A mathematical description of the myogenic response in the microcirculation

Multi-Channel Bioimpedance System for Detecting Vascular Tone in Human Limbs: An Approach

Vascular tone plays a vital role in regulating blood pressure and coronary circulation, and it determines the peripheral vascular resistance. Vascular tone is dually regulated by the perivascular nerves and the cells in the inside lining of blood vessels (endothelial cells). Only a few methods for measuring vascular tone are available. Because of this, determining vascular tone in different arteries of the human body and monitoring tone changes is a vital challenge. This work presents an approach for determining vascular tone in human extremities based on multi-channel bioimpedance measurements. Detailed steps for processing the bioimpedance signals and extracting the main parameters from them have been presented. A graphical interface has been designed and implemented to display the vascular tone type in all channels with the phase of breathing during each cardiac cycle. This study is a key step towards understanding the way vascular tone changes in the extremities and how the nervous system regulates these changes. Future studies based on records of healthy and diseased people will contribute to increasing the possibility of early diagnosis of cardiovascular diseases.

Effect of perivascular low dose ethanol on rat femoral vessels: Preliminary study

Vasospasm is one of the important causes of morbidity in free flap and replantation surgery. In secondary Raynaud's phenomenon, nearly half of the patients experience digital ulceration, pain and loss of function at least once in their lifetime. The aim of this study is to investigate the vasodilation effect of ethanol-mediated chemical denervation on peripheral vessels by topical administration. In this study, 27 Wistar albino male rats weighing 250-300 grams were used. The rats were randomly divided into three groups: saline (group S, n = 8), lidocaine (group L, n = 9) and 96% ethanol (group E, n = 9). According to group, 0.1 mL saline, 0.1 mL lidocaine and 0.1 mL ethanol were applied around the rat femoral neurovascular bundle. After the application, on the 0th day and 3th weeks, femoral artery and vein diameters were measured. After 3. weeks, histopathological samples from femoral artery, vein and nerve were evaluated. On the 0th day, the mean diameter of the femoral artery and vein was similar in group E and L and higher than group S. After three weeks, the vasodilatation effect of ethanol was increased in group E. In Group L and S, the vasodilatation effect was lost. Histopathological examination showed that ethanol significantly caused perivascular inflammation and nerve degeneration compared to other agents and did not cause endothelial damage. Vasodilatation obtained by ethanol is a rapid onset and long-lasting effect. It is also inexpensive and effective for peripheral vasodilatation.

Autoregulation and mechanotransduction control the arteriolar response to small changes in hematocrit

Here, we present an analytic model of arteriolar mechanics that accounts for key autoregulation mechanisms, including the myogenic response and the vasodilatory effects of nitric oxide (NO) in the vasculature. It couples the fluid mechanics of blood flow in arterioles with solid mechanics of the vessel wall and includes the effects of wall shear stress- and stretch-induced endothelial NO production. The model can be used to describe the regulation of blood flow and NO transport under small changes in hematocrit and to analyze the regulatory response of arterioles to small changes in hematocrit. Our analysis revealed that the experimentally observed paradoxical increase in cardiac output with small increases in hematocrit results from the combination of increased NO production and the effects of a strong myogenic response modulated by elevated levels of WSS. Our findings support the hypothesis that vascular resistance varies inversely with blood viscosity for small changes in hematocrit in a healthy circulation that responds to shear stress stimuli. They also suggest beneficial effects independent of changes in O(2) carrying capacity associated with the postsurgical transfusion of one or two units of blood.

Perspective: physiological role(s) of the vascular myogenic response

The vascular myogenic response is an inherent property of VSM in the walls of small arteries and arterioles, allowing these principal resistance segments of the microcirculation to respond to changes in transmural pressure. Elevated intraluminal pressure leads to myogenic constriction, whereas reduced pressure leads to myogenic dilation. This review focuses on the physiological significance of the myogenic response in microvascular networks. First, historical concepts related to the detection of stretch by the vessel wall are reviewed, including the wall tension hypothesis, and the implications of the proposal that the arteriolar network responds to Pp changes as a system of series-coupled myogenic effectors. Next, the role of the myogenic response in the local regulation of blood flow and/or Pc is examined. Finally, the interaction of myogenic constriction and dilation with other local control mechanisms, including metabolic, neural and shear-dependent mechanisms, is discussed. Throughout the review, an attempt is made to integrate historical and current literature with an emphasis on the physiological role, rather than the underlying signaling mechanisms, of this important component of vascular control.

