The microvasculature in skeletal muscle. VI. Adrenergic innervation of arterioles in normotensive and spontaneously hypertensive rats

Hyperspectral analysis for perioperative perfusion monitoring-a clinical feasibility study on free and pedicled flaps

OBJECTIVES

In reconstructive surgery, flap monitoring is crucial for early identification of perfusion problems. Using hyperspectral imaging (HSI), this clinical study aimed to develop a non-invasive, objective approach for perfusion monitoring of free and pedicled flaps.

MATERIAL AND METHODS

HSI of 22 free (FF) and 8 pedicled flaps (PF) in 30 patients was recorded over time. Parameters assessed were tissue oxygenation/superficial perfusion (0-1 mm) (StO2 (0-100%)), near-infrared perfusion/deep perfusion (0-4 mm) (NIR (0-100)), distribution of haemoglobin (THI (0-100)), and water (TWI (0-100)). Measurements up to 72 h were correlated to clinical assessment.

RESULTS

Directly after flap inset, mean StO2 was significantly higher in FF (70.3 ± 13.6%) compared with PF 56.2 ± 14.2% (p = 0.05), whereas NIR, THI, and TWI were similar (NIR_p = 0.82, THI_p = 0.97, TWI_p = 0.27). After 24 h, StO2, NIR, THI, and TWI did not differ between FF and PF. After 48 h, StO2, NIR, and TWI did not differ between FF and PF whereas THI was significantly increased in FF compared with PF(p = 0.001). In three FF, perfusion decreased clinically and in HSI, 36(1), 40(2), 5(3), and 61(3) h after flap inset which was followed by prompt intervention.

CONCLUSIONS

StO2 < 40%, NIR < 25/100, and THI < 40/100 indicated arterial occlusion, whereas venous problems revealed an increase of THI. In comparison with FF, perfusion parameters of PF were decreased after flap transfer but remained similar to FF later on.

CLINICAL RELEVANCE

HSI provides objective and non-invasive perfusion monitoring after flap transplantation in accordance to the clinical situation. With HSI, signs of deterioration can be detected hours before clinical diagnosis.

Skeletal Muscle Microvasculature: A Highly Dynamic Lifeline

Skeletal muscle is highly irrigated by blood vessels. Beyond oxygen and nutrient supply, new vessel functions have been identified. This review presents vessel microanatomy and functions at tissue, cellular, and molecular levels. Mechanisms of vessel plasticity are described during skeletal muscle development and acute regeneration, and in physiological and pathological contexts.

Neural Control of Vascular Function in Skeletal Muscle

The sympathetic nervous system represents a fundamental homeostatic system that exerts considerable control over blood pressure and the distribution of blood flow. This process has been referred to as neurovascular control. Overall, the concept of neurovascular control includes the following elements: efferent postganglionic sympathetic nerve activity, neurotransmitter release, and the end organ response. Each of these elements reflects multiple levels of control that, in turn, affect complex patterns of change in vascular contractile state. Primarily, this review discusses several of these control layers that combine to produce the integrative physiology of reflex vascular control observed in skeletal muscle. Beginning with three reflexes that provide somewhat dissimilar vascular patterns of response despite similar changes in efferent sympathetic nerve activity, namely, the baroreflex, chemoreflex, and muscle metaboreflex, the article discusses the anatomical and physiological bases of postganglionic sympathetic discharge patterns and recruitment, neurotransmitter release and management, and details of regional variations of receptor density and responses within the microvascular bed. Challenges are addressed regarding the fundamentals of measurement and how conclusions from one response or vascular segment should not be used as an indication of neurovascular control as a generalized physiological dogma. Whereas the bulk of the article focuses on the vasoconstrictor function of sympathetic neurovascular integration, attention is also given to the issues of sympathetic vasodilation as well as the impact of chronic changes in sympathetic activation and innervation on vascular health. © 2016 American Physiological Society.

