial cells of open and functioning blood vessels were labeled by this lectin for 3 min and detected by immunostaining for lectin. The percentage of open and functioning capillaries of leg muscles by the avidin-biotin method was 89.8±3.3% at 37°C, while capillaries were unclear or unstained at 20 and 30°C, probably due to a decrease of blood flow. The results using the tyramide-dinitrophenol method were 58.6±15.0% at 20°C, 68.5±12.3% at 30°C, 83.8±5.7% at 37°C and 83.3±7.8% at 40°C. The value at 20°C was significantly different from those at 37 and 40°C. The results by the tyramide-biotin method were 85.5±5.3% at 20°C, 87.3±9.7% at 30°C, 94.7±3.6% at 37°C and 92.5±2.1% at 40°C. Based on these results, it was concluded that the blood flow of each capillary considerably decreased at 20 and 30°C and probably increased at 40°C, whereas the proportion of open and functioning capillaries was essentially unchanged.

In vivo determination of diffusive transport parameters in a superfused tissue

To address the hypothesis that functional changes in tissue transport can be related to structural alterations, we combined mathematical modeling with in vivo experimentation. The model concept includes interstitial diffusion and removal by a distributed microvasculature. Transport of solute and water across the peritoneum is measured via a plastic chamber affixed to the abdominal wall of anesthetized Sprague-Dawley rats. Solutions containing [14C]mannitol, with or without vasoactive compounds [control (C; n = 10), C + nitroprusside (NP; n = 10), C + norepinephrine (NE; n = 10)], were infused into the chamber, and the volume and tracer concentrations were determined over 60 min to calculate the mass transfer coefficient (MTC) and the water flux. At 60 min, FITC-dextran (500 kDa) was given to mark the perfused vasculature. After euthanasia, the tissue under the chamber was frozen, dried, sliced with a cryomicrotome, and examined with fluorescent microscopy and quantitative autoradiography. The microvessel density (×103/cm2: NE, 50 ± 10; C, 180 ± 7.0; NP, 225 ± 15) resulted in marked differences ( P < 0.05) in water flux (μl·min−1·cm−2: NE, 0.1 ± 0.1; C, 1.6 ± 0.4; NP, 1.0 ± 0.2) and in mannitol MTC (×103cm/min: NE, 0.9 ± 0.3; C, 3.8 ± 0.3; NP, 3.6 ± 0.6). Concentration profiles and calculated capillary permeability and tissue diffusivity were significantly different among the groups. These results demonstrate a direct correlation of mass transfer, diffusion, capillary permeability, and water flux with peritoneal vascular density and validate a method by which mechanistic changes in transport may be measured.

Dissociation between volume blood flow and laser-Doppler signal from rat muscle during changes in vascular tone

Although the laser-Doppler flowmetry (LDF) signal from skeletal muscle has been shown to provide a good measure of blood flow under some conditions, its behavior during administration of vasoactive substances has never been addressed. The aims of this study were to compare 1) changes in LDF signal with those in total muscle blood flow measured with radioactive microspheres after ganglionic blockade (chlorisondamine) and during administration of angiotensin II (ANG II), phenylephrine (PE), and isoproterenol (Iso) and 2) changes in vascular resistance estimated by the two techniques. The LDF signal from the biceps femoris muscle was investigated in anesthetized male Wistar rats. Ganglionic blockade led to a significant (P < 0.05) fall in mean arterial pressure (MAP) [medians (lower, upper quartiles): 78 (72, 83) vs. 127 (114, 138) mmHg under basal conditions], muscle blood flow (MBF, microsphere technique; 61%), and the LDF signal (29%). Muscle vascular resistance (MVR = MAP/MBF) was increased (64%, P < 0.05), but vascular resistance estimated as MAP/LDF signal (MVRLDF) was unchanged. During ANG II and PE infusions, MAP rose (P < 0.05) to 178 (155, 194) and 127 (124, 142) mmHg, respectively; MBF did not change compared with the preinfusion (postganglionic blockade) level and remained significantly (P < 0.05) lower than baseline, whereas the LDF signal increased up to a level not different from baseline. MVR rose and was significantly (P < 0.05) higher than baseline, whereas MVRLDF did not differ significantly from baseline. During Iso infusion, MAP fell [58 (56, 60) vs. 94 (92, 102) mmHg, P < 0.05], the LDF signal was reduced (49%, P < 0.05) despite a large increase in MBF (139%, P < 0.05), and MVR fell (74%, P < 0.05), whereas MVRLDF did not change vs. preinfusion level. Our results suggest that 1) changes in the LDF signal from muscle may not correlate with changes in total muscle blood flow measured by the microsphere technique during infusion of vasoactive substances and 2) the use of LDF data for estimation of MVR during changes in vascular tone in rat skeletal muscle is probably not appropriate.