A steady-state electrochemical model of vascular smooth muscle cells

A model of the steady-state electrochemical response of vascular smooth muscle cells to external stimuli is presented, which accounts for K, Na, and Ca fluxes. The results of the model are broadly in accordance with experimental data 1), at various transmural pressures; 2), with channel and pump blockade; and 3), under manipulation of external ionic concentrations. The model exhibits dual stable states which sometimes coexist, and abrupt transitions between these states may account for nongraded responses in arteries as external potassium or pressure is varied. The simulations suggest that changes in the intracellular sodium concentration ([Na]i) often accompany smooth muscle responses. For example, [Na]i values vary threefold over the range of pressures from 10 to 100 mmHg.

The myogenic response in isolated rat cerebrovascular arteries: vessel model

We develop an integrated model of isolated rat arterial resistance vessel (RV), which can simulate its major property of myogenic response. The vascular smooth muscle cell is an important component of the wall of this vessel, and serves as a vasomotor organ providing the active tension generation that underlies the myogenic response of the wall to stretch. In the previous study, we focused on the development of a smooth muscle cell model that can mimic the strain-sensing and force-generating features of the myogenic mechanism. In the current model, we embed this cell model in a larger vessel wall configuration, and couple the time course of cellular contractile activation to macroscopic changes in vessel diameter. The integrated model is used to mimic published pressure-vessel diameter data obtained from isolated RVs that are mounted in a hydraulic test apparatus. The model provides biophysically based insights into the myogenic mechanism as it responds to changes in transmural pressure, in the presence and absence of Ca2+ blockers applied to the bathing fluid.It mimics measured data very well and provides a model that is able to link events at subcellular level to macroscopic changes in vessel diameter. The model initiates a mechanistic approach to investigate myogenic response, which has not been taken previously by any other models.

Myogenic tone, reactivity, and forced dilatation: a three-phase model of in vitro arterial myogenic behavior

Myogenic behavior, prevalent in resistance arteries and arterioles, involves arterial constriction in response to intravascular pressure. This process is often studied in vitro by using cannulated, pressurized arterial segments from different regional circulations. We propose a comprehensive model for myogenicity that consists of three interrelated but dissociable phases: 1) the initial development of myogenic tone (MT), 2) myogenic reactivity to subsequent changes in pressure (MR), and 3) forced dilatation at high transmural pressures (FD). The three phases span the physiological range of transmural pressures (e.g., MT, 40-60 mmHg; MR, 60-140 mmHg; FD, >140 mmHg in cerebral arteries) and are characterized by distinct changes in cytosolic calcium ([Ca(2+)](i)), which do not parallel arterial diameter or wall tension, and therefore suggest the existence of additional regulatory mechanisms. Specifically, the development of MT is accompanied by a substantial (200%) elevation in [Ca(2+)](i) and a reduction in lumen diameter and wall tension, whereas MR is associated with relatively small [Ca(2+)](i) increments (<20% over the entire pressure range) despite considerable increases in wall tension and force production but little or no change in diameter. FD is characterized by a significant additional elevation in [Ca(2+)](i) (>50%), complete loss of force production, and a rapid increase in wall tension. The utility of this model is that it provides a framework for comparing myogenic behavior of vessels of different size and anatomic origin and for investigating the underlying cellular mechanisms that govern vascular smooth muscle mechanotransduction and contribute to the regulation of peripheral resistance.

Influence of baroreflex on volume elasticity of heart and aorta in the rabbit

Optimal ventriculoaortic coupling includes tuning of elastic properties. The ratio of effective arterial elastance and left ventricular endsystolic elastance is often taken as a measure for mechanical and energetical efficiency. The present study determined the time course of ventricular and aortic volume elasticity (VE = dp/dV) throughout a complete heartbeat. This was achieved by using changes of eigenfrequency of two catheter-transducer systems under closed chest conditions in rabbits. Short-term VE modulation was studied by a baroreflex response, as induced by pressure changes applied to the carotid sinus. Long-term changes were studied in atherosclerotic rabbits (12 wk of high-cholesterol feeding). The time course and mean values of ventricular and aortic VE were changed by the baroreflex stimulus. Cholesterol feeding diminished the response. The degree of ventriculoaortic coupling, as quantified by VE(Aorta)/VE(Ventricle) ratio, varied during a single ejection period. The large span allows either maximal energetical efficiency or maximal stroke work. Although normal rabbits adjusted their ventriculoaortic coupling during baroreflex input, the cholesterol-fed rabbits failed to do so.