Effects of a hemoglobin-based oxygen carrier (HBOC-201) and derivatives with altered oxygen affinity and viscosity on systemic and microcirculatory variables in a top-load rat model

The effects of a polymerized bovine hemoglobin-based oxygen carrier (HBOC) and two derivatives on arteriolar vasoactivity and tissue oxygen tension were explored by administering HBOC in a dose-response fashion to normovolemic rats. The effect of oxygen affinity (P50) and viscosity was also explored, where the P50 and viscosity of the parent compound (HBOC-201) and its modifications (MP50 and LP50A) were as follows: 40mmHg and 3.0cP (HBOC-20l); 18mmHg and 4.4cP (MP50); and 17mmHg and 12.1cP (LP50A). Anesthetized male Sprague-Dawley rats (N=32) were randomized to receive one of the HBOC solutions, and were administered four infusions that increased in concentration for each dose (2, 22, 230 and 780mg/kg, IV). Data were compared to rats receiving an equivalent volume for each of the four infusions (0.4, 0.4, 3.8, 13.1ml/kg, IV) of iso-oncotic 5.9% human serum albumin (HSA). Increasing doses of either HBOC solutions or HSA were associated with increasing MAP. Doses 3 and 4 of HBOC-201, MP50 and HSA produced significant increases in MAP, whereas similar increases began at a lower dose (Dose 2) with LP50A. There were no significant changes in arteriolar diameters at any dose for any group. Interstitial partial pressure of oxygen (ISF PO2) remained unchanged for HBOC-201, MP50 and HSA, but LP50A caused a significant decrease in ISF PO2 compared to baseline after Doses 3 and 4. In conclusion, there was no evidence that HBOC-201 would perform better with increased oxygen affinity (40 to 18mmHg) or viscosity (3.0 to 4.4cP).

Sympathetic nerve-derived ATP regulates renal medullary vasa recta diameter via pericyte cells: a role for regulating medullary blood flow?

Pericyte cells are now known to be a novel locus of blood flow control, being able to regulate capillary diameter via their unique morphology and expression of contractile proteins. We have previously shown that exogenous ATP causes constriction of vasa recta via renal pericytes, acting at a variety of membrane bound P2 receptors on descending vasa recta (DVR), and therefore may be able to regulate medullary blood flow (MBF). Regulation of MBF is essential for appropriate urine concentration and providing essential oxygen and nutrients to this region of high, and variable, metabolic demand. Various sources of endogenous ATP have been proposed, including from epithelial, endothelial, and red blood cells in response to stimuli such as mechanical stimulation, local acidosis, hypoxia, and exposure to various hormones. Extensive sympathetic innervation of the nephron has previously been shown, however the innervation reported has focused around the proximal and distal tubules, and ascending loop of Henle. We hypothesize that sympathetic nerves are an additional source of ATP acting at renal pericytes and therefore regulate MBF. Using a rat live kidney slice model in combination with video imaging and confocal microscopy techniques we firstly show sympathetic nerves in close proximity to vasa recta pericytes in both the outer and inner medulla. Secondly, we demonstrate pharmacological stimulation of sympathetic nerves in situ (by tyramine) evokes pericyte-mediated vasoconstriction of vasa recta capillaries; inhibited by the application of the P2 receptor antagonist suramin. Lastly, tyramine-evoked vasoconstriction of vasa recta by pericytes is significantly less than ATP-evoked vasoconstriction. Sympathetic innervation may provide an additional level of functional regulation in the renal medulla that is highly localized. It now needs to be determined under which physiological/pathophysiological circumstances that sympathetic innervation of renal pericytes is important.

Are the precapillary sphincters and metarterioles universal components of the microcirculation? An historical review

The microcirculation is a major topic in current physiology textbooks and is frequently explained with schematics including the precapillary sphincters and metarterioles. We re-evaluated the validity and applicability of the concepts precapillary sphincters and metarterioles by reviewing the historical context in which they were developed in physiology textbooks. The studies by Zweifach up until the 1950s revealed the unique features of the mesenteric microcirculation, illustrated with impressive schematics of the microcirculation with metarterioles and precapillary sphincters. Fulton, Guyton and other authors introduced or mimicked these schematics in their physiology textbooks as representative of the microcirculation in general. However, morphological and physiological studies have revealed that the microcirculation in the other organs and tissues contains no metarterioles or precapillary sphincters. The metarterioles and precapillary sphincters were not universal components of the microcirculation in general, but unique features of the mesenteric microcirculation.