Regulation of blood flow in the mammary microvasculature

Although milk yield of cows and goats is known to be closely related to the total flow of blood through the udder, a number of studies suggest that milk yield can vary independently. No studies have attempted to measure the proportion of total flow that is nutritive. Within the mammary gland, capillary networks form a basket-like architecture surrounding each alveolus. Notably, flow in individual capillaries is not constant and varies among capillaries. Capillary flow (measured by intravital microscopy) was decreased by oxytocin, which generally increased total flow in the mammary artery, suggesting that the proportion of total flow that is nutritive can vary. In addition to classic metabolic regulators (e.g., carbon dioxide and oxygen) of tissue blood flow, the mammary gland produces a number of vasodilatory compounds, including parathyroid hormone-related protein, insulin-like growth factor-I, prostacyclin, nitric oxide, and endothelin. All of these compounds have been shown to alter mammary blood flow. Mammary tissue also contains kallikrein and angiotensin-converting enzyme, which convert circulating kinins and angiotensin, respectively, into potent vasoactive compounds. A number of these compounds are produced by epithelial cells themselves, providing a mechanism for the functioning epithelium to control its own blood supply and, hence, nutrient flow for milk synthesis. In this review, we examine the nature of the mammary microcirculation, its behavior under different conditions, and some of the regulatory features of the mammary microvasculature.

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.

The influence of muscle contraction on the degree of microvascular perfusion in rat skeletal muscle following transcutaneous neuromuscular electrical stimulation

Electrotherapy is used clinically according to a variety of protocols and at various intensities with the intent of effecting any number of physiological changes. The purpose of this study was to determine if the increased degree of microvascular perfusion observed following 2,500 Hz transcutaneous neuromuscular electrical stimulation (TNMES) is dependent on evoked muscle contractions. The tibialis anterior (TA) and extensor digitorum longus (EDL) muscles from 30 male rats were analyzed. Six animals were untreated and served as controls, while the TA and EDL muscles of six animals were treated with TNMES at current intensities three times that needed to evoke a minimum visible contraction in the TA (M-TNMES). The remaining animals were treated with gallamine, which effectively blocked neurally mediated muscle contraction. The TA and EDL muscles of six gallamine-treated rats received no TNMES and served as shams (G-Sham), six received M-TNMES (GM-TNMES), and six received TNMES at intensities sufficient to produce sustained muscle contraction with a neuromuscular blockade in place (G-HIS). Perfused microvessels were labeled with fluorescein isothiocyanate-bovine serum albumin. The degree of microvascular perfusion was determined by calculating perfused microvessel/muscle fiber ratios (PV/F). The mean PV/F ratios of all groups were compared using Fisher's LSD (alpha = 0.05). When compared to controls, the PV/F ratios of the TA and EDL muscles in M-TNMES and G-HIS groups showed a significant (p < or = 0.05) increase while the G-Sham and GM-TNMES groups were similar to controls.(ABSTRACT TRUNCATED AT 250 WORDS)

An improved method for studying microvascular geometry using fluorescent dyes: preventing dye extravasation, preserving dye integrity and enhancing tissue morphometry

A procedure for stabilizing fluorescent markers used to study the microvascular geometry and morphometry of muscle tissue is described. The procedure involves fluorescent labeling of plasma, fixation of muscle tissue in 10% buffered formalin, and quick freezing. This procedure prevents extravasation of the fluorescent dyes out of the capillaries as frequently seen in other muscle microvascular techniques, thereby greatly increasing the time that capillaries are visible. We found that formalin may actually increase the rate of fluorochrome bleaching by photo-oxidation, but the increased rate of bleaching is more than offset by the greater concentration of dye trapped in the capillaries. Further, formalin fixation results in little distortion of the muscle fibers themselves, making this approach ideal for morphometric studies.