Biomechanics of microcirculatory blood perfusion

The microcirculation represents a region of the circulation in which blood vessels are directly surrounded by the tissue and cells to which they supply nutrients and from which they collect metabolites. The cellular elements that make up the microcirculation have now been identified, and a large body of evidence has become available that provides molecular definitions of these elements. The blood flow is in a domain in which viscous stresses dominate, but the viscoelastic and active properties of cells lead to nonlinear problems. The ability of cells to actively control cytoplasmic mechanical properties and shape, as well as their membrane adhesion, leads to unique cell behavior in microvessels that has a direct influence on organ transport and function. There is also increasing evidence that the properties of the cells are in turn influenced by fluid shear stresses. These issues have greatly expanded the scope of microvascular analysis. The microcirculation is one of the sites in which diseases manifest themselves at an early stage. The application of biomechanical analysis of the microcirculation is starting to focus on diseases. The field is rich with problems of high significance.

Mathematical modelling of responses of cerebral blood vessels to changing intraluminal pressure

The authors have designed a mathematical model to investigate the influences of the physical and chemical properties of the cerebral blood vessel resistance on vessel diameter. The model is based on the way the total tension within the blood vessel walls varies due to specific ions interacting and affecting the vascular smooth muscle cells and the vascular walls. In particular, we shall model a series of calcium sites and derive a generalized equation of the diameter as a function of pressure. The model includes the action of the vascular smooth muscle cells and the elasticity of the vascular walls, the pressure exerted on the walls by the blood and the effect of alterations to their properties within the blood vessel. They are formulated in terms of three parameters: the diameter at zero pressure, the myogenic response as the pressure tends to zero and a term associated with the myogenic tone. All three parameters may be reliably extracted from diameter-pressure measurements. The model was successfully used in quantifying diameter oscillations and dynamic myogenic responses that are frequently observed both in vivo and in vitro. Finally, we tested the model on experimental data obtained from the resistance of cerebral vessels that have been isolated from rats. In particular, we have first shown that the blood vessel characteristics are such that the diameter change due to calcium ion variations is at a maximum value. Second, we have shown that blood flow affects the myogenic response and third, we can explain the affect of ATP on the vessel diameter.

Time-dependent autoregulation of renal blood flow in conscious rats

Response of renal vasculature to changes in renal perfusion pressure (RPP) involves mechanisms with different frequency characteristics. Autoregulation of renal blood flow is mediated by a rapid myogenic response and a slower tubuloglomerular feedback mechanism. In 25 male conscious rats, ramp-shaped changes in RPP were induced to quantify dynamic properties of autoregulation. Decremental RPP ramps immediately followed by incremental ramps were made for four different rates of change, ranging from 0.118 to 1.056 mmHg/s. Renal blood flow and cortical and medullary fluxes were assessed, and the corresponding relative conductance values were calculated continuously. During RPP decrements, conductance increased. With increasing rate of change of RPP decrements, maximum conductance increased from 10% to 80%, as compared with control. This response, which indicates the magnitude of autoregulation, was more pronounced in cortical versus medullary vasculature. Pressure at maximum conductance decreased with increasing rate of change of RPP decrements from 88 to 72 mmHg. During RPP increments, dependence of maximum conductance changes on the rate of change was enhanced (-20 to 110% of control). Thus, a hysteresis-like asymmetry between RPP decrements and increments, a resetting of autoregulation, was observed, which in direction and magnitude depended on the rate of change and duration of RPP changes. In conclusion, renal vascular responses to changes in RPP are highly dependent on the dynamics of the error signal. Furthermore, the method presented allows differentiated stimulation of various static and dynamic components of pressure-flow relationship and, thus, a direct assessment of the magnitudes and operating pressure range of active mechanisms of pressure-flow relationships.