Whole microvascular unit deletions in dermatomyositis

OBJECTIVES

The pathophysiology of dermatomyositis (DM) remains unclear, combining immunopathological mechanisms with ischaemic changes regarded as a consequence of membranolytic attack complex (MAC)-induced capillary destruction. The study is a reappraisal of the microvascular involvement in light of the microvascular organisation in normal human muscle.

METHODS

Muscle microvasculature organisation was analysed using 3D reconstructions of serial sections immunostained for CD31, and histoenzymatic detection of endogenous alkaline phosphatase activity of microvessels. An unbiased point pattern analysis-based method was used to evaluate focal capillary loss. Double immunostainings identified cell types showing MAC deposits.

RESULTS

The normal arterial tree includes perimysial arcade arteries, transverse arteries penetrating perpendicularly into the endomysium and terminal arterioles feeding a microvascular unit (MVU) of six to eight capillaries contacting an average of five myofibres. Amyopathic DM cases (n=3) and non-necrotic fascicles of early DM cases (n=27), showed patchy capillary loss in the form of 6-by-6 capillary drop-out, corresponding to depletion of one or multiple MVUs. MAC deposits were also clustered (5-8 immunostained structures, including endothelial cells, but also pericytes, mesenchymal cells and myosatellite cells).

CONCLUSIONS

Capillary loss may not be the primary cause of muscle ischaemia in DM. The primary event rather stands upstream, probably at the level of perimysial arcade arteries around which inflammatory infiltrates predominate and which lumen may show narrowing in chronic DM. Ischaemia-reperfusion injury, which is favoured by autoimmune backgrounds in experimental models and which activates the complement cascade in capillaries, could represent an hitherto unsuspected (and potentially preventable) mechanism of muscle damage in DM.

Characterization of the thoracodorsal artery: morphology and reactivity

OBJECTIVES

In this paper, we describe the histological and contractile properties of the thoracodorsal artery (TDA), which indirectly feeds the spinotrapezius muscle.

METHODS

We used immunolabelling techniques to histologically characterize the TDA while the contractile properties were assessed using pressure arteriography.

RESULTS

Our results demonstrate that the TDA is composed of approximately one to two layers of smooth muscle cells, is highly innervated with adrenergic nerves, and develops spontaneous tone at intraluminal pressures above 80 mmHg. The reactivity of the TDA in response to various contractile agonists such as phenylephrine, noradrenaline, angiotensin II, serotonin, endothelin 1, and ATP, as well as vasodilators, shows that the TDA exhibits a remarkably comparable reactivity to what has been observed in mesenteric arteries. We further studied the different components of the TDA response to acetylcholine, and found that the TDA was sensitive to TRAM 34, a blocker of the intermediate conductance potassium channel, which is highly suggestive of an endothelium-dependent hyperpolarization.

CONCLUSIONS

We conclude that the TDA exhibits comparable characteristics to other current vascular models, with the additional advantage of being easily manipulated for molecular and ex vivo vasoreactivity studies.

Changes in muscle sympathetic nerve activity and vascular responses evoked in the spinotrapezius muscle of the rat by systemic hypoxia

Responses evoked in muscle sympathetic nerve activity (MSNA) by systemic hypoxia have received relatively little attention. Moreover, MSNA is generally identified from firing characteristics in fibres supplying whole limbs: their actual destination is not determined. We aimed to address these limitations by using a novel preparation of spinotrapezius muscle in anaesthetised rats. By using focal recording electrodes, multi-unit and discriminated single unit activity were recorded from the surface of arterial vessels.This had cardiac- and respiratory-related activities expected of MSNA, and was increased by baroreceptor unloading, decreased by baroreceptor stimulation and abolished by autonomic ganglion blockade. Progressive, graded hypoxia (breathing sequentially 12, 10, 8% O2 for 2min each) evoked graded increases in MSNA.In single units, mean firing frequency increased from 0.2±0.04 in 21% O2 to 0.62 ± 0.14 Hz in8% O2, while instantaneous frequencies ranged from 0.04–6Hz in 21% O2 to 0.09–20 Hz in 8%O2. Concomitantly, arterial pressure (ABP), fell and heart rate (HR) and respiratory frequency(RF) increased progressively, while spinotrapezius vascular resistance (SVR) decreased (Spinotrapezius blood flow/ABP), indicating muscle vasodilatation. During 8% O2 for 10 min, the falls in ABP and SVR were maintained, but RF, HR and MSNA waned towards baselines from the second to the tenth minute. Thus, we directly show that MSNA increases during systemic hypoxia to an extent that is mainly determined by the increases in peripheral chemoreceptor stimulation and respiratory drive, but its vasoconstrictor effects on muscle vasculature are largely blunted by local dilator influences, despite high instantaneous frequencies in single fibres.