Causative role of coronary microvessels for the development and progression of chronic myocardial lesions in spontaneously hypertensive rats (SHR)

The pathomechanisms responsible for the development and progression of myocardial alterations in hypertensive heart disease are largely unknown. Using newly developed preparation and measuring procedures in 78 SHR and 82 controls aged 3-78 weeks, topological relations were detected between focal morphological appearances of chronic myocardial ischemia (fml.) and pathological microvessel (mv.) reactions characterized by morphometric signs of chronic contractions. The smallest ramifications are of particular pathogenic importance. A generalized peak of pathological mv. reactions between the 16th and 24th weeks is responsible for the development of first fml. The further progression of the area density of fml. from 1.26 +/- 0.85% (24th week) to 31.82 +/- 8.60% (78th week) is attributable to the further increase in pathological mv. reactions caused by organ-specific influences. The histological and morphometric findings suggest that the pathological mv. reactions are aggravated by their own effects at the local level.

Changes in capillary perfusion induced by different patterns of activity in rat skeletal muscle

The proportion of "perfused" capillaries was evaluated in rat tibialis anterior at rest and during two different types of contraction after timed injection of thioflavine S. Capillary/fibre (C/F) ratio was estimated for "perfused" capillaries--those filled with fluorochrome--(Cp) from photomicrographs. Sections were subsequently stained for alkaline phosphatase and C/F ratio was estimated for all capillaries (Ct). At 7.5 sec after injection of fluorochrome, Cp:Ct at rest was 0.32 +/- 0.092 in the oxidative core and 0.43 +/- 0.058 in the glycolytic cortex (means +/- SEM). This increased to 0.83 +/- 0.045 and 0.88 +/- 0.026, respectively, during selective activation of glycolytic fibres. Activation of all fibres led to a modest further increase (0.92 +/- 0.040 in the core and 0.91 +/- 0.035 in the cortex). Blood flow (measured by radiolabelled microspheres increased to a similar extent (fivefold) in both regions of the muscle during activation of glycolytic fibres; the further increase during maximal activation was much smaller in the cortex (from 4 to 41 ml/100 g/min) than in the core (from 7 to 196 ml/100 g/min). Increased capillary perfusion during muscle contractions was thus independent of the type of activity, while muscle blood flow increased more in oxidative than in glycolytic regions during maximal activation. Thus the increase in muscle blood flow with maximal activation cannot be accounted for by further recruitment of "unperfused" capillaries and must result from a significant increase in the velocity of capillary blood flow.

Reversible changes of skeletal muscular capillaries after application of a tourniquet

Morphological studies of skeletal muscular capillaries after a 60-min tourniquet application were performed in the rat extensor hallucis proprius muscle. On the basis of the period of reflow, the rats were divided into three groups (15-min group, 24-hr group, and 7-day group). FITC-albumin was injected intravenously to detect open capillaries. The percentage of interstitial space of the muscle was high in the 24-hr group, but had recovered in the 7-day group. Capillary density, calculated under a fluorescence microscope, was low in both the 15-min and 24-hr groups, but in the 7-day group, it had recovered. Intramuscular pressure was high in the 15-min group, whereas it was almost normal in the 7-day group. Electron microscopic studies showed stenosis or collapse of the capillaries in the 15-min group, whereas it was almost normal in the 7-day group. We consider that the changes in the microcirculatory system in the rat extensor hallucis proprius muscle after a 60-min tourniquet application are reversible.

Capillary rarefaction characteristic of the skeletal muscle of hypertensive patients

There is evidence that the rarefaction of the capillary bed is typical for the skeletal muscle of spontaneously hypertensive rats. We were therefore interested to learn whether there is also a rarefaction in skeletal muscle of human hypertensives. The number of capillaries was morphometrically analysed and counted in the quadriceps and the pectoralis major muscles of human normotensives (n = 12) and hypertensives (n = 15). The clinical diagnosis and certain pathological criteria, such as blood pressure (with or without antihypertensive therapy), heart weight, left ventricular wall thickness, the state of kidney arterioles and brain, and heart vessels, were used to classify the patients into two groups. The dissected tissue samples were prepared according to the GMA method and the capillary numbers per area were counted using light microscopy (250 x). The quadriceps muscle had a capillary density (per 2.5 mm2) of 442 +/- 51 in normotensives and 277 +/- 41 in hypertensive patients; in the pectoralis major muscle we counted 477 +/- 30 in controls and 232 +/- 28 in hypertensives. The rarefaction in the quadriceps muscle ranged by about 37%, in the pectoralis major muscle by about 51%. It is suggested that the reduction of the capillary surface area caused by the capillary rarefaction reduces the transcapillary fluid exchange and in that way prevents an overperfusion of the terminal vascular bed.