Model-based assessment of pressure and flow-dependent coronary responses following abrupt pressure drops

The response of the coronary vasculature to an experimental manoeuvre of step-like decrease of the perfusion pressure has been investigated with a model. The coronary vasculature was simulated using a 'windkessel' scheme. Proximal resistance and compliance were assumed to be pressure-independent. The distal resistance, on the contrary, was controlled by a feed-back loop which accounts for the smooth muscle activation induced by the pressure variation. Three more parameters were introduced, and namely the smooth muscle activation time constant and the pressure-induced and flow-induced gains. The parameter values were assessed by comparing the model predicted coronary flow with the one actually measured in animals.

Mathematical considerations for modeling cerebral blood flow autoregulation to systemic arterial pressure

The shape of the autoregulation curve for cerebral blood flow (CBF) vs. pressure is depicted in a variety of ways to fit experimentally derived data. However, there is no general empirical description to reproduce CBF changes resulting from systemic arterial pressure variations that is consistent with the reported data. We analyzed previously reported experimental data used to construct autoregulation curves. To improve on existing portrayals of the fitting of the observed data, a compartmental model was developed for synthesis of the autoregulation curve. The resistive arterial and arteriolar network was simplified as an autoregulation device (ARD), which consists of four compartments in series controlling CBF. Each compartment consists of a group of identical vessels in parallel. The response of each vessel category to changes in perfusion pressure was simulated using reported experimental data. The CBF-pressure curve was calculated from the resistance of the ARD. The predicted autoregulation curve was consistent with reported experimental data. The lower and upper limits of autoregulation (LLA and ULA) were predicted as 69 and 153 mmHg, respectively. The average value of the slope of the CBF-pressure curve below LLA and beyond ULA was predicted as 1.3 and 3.3% change in CBF per mmHg, respectively. Our four-compartment ARD model, which simulated small arteries and arterioles, predicted an autoregulation function similar to experimental data with respect to the LLA, ULA, and average slopes of the autoregulation curve below LLA and above ULA.

Biomechanical model for the myogenic response in the microcirculation: Part I--Formulation and initial testing

The pressure dependent or myogenic contraction of arterioles is one of the most fundamental control mechanisms of microvascular perfusion. While many experimental observations have been obtained on the myogenic response, no generally accepted biomechanical model has been formulated. A novel biomechanical theory is proposed based on two fundamental assumptions: the arteriolar wall exhibits viscoelastic properties before and during myogenic contractions, and the contraction is achieved by a pressure dependent change of reference length. The formulation of the model and its application to different experimental procedures on microvascular smooth muscle in the literature is presented. The model describes closely a broad spectrum of steady and unsteady pressure dependent diameter variations of arterioles under a pressure dependent stimulus.

Role of the myogenic mechanism in the genesis of microvascular oscillations (vasomotion): analysis with a mathematical model

The possibility that spontaneous oscillations in microvessel caliber, called vasomotion, arise from the activity of the local myogenic mechanism is analyzed in this work using an original mathematical model. According to experimental results, the model assumes that the myogenic response in microcirculation (transverse arterioles and terminal precapillary-postcapillary microvessels) is characterized by both a static and a dynamic (i.e., rate-dependent) component. Computer simulations demonstrate that the myogenic mechanism of action, thanks to its strong rate-dependent component in terminal arterioles, can produce vascular instability and oscillations of vessel caliber without the need to assume the existence of a local pacemaker in smooth muscle cells. Moreover, these oscillations turn out similar, both in frequency and in shape, to those experimentally observed in microvascular networks. Finally, according to experimental data, several kinds of vasodilatory stimuli (such as arterial hypotension, increase in the tissue metabolic rate, and postischemic reactive hyperemia) cause stoppage of vasomotion and stabilization of vessel caliber.

Myogenic vasoconstriction in the rat kidney elicited by reducing perirenal pressure