Tyrosine hydroxylase immunoreactivity as indicator of sympathetic activity: simultaneous evaluation in different tissues of hypertensive rats

Vasomotor control by the sympathetic nervous system presents substantial heterogeneity within different tissues, providing appropriate homeostatic responses to maintain basal/stimulated cardiovascular function both at normal and pathological conditions. The availability of a reproducible technique for simultaneous measurement of sympathetic drive to different tissues is of great interest to uncover regional patterns of sympathetic nerve activity (SNA). We propose the association of tyrosine hydroxylase immunoreactivity (THir) with image analysis to quantify norepinephrine (NE) content within nerve terminals in arteries/arterioles as a good index for regional sympathetic outflow. THir was measured in fixed arterioles of kidney, heart, and skeletal muscle of Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) (123 ± 2 and 181 ± 4 mmHg, 300 ± 8 and 352 ± 8 beats/min, respectively). There was a differential THir distribution in both groups: higher THir was observed in the kidney and skeletal muscle (∼3-4-fold vs. heart arterioles) of WKY; in SHR, THir was increased in the kidney and heart (2.4- and 5.3-fold vs. WKY, respectively) with no change in the skeletal muscle arterioles. Observed THir changes were confirmed by either: 1) determination of NE content (high-performance liquid chromatography) in fresh tissues (SHR vs. WKY): +34% and +17% in kidney and heart, respectively, with no change in the skeletal muscle; 2) direct recording of renal (RSNA) and lumbar SNA (LSNA) in anesthetized rats, showing increased RSNA but unchanged LSNA in SHR vs. WKY. THir in skeletal muscle arterioles, NE content in femoral artery, and LSNA were simultaneously reduced by exercise training in the WKY group. Results indicate that THir is a valuable technique to simultaneously evaluate regional patterns of sympathetic activity.

Compartmentalized connexin 43 s-nitrosylation/denitrosylation regulates heterocellular communication in the vessel wall

OBJECTIVE

To determine whether S-nitrosylation of connexins (Cxs) modulates gap junction communication between endothelium and smooth muscle.

METHODS AND RESULTS

Heterocellular communication is essential for endothelium control of smooth muscle constriction; however, the exact mechanism governing this action remains unknown. Cxs and NO have been implicated in regulating heterocellular communication in the vessel wall. The myoendothelial junction serves as a conduit to facilitate gap junction communication between endothelial cells and vascular smooth muscle cells within the resistance vasculature. By using isolated vessels and a vascular cell coculture, we found that Cx43 is constitutively S-nitrosylated on cysteine 271 because of active endothelial NO synthase compartmentalized at the myoendothelial junction. Conversely, we found that stimulation of smooth muscle cells with the constrictor phenylephrine caused Cx43 to become denitrosylated because of compartmentalized S-nitrosoglutathione reductase, which attenuated channel permeability. We measured S-nitrosoglutathione breakdown and NO(x) concentrations at the myoendothelial junction and found S-nitrosoglutathione reductase activity to precede NO release.

CONCLUSIONS

This study provides evidence for compartmentalized S-nitrosylation/denitrosylation in the regulation of smooth muscle cell to endothelial cell communication.

Perfusion vs. oxygen delivery in transfusion with "fresh" and "old" red blood cells: the experimental evidence

We review the experimental evidence showing systemic and microvascular effects of blood transfusions instituted to support the organism in extreme hemodilution and hemorrhagic shock, focusing on the use of fresh vs. stored blood as a variable. The question: "What does a blood transfusion remedy?" was analyzed in experimental models addressing systemic and microvascular effects showing that oxygen delivery is not the only function that must be addressed. In extreme hemodilution and hemorrhagic shock blood transfusions simultaneously restore blood viscosity and oxygen carrying capacity, the former being critically needed for re-establishing a functional mechanical environment of the microcirculation, necessary for obtaining adequate capillary blood perfusion. Increased oxygen affinity due to 2,3 DPG depletion is shown to have either no effect or a positive oxygenation effect, when the transfused red blood cells (RBCs) do not cause additional flow impairment due to structural malfunctions including increased rigidity and release of hemoglobin. It is concluded that fresh RBCs are shown to be superior to stored RBCs in transfusion, however increased oxygen affinity may be a positive factor in hemorrhagic shock resuscitation. Although experimental studies seldom reproduce emergency and clinical conditions, nonetheless they serve to explore fundamental physiological mechanisms in the microcirculation that cannot be directly studied in humans.