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.

An examination of the measurement of flow heterogeneity in striated muscle

This review leads us to a number of conclusions and suggestions for further study. First, we find wide differences in the meaning of flow heterogeneity, arising as a result of the different methods used. These differences will have to be reconciled to form a comprehensive view of the role of heterogeneity in determining vascular function. Second, in the future, the meaning of heterogeneity must be clearly defined and related to a particular microvascular component, and it is imperative that the differences in scale of heterogeneity be appreciated when comparing data from various laboratories. These heterogeneities have different implications for function, and failure to distinguish among them leads to confusion. Third, the degree to which perfusion heterogeneity is regulated in the microcirculation remains in doubt. Reports of variations in flow heterogeneity in response to physiological stimuli are for the most part based on highly questionable indirect methods. Fourth, the heterogeneity that can be demonstrated at the capillary level within striated muscle does not appear to be large relative to the capacity for the microcirculation to exchange most diffusible solutes. Thus, the inferences regarding heterogeneity, as evidenced by diffusible indicators, are likely to be the result of different preparations, damage to the preparations, or perhaps large-scale heterogeneities in the tissue. An alternate possibility would be that the heterogeneity occurs at the microvascular level but reflects some other aspect of microcirculatory function, such as length or hematocrit heterogeneities, but not flow heterogeneities. Fifth, flow heterogeneity within microvessels implies important consequences for capillary exchange and tissue oxygenation. Heterogeneities of velocity of a magnitude comparable to those observed by direct visualization of microcirculation can clearly produce reductions in oxygen supply to small tissue regions of a degree that may limit oxygen delivery, and thereby, tissue function. Sixth, flow heterogeneity may also influence capillary hematocrit and/or red cell spacing by producing cell separation at bifurcations and a resultant reduction in mean capillary tube hematocrit. There is as yet no agreement on why and how these hematocrits influence tissue oxygenation and function. Although several hypotheses are advanced to explain the distribution of blood flow and red cells within microcirculation, each lacks a critical experimental test at present.(ABSTRACT TRUNCATED AT 400 WORDS)

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

The two objectives of this study were to evaluate the spatial distribution of velocities measured simultaneously in a population of capillaries at the surface of a sartorius muscle in anesthetized frogs, and to estimate the cross-sectional density of capillaries perfused with red cells in this muscle at rest and after supramaximal stimulation. In each of 10 muscles studied, the mean velocity associated with this distribution increased significantly after the stimulation (overall increase from 0.12 mm/sec in control to 0.46 mm/sec at the peak of hyperemia). The coefficient of variation (i.e., ratio of standard deviation to mean velocity), however, decreased, indicating that the individual velocities became relatively more homogeneous and that microvascular adjustments occurred at the capillary level. Additionally, the proportion of capillaries with zero or very low velocities (0-0.1 mm/sec) was reduced practically in all muscles suggesting an improved oxygen exchange after every stimulation. In contrast to these consistent velocity responses in all muscles, capillary recruitment was detected only in 5 out of 10 muscles (overall density increase from 104 to 134 cap/mm2). This partial occurrence of recruitment (range, 23-149%) was associated with control mean velocities less than 0.1 mm/sec. Since, in light of the velocity data, the absence of recruitment in the 5 remaining muscles did not necessarily signify the lack of microvascular adjustments at the capillary level, it is proposed that more sensitive indexes, such as the reduction in the proportion of low flow capillaries and the increase in relative homogeneity, be used in addition to or instead of recruitment, to describe more completely the microvascular response to stimulation.

Distribution patterns of blood flow in the rabbit tenuissimus muscle in response to brief ischemia and muscular contraction

The distribution of the microvascular blood flow in the tenuissimus muscle of the rabbit was examined by intravital microscopy during postocclusion (reactive) hyperemia (RH) elicited by 5 min of circulatory arrest and postexercise (functional) hyperemia (FH) elicited by 1 min of muscular contraction. The main feeding arterioles in this muscle supply two vascular areas--the muscle capillaries and vessels in adjacent connective tissue. From flow measurements made at two different sites in these arterioles, the fractional distribution of the blood flow between the muscle capillaries and the connective tissue at rest and during hyperemia was determined. Both stimuli elicited profound hyperemic responses lasting for approximately 3 min with peak flow values of 7.6 (RH) and 5.5 (FH) times the control flow. There was no major change in the fractional distribution of the hyperemic blood flow in response to the brief ischemia from that observed under resting conditions. Muscular contraction, on the other hand, resulted in a considerably greater increase in the flow to the muscle capillaries than in that to the connective tissue. Differences in reactivity between larger and smaller arteriolar segments within the muscle tissue were considered to underlie the fractional redistribution seen in response to this stimulus. Differential control of arteriolar blood supply and capillary perfusion may thus be exerted by these vascular segments and may be of functional importance in the adjustment of flow to increased tissue oxygen demand.