Quantifying perivascular sympathetic innervation: regional differences in male C57BL/6 mice at 3 and 20 months

Perivascular sympathetic innervation density (PSID) is a key determinant of vasomotor responses to sympathetic nerve activity. However, total axonal length (for en passant neurotransmission) per vessel surface area has not been well defined, particularly while preserving 3-dimensional vascular structure. We developed a novel method for quantifying PSID using 3-dimensional anatomical reconstruction and compare a variety of blood vessels in Young (3 months) and Old (20 months) male C57BL/6 mice. Individual vessels were dissected and immunolabeled for tyrosine hydroxylase. The total length of fluorescent axons in defined vessel surface areas was quantified by mapping Z-stack images (magnification=760x). For Young mice, innervation densities (mum axon length/mum(2) vessel surface area) in mesenteric (0.075+/-0.002) and femoral (0.080+/-0.003) arteries were greater (P<0.05) than mesenteric veins (0.052+/-0.002) and gracilis muscle feed arteries (0.040+/-0.002). Carotid arteries and gracilis muscle veins were not immunoreactive nor were there significant differences in PSID between Young and Old animals. We demonstrate a novel approach to quantify sympathetic innervation of the vasculature while preserving its 3-dimensional structure and document regional variation in PSID that persists with aging in mice. This analytical approach may be used for quantifying PSID in other tissues that have superficial vessels which can be studied in situ or from which embedded vessels can be excised. With appropriate visualization of neuronal projections, it may also be applied to tissues that have other sources of superficial innervation.

Microvascular oxygen distribution in awake hamster window chamber model during hyperoxia

The microvascular effects and hemodynamic events following exposure to normobaric hyperoxia (because of inspiration of 100% O2) were studied in the awake hamster window chamber model and compared with normoxia. Hyperoxia increased arterial blood Po2 to 477.9 +/- 19.9 from 60.0 +/- 1.2 mmHg (P < 0.05). Heart rate and blood pressure were unaltered, whereas cardiac index was reduced from 196 +/- 13 to 144 +/- 31 ml.min-1.kg-1 (P < 0.05) in hyperoxia. Direct measurements in the microcirculation showed there was arteriolar vasoconstriction, reduction of microvascular flow (83% of control, P < 0.05), and functional capillary density (FCD, 74 +/- 16% of control), the latter change being significant (P < 0.05). Calculations of oxygen delivery and oxygen consumption based on the measured changes in microvascular blood flow velocity and diameter and estimates of oxygen saturation corrected for the Bohr effect due to the lowered pH and increased Pco2 showed that oxygen transport in the microvascular network did not change between normal and hyperoxic condition. The congruence of systemic and microvascular hemodynamics events found with hyperoxia suggests that the microvascular findings are common to most tissues in the organism, and that hyperoxia, due to vasoconstriction and the decrease of FCD, causes a maldistribution of perfusion in the microcirculation.

Oxygen gradients in the microcirculation

As arterialized blood transits from the central circulation to the periphery, oxygen exits through the vessel walls driven by radial oxygen gradients that extend from the red blood cell column, through the plasma, the vessel wall, and the parenchymal tissue. This exit determines a longitudinal gradient of blood oxygen saturation whose extent is inversely related to the level of metabolic activity of the tissue, being small for the brain and considerable for skeletal muscle at rest where hemoglobin is only half-saturated with oxygen when blood arrives to the capillaries. Data obtained by a variety of methods show that the oxygen loss is too great to be explained by diffusion alone, and oxygen gradients measured in the arteriolar wall provide evidence that this structure in vivo is a very large oxygen sink, and suggests a rate of oxygen consumption two orders of magnitude greater than seen in in vitro studies. Longitudinal gradients in the capillary network and radial gradients in surrounding tissue also show a dependence on the metabolic rate of the tissue, being more pronounced in brain than in resting skeletal muscle and mesentery. Mean PO2 values increase from the postcapillary venules to the distal vessels of this network while radial gradients indicate additional oxygen loss. This circumstance may be due to pathways with higher flow having higher oxygen content than low flow pathways as well as possible oxygen uptake from adjacent arterioles. Taken together, these newer findings on oxygen gradients in the microcirculation require a reexamination of existing concepts of oxygen delivery to tissue and the role of the capillaries in this process.