Turnover rate of interstitial albumin in rat skin and skeletal muscle. Effects of limb movements and motor activity

Fractional removal rate (FRR) of radioactive-labelled human serum albumin (I-HSA) injected subcutaneously or intramuscularly was determined by external gamma-detecting equipment. Radioactivity over the injection site fell monoexponentially during registration periods up to 6 h. The FRR was calculated as the turnover rate constant of the radioactivity removal. The FRR fell into one of two ranges: in anaesthetized rats FRR was 0.02-0.03 h-1, and in awake and freely moving rats FRR was 0.08-0.11 h-1. In awake rats, FRR was similar during day and night (spontaneous motor activity is four times higher during the night). Passive limb movements at 1 Hz in anaesthesia increased FRR in skin to that in awake rats, while FRR in skeletal muscle was unchanged. Immobilization resulted in FRR similar to that in anaesthesia. Interstitial albumin mass did not change during 6 h of anaesthesia. It is concluded that the observed FRR reflects steady state changes in albumin turnover. In the awake and freely moving rats at least 3/4 of the removal of albumin is by the lymphatics. Calculated lymph flow was 10 microliters g-1 h-1 and 40 microliters g-1 h-1 in skeletal muscle and skin respectively with corresponding figures during anaesthesia of 3 microliters g-1 h-1 and 10 microliters g-1 h-1 respectively.

Transcapillary extravasation rate of albumin in rat skeletal muscle. Effect of motor activity

Transcapillary extravasation of albumin in hindlimb and neck skeletal muscle of rats was determined as extravascular plasma equivalent volumes (I-HSAev), 1-168 h after a single intravenous injection of radiolabelled human serum albumin (I-HSA). One experimental group was immobilized in pentobarbital anaesthesia for the first 8 h after injection of I-HSA. With longer extravasation periods, the rats then woke up and moved freely. In the other experimental group, injections were made in ether anaesthesia allowing the rats to move freely within 10 min after injection. One hour after injection, I-HSAev in the immobilized group averaged 1.5 10(-3) ml g-1 in neck and hindlimb muscle. In the mobile group, I-HSAev was two to three times higher than in the immobilized rats up to 12 h in both muscle groups (P less than 0.05). At 24 h and later I-HSAev was similar in mobile and immobile rats (P greater than 0.05). The I-HSAev increased up to 48 h, levelling off at about 0.06 and 0.08 ml g-1 in hindlimb and neck skeletal muscle, respectively. Corresponding interstitial albumin concentration, calculated from steady-state I-HSAev and interstitial fluid volume (51Cr-EDTA space), averaged 48% and 34% of plasma albumin concentration, respectively.

Sequential perfusion of skeletal muscle capillaries

Rats were injected intraarterially with a fluorescent dye that binds to capillary endothelium, thereby labeling any capillary through which it has passed. After 10, 15, or 30 sec of circulation of the dye blood flow was interrupted, the gastrocnemius was frozen, and the density and distribution of labeled capillaries were measured in transverse sections of the central portion of the medial head. These tissue sections were then counterstained by the myosin ATPase method for capillaries to mark all capillaries. After 10 sec, 45% of all capillaries were labeled and after 15 sec, 59% of all capillaries were labeled. Thirty seconds after injection, all capillaries were labeled with the fluorescent dye. In all three time intervals, the distributions of labeled capillaries were ordered, suggesting that there is a tissue-level control mechanism for regulating capillary perfusion to maintain relatively short maximal oxygen diffusion distances.

Flow and distribution of India ink in microvessels of the frog

Velocities of an India ink front and of RBCs moving ahead of it were studied by intravital microscopy in capillaries of frog mesentery and skeletal muscle. Measurements were made during microperfusion of single vessels or groups of connected vessels in mesentery, and following intravenous ink injection in both tissues. The presence of ink did not appear to interfere with microvascular flow or vasomotion during the period of observation. On the average, the ink spearhead moved only slightly faster than the RBCs. There was substantial variation in relative velocities of RBCs in the same vessel and in the relative velocities of ink front and RBCs. The time course of ink filling showed substantial heterogeneity of flow in mesentery, more nearly uniform flow in skeletal muscle. Comparison of the measured velocity ratios of ink to RBCs with published observations on relative velocities of RBC to blood suggest that the advancing, apparently parabolic front of ink moves at less than twice the mean blood velocity. This is due in small part to diffusive dispersion of the ink particles in the laminar flow gradient, but more largely to stochastic dispersion of the front by interaction with RBCs and by displacement at branches.

Non-homogeneous blood flow distribution in the rabbit tenuissimus muscle. Differential control of total blood flow and capillary perfusion

Structural and functional relationships underlying the blood flow distribution in the rabbit tenuissimus muscle were examined by means of intravital microscopy. A majority of the main feeding arterioles (transverse arterioles) continued into adjacent connective tissue, after giving off branches (terminal arterioles) within the muscle tissue to supply the muscle capillaries. The transverse arterioles thus supplied two vascular areas, although the major part of the arteriolar flow, under normal resting conditions, was distributed to the muscle capillaries--a flow fraction over which the terminal arterioles exerted ultimate control. The fractional distribution of the blood flow between muscle and connective tissue was determined by the relative contributions of the transverse and terminal arterioles to the vascular resistance. These arteriolar segments showed a differential response to an increase in oxygen availability (elevated ambient pO2), resulting in a total reduction of muscle capillary flow, but no concomitant change in the flow to connective tissue. A decrease in perfusion pressure, on the other hand, led to similar flow changes in the muscle and connective tissue circulation, which was attributed to proportionate resistance changes in the transverse and terminal arterioles. Differences between the larger transverse and smaller terminal arterioles in their sensitivity to various stimuli may form a functional basis for a differential control of arteriolar blood supply and capillary perfusion in this muscle.

The preparation and use of fluorescent-protein conjugates for microvascular research

A procedure is described for making large quantities (100 ml) of fluorochrome-labeled albumin. Chromatographic techniques are described for the purification of commercial albumin (BSA) and the purification of albumin from serum. We report experimentally determined optimal conditions for the covalent attachment of fluorescent dyes (rhodamine isothiocyanate (RITC) and fluorescein isothiocyanate (FITC] to albumin. Subsequent removal of all unreacted fluorescent material (UFM) was achieved using charcoal adsorption. We observed no loss of protein following charcoal treatment. The final protein conjugate was analyzed by polyacrylamide gel electrophoresis, gel chromatography, and isoelectric focusing. The conjugates were determined to be free of UFM and homogeneous with respect to molecular weight. However, FITC conjugation lowered the average isoelectric point of albumin by 0.1 to 0.3 pH units. Illustrations of combining fluorescence microscopy with FITC-BSA and RITC-BSA to view microvascular phenomena in skeletal muscle and the heart are given. Knowledge of the biochemical characteristics of the fluorochrome employed is important for proper interpretation of experimental results using this technique.

Distribution of capillary blood flow in the microcirculation of the hamster: an in vivo study using epifluorescent microscopy

In vivo epifluorescent microscopy (EPI) was used to study capillary perfusion in superfused hamster cheek pouch, and cremaster and sartorius muscle preparations. In cheek pouches and cremaster muscles, in vivo epifluorescence microscopy (EPI) was compared to in vivo transillumination microscopy (TRANS) and the former was found to allow detection of a larger number of capillaries--34% more capillaries in cheek pouch and 21% more in cremasters were observed with EPI. The fraction of capillaries containing erythrocytes alone, plasma alone or plasma plus red cells was determined in all three tissues. Also, the fraction of capillaries unperfused was noted. Less than 2% of the capillaries contained plasma alone. The number of capillaries which contained stationary erythrocytes varied with vasomotor tone. In control cremaster and sartorius muscles 17 and 13% of observed capillaries were unperfused but contained erythrocytes. Ninety-eight percent of capillaries contained stationary erythrocytes in cremaster muscles vasoconstricted with 21% oxygen. From these observations we conclude (1) functional capillary counts obtained with TRANS may represent underestimates of the true number; (2) plasma shunts do not appear to be a significant factor in normal microcirculatory function in the hamster cheek pouch, or in cremaster and sartorius muscles; (3) since unperfused capillaries contain red cells, red cell counts in histological sections will overestimate functional capillarity. The magnitude of the overestimation will be a function of vascular tone and capillary hematocrit.