Transmitter characteristics of cutaneous, renal and skeletal muscle small arteries in the rat

AIM

We studied transmitter characteristics of proximal and distal arteries supplying skin (saphenous artery and its medial tarsal branch), kidneys (terminal branches of renal artery and interlobar arteries) and skeletal muscle (proximal and distal sections of external sural artery).

METHODS

Artery segments were mounted in an isometric myograph and intramural nerves were activated by electrical field stimulation. Adrenergic and purinergic components of the neurogenic response were blocked using phenoxybenzamine and alpha,beta-methylene adenosine triphosphate (mATP), respectively.

RESULTS

Arteries from skin or kidney developed rapid and prominent neurogenic contractile responses, with half-maximal amplitude reached within 5-15 s; responses in proximal vessels were greater than in distal vessels. Arteries from skeletal muscle responded to sympathetic stimulation with a moderate contraction developing over 1 min or more, the response of distal segments was greater than that of proximal segments. In skeletal muscle vessels the sympathetically evoked contraction was completely blocked by phenoxybenzamine, whereas in skin and renal vessels it was the combined effect of noradrenaline and adenosine triphosphate (ATP). Given alone, mATP did not change the magnitude of the response to nerve stimulation, but increased its latency and also potentiated the response to exogenous noradrenaline. In all vascular beds, distal vessels were more sensitive to noradrenaline and mATP.

CONCLUSION

It thus appears that the noradrenaline/ATP ratio of the sympathetic vasoconstrictor response differs between vascular beds in a way that is consistent with known differences in the selective regulation of regional vascular resistance by the sympathetic nervous system.

Diminished arteriolar responses in nitrate tolerance involve ROS and angiotensin II

Our purpose was to evaluate hyporesponsivity to nitric oxide (NO)-induced dilation in small arterioles during nitrate tolerance. An Alza osmotic pump was implanted in the left flank of adult rats (n = 56) for continuous administration of nitroglycerin (140 microg/h) or vehicle (propylene glycol). On postoperative day 3, arcade (approximately 50-microm diameter) and terminal (approximately 20 microm) arterioles were observed in the cremaster preparation with in vivo video microscopy. Local vascular responses were obtained with micropipette-applied NO donors, with and without superoxide dismutase (SOD), Mn(III) tetrakis(4-benzoic acid) porphyrin chloride (MnTBAP), or losartan. On day 3, NO-mediated dilation was significantly attenuated in nitroglycerin-treated rats. Attenuation was greater in the terminal arterioles compared with the arcades. Control responses were restored by SOD, MnTBAP, or losartan, suggesting a role for elevated angiotensin II and reactive oxygen species (ROS) as mediators of the attenuated NO dilation (nitrate tolerance). Addition of losartan to the drinking water likewise prevented nitrate tolerance. In summary, terminal arterioles are affected by nitrates to a greater extent than the arcade arterioles that feed them, in a process dependent on angiotensin II and ROS.

Blood substitutes

Red cell substitutes are a group of oxygen carriers designed to temporarily replace transfused blood. Each product is unique in its limitations and advantages. Research and development has been slow because of the far-reaching consequences of replacing an oxygen carrier outside of the red cell. Nevertheless, a number of products are in advanced clinical trials and nearing the market. When they are available for use it is likely that development will accelerate and even better products will substantially alleviate the world-wide shortage of blood for transfusion and enable the delivery of medical care to underserved populations. An important consequence of the development of these products has been a better understanding of how oxygen is delivered to tissues.

Haemodynamics and oxygen tension in the microcirculation of ischaemic skin flaps after neural blockade and haemodilution

The aim of this study was to evaluate the effects of neural blockade and extended normovolaemic haemodilution on haemodynamics and oxygenation in the microcirculation of ischaemic skin flaps using a recently developed island flap on the back of Syrian golden hamsters. One part of the flap was made ischaemic by inducing a random perfusion pattern following interruption of the axial blood supply. The model permitted quantitative assessment of the microhaemodynamics and oxygen tension in all microvascular segments by the use of intravital microscopy. Oxygen tension was measured with the phosphorescence decay method. Neural blockade was induced by applying 2% lidocaine to the neurovascular flap pedicle. Haemodilution was achieved by isovolaemic exchange of 50% of the total blood volume with dextran 70. One hour after surgery (baseline), centreline velocity was significantly reduced to 20-44% in all the microvessels in the randomly as compared to the axially perfused part, whereas the diameters were slightly larger (ns). In the control group, blood flow declined by 20-75% (P< 0.01 vs. baseline) over time in the entire flap. Flow reduction was significantly attenuated by haemodilution in the entire flap, but more pronounced in the ischaemic part. Neural blockade caused marked vasodilatation and significantly improved blood flow in the axially but not in randomly perfused microvessels. After 8 h, oxygen tension ranged from 4.0 to 6.1 mmHg in the axial part (means, ns between groups), whereas in the ischaemic part, it was 0.8-1.0 mmHg (P< vs. axial) in the control and neural blockade groups, and 1.7 mmHg (ns vs. axial and between groups) after haemodilution. Our findings indicate that neural blockade does not improve microcirculation and oxygenation in randomly perfused flap tissue because the sympathetic regulation of its microcirculation is overruled by autoregulatory mechanisms. Normovolaemic haemodilution, even after a 50% exchange, augments oxygenation in ischaemic flap tissue due to increased blood flow particularly in the randomly perfused tissue.

Myogenic reactivity of rat epineurial arterioles: potential role in local vasoregulatory events

Local control of neural blood flow is considered to reside in innervation of epineurial and endoneurial arterioles rather than in intrinsic autoregulatory mechanisms. With the use of an isolated vessel preparation and an in vivo approach, the present studies examined intrinsic vasomotor responsiveness of epineurial arterioles. Segments of epineurial arterioles, cannulated on glass micropipettes (40 micrometers) and pressurized in the absence of intraluminal flow, showed sustained pressure-dependent (30-90 mmHg) vasoconstriction and acute myogenic reactivity. Myogenic tone was unaffected by phentolamine (10(-6) M). Removal of extracellular Ca(2+) resulted in loss of spontaneous tone and passive behavior. Concentration-response curves for norepinephrine (10(-9)-3 x 10(-6) M) and relaxation to both acetylcholine (10(-8)-10(-5) M) and adenosine (10(-8)-10(-4) M) were obtained. Acetylcholine dilator responses were inhibited by N(G)-nitro-L-arginine methyl ester. Epineurial blood flow was measured in vivo using a laser-Doppler flow probe. Blood flow declined over a 2-h period after surgery, and during this time preparations developed responsiveness to the dilator acetylcholine. Phentolamine blocked vasoconstrictor responses to exogenous norepinephrine but only partially reversed the in vivo baseline tone. The time-dependent decline in epineurial blood flow was observed despite the presence of tetrodotoxin (1 microM), further confirming that tone was predominantly caused by myogenic rather than neurogenic mechanisms. It is concluded that because epineurial arterioles exhibit intrinsic myogenic reactivity, they have the potential to participate in local regulation of neural hemodynamics independently of their own innervation.

New transfusion strategies: red cell substitutes

Red cell substitutes are solutions that can potentially be used in emergencies or during surgery when rapid expansion of the blood volume with an oxygen carrier is needed. The three main types of products in development are based on cell-free hemoglobin, perfluorocarbon emulsions, or liposome-encapsulated hemoglobin. None is currently approved for clinical use, but several are in advanced clinical trials. Outside the red blood cell, hemoglobin is subject to degradation and heme loss. It readily diffuses in the plasma space and effectively scavenges nitric oxide. These properties must be understood and controlled if hemoglobin-based products are to fulfill their promise. The development of red cell substitutes affords us a deeper insight into how oxygen is delivered to tissues in the microcirculation and how blood-flow distribution is regulated within and between organs. As red cell substitutes become available to clinicians and scientists, clinical applications are expected to expand.

Altered muscle metabolism associated with vasoconstriction in spontaneously hypertensive rats

In the rat muscle vascular bed, vasoconstrictors either increase or decrease oxygen consumption (VO2). The present study compared the effects of norepinephrine (NE), angiotensin II (ANG II), and 5-hydroxytryptamine (5-HT) on vasoconstriction-associated metabolism in the constant-flow perfused hindlimb of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) in the absence of insulin. Basal perfusion pressure, VO2, glucose uptake, and lactate production were increased by 21.4, 11.9, 46.4, and 44.9% (P < 0.05 for all), respectively, in SHR, which also had higher blood pressure and metabolic rate (P < 0.05) in vivo. Dose-response curves for NE-induced perfusion pressure, VO2, and lactate production in SHR were shifted to the left compared with WKY. Associated with the increased perfusion pressure, NE-induced VO2 and glucose uptake were both decreased (P < 0.01), particularly at high concentrations. These differences were unaffected by 10 microM propranolol but were all diminished by further addition of prazosin (2.5 nM). ANG II stimulated VO2, glucose uptake, and lactate production in both strains, but the increased lactate production was smaller in SHR (P < 0.05) with a proportional decrease (P < 0.05) in glucose uptake. Conversely, 5-HT decreased VO2 in both strains (P < 0.01), and this effect was greater in SHR (P < 0.01). These data suggest that SHR muscle thermogenesis and glucose uptake are impaired during vasoconstriction, especially in response to NE.

Modulation of arteriolar sympathetic constriction by local nitric oxide: onset during rapid juvenile growth

The goal of this study was to determine if the endogenous activity of nitric oxide (NO) and/or prostanoids can limit arteriolar responses to increased sympathetic nerve activity in striated muscle, and to explore possible changes in these influences during rapid juvenile growth. Using intravital microscopy, arteriolar responses to 2-16 Hz sympathetic nerve stimulation were studied in the superfused spinotrapezius muscle of weanling (4-5 weeks old) and juvenile (7-8 weeks old) rats. Nerve stimulation elicited frequency-dependent arteriolar constrictions that were abolished in both age groups by the fast Na+-channel blocker tetrodotoxin or the alpha-antagonist phentolamine. Diameter and flow responses to 2-8 Hz stimulation were greater in juvenile rats than in weanling rats. In juvenile rats but not in weanling rats, the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) reduced arteriolar diameters and blood flow at rest and enhanced the arteriolar diameter and flow responses to sympathetic nerve stimulation. The cyclooxygenase inhibitor meclofenamate reduced resting arteriolar diameters in both age groups, but had no effect on responses to sympathetic nerve stimulation in either group. These results suggest that juvenile growth is accompanied by an overall increase in arteriolar responsiveness to sympathetic nerve activity, and by the onset of local NO activity that limits this increased responsiveness.

Serotonin-mediated acute insulin resistance in the perfused rat hindlimb but not in incubated muscle: a role for the vascular system

We have recently shown that the vasoconstrictor serotonin (5-HT) inhibits oxygen uptake in perfused hindlimb possibly due to vascular shunting. Thus in the present study the effect of 5-HT on insulin-mediated glucose uptake was assessed. Rat hindlimbs were perfused at constant flow with medium containing 8.3 mM glucose and a tracer amount of 2-deoxy-D-[1-3]glucose (2DG) with and without 10 microM 5-HT, 15 nM insulin and a combination of the two. 5-HT inhibited insulin-mediated stimulation of glucose uptake by 30.4% when added after insulin and 34.4% when added before insulin. In addition, 5-HT inhibited insulin-mediated 2DG uptake by perfused muscles with inhibition ranging from 32% (soleus) to 80% (extensor digitorum longus). The effects of 5-HT on insulin-mediated glucose uptake were partially reversed by vasodilation with carbachol. In contrast to the results for the hindlimb, 10 microM 5-HT had no significant effect on either basal glucose uptake or the stimulation of glucose uptake mediated by 15 nM insulin by isolated incubated soleus or extensor digitorum longus muscles. It is concluded that 5-HT impairs insulin-mediated glucose uptake in the perfused rat hindlimb that may derive from vascular shunting not apparent when muscles are incubated with 5-HT in vitro. These findings may have implications for the link between hypertension and diabetes.