Autoregulation of renal blood flow is generally believed to result from tubuloglomerular feedback and/or a vascular myogenic mechanism, but there is no consensus on the relative importance of these mechanisms. We designed an experiment in which tubuloglomerular feedback would tend to oppose a myogenic response: the denervated kidney in situ was enclosed in an airtight chamber and exposed to a 35 mmHg subatmospheric pressure for 1 to 10 minutes. Renal blood flow recorded by an electromagnetic flowmeter fell by 33% in the course of a few seconds. Renal venous concentration of inulin showed no consistent change, indicating similar reduction in glomerular filtration rate. Since urine flow also fell, it is likely that the tubular flow rate was reduced. The kidney volume expanded by 10-20%, and subcapsular interstitial fluid pressure was reduced from 6.8 to -8.6 mmHg. Arterial pressure remained unchanged, while renal venous pressure inside the chamber fell from 9.4 to 5.8 mmHg. Normalization of perirenal pressure gave rapid normalization of all parameters. Elevation of ureteral pressure attenuated or even prevented the renal blood flow reduction. Renal decapsulation or sympathetic blockade by phentolamine, or infusion of furosemide or 0.9% NaCl to inactivate tubuloglomerular feedback, did not prevent the renal blood flow reduction. We interpret the results to indicate that myogenic vasoconstriction greatly overpowered TGF and even surpassed the constriction predicted by a mathematical model based on maintenance of the preglomerular wall tension as estimated from transmural pressure.

Myogenic vascular regulation in skeletal muscle in vivo is not dependent of endothelium-derived nitric oxide

The hypothesis, based on in vitro experiments on large conduit arteries, that endothelium-derived nitric oxide is a mediator of vascular myogenic reactivity was tested in cat gastrocnemius muscle in vivo. This was done by comparing, in the absence and presence of effective endothelium-derived nitric oxide blockade by the specific inhibitors NG-monomethyl-L-arginine or NG-nitro-L-arginine methyl ester, myogenic responses in defined consecutive vascular sections to dynamic vascular transmural pressure stimuli, to arterial occlusion (reactive hyperaemia), and to arterial pressure changes (autoregulation of blood flow and capillary pressure). The results demonstrated that the myogenic vascular reactivity to quick ramp transmural pressure stimuli was not attenuated by endothelium-derived nitric oxide blockade, but rather reinforced. The amplitude of the reactive hyperaemia response was unaffected by endothelium-derived nitric oxide blockade, but its duration was shortened because of faster myogenic constriction, especially of large-bore arterial resistance vessels greater than 25 microns, in the recovery phase. Both the improved myogenic responsiveness to transmural pressure stimuli and the shortening of the reactive hyperaemia by endothelium-derived nitric oxide blockade suggested that endothelium-derived nitric oxide released in vivo acts as a 'metabolic' factor which certainly does not improve, but rather depresses myogenic vascular reactivity. Autoregulation of blood flow and capillary pressure were well preserved in the presence of endothelium-derived nitric oxide blockade. It was concluded from the results of these multifaceted tests that myogenic vascular regulation in skeletal muscle in vivo seems independent of endothelium-derived nitric oxide.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between hyperaemic response and viscoelastic properties in the coronary circulation of the dog

A sudden reduction in perfusion pressure evokes a transient hyperaemic response in the coronary arteries of anaesthetized dogs; its characteristics depend on the vasomotor tone. A heuristic model, which mimics the vascular bed with a lumped second-order system on the lines of the well-known Windkessel model, but accounting for the blood inertia, is proposed to describe that response and to quantify the viscoelastic properties of the system.

A single-tube mathematical model of reactive hyperaemia

A mathematical model of reactive hyperaemia is developed using quasi-steady flow in a single tube to represent blood flow in the vascular bed. The role of the myogenic response during reactive hyperaemia is examined by suggesting a linear relationship between tube cross-sectional area S and pressure p, in which S decreases as p increases, thereby modelling the response of the smooth muscle in the blood vessel walls to increases in p which the myogenic mechanism proposes. However, this simple relationship, together with the equations of continuity and Poiseuille flow, lead to an unstable equation for p which is inconsistent with the known boundary conditions. It is necessary to make S a function of p and delta p/delta t in order to achieve a stable response which implies that the myogenic response must be rate sensitive to pressure changes. The resulting equations are then solved for p, S, and flow Q by numerical integration and give results for Q which are in broad agreement with experiment. The model also suggests that the changing pressure gradient governs the flow in reactive hyperaemia rather than changes in the resistance of the blood vessels.

Dynamic response of the coronary circulation to a rapid change in its perfusion in the anaesthetized goat

1. We tested predictions of a mathematical formulation of a hypothesis of dynamic control of coronary blood flow by tissue oxygen tension. 2. The rate of change of adjustment of the coronary circulation to a step change in arterial perfusion was analysed in the cannulated main stem preparation of the anaesthetized goat. The variable studied was the ratio between driving pressure and coronary flow, each averaged per heart beat. The response of this ratio was measured following a sudden change in perfusion pressure with constant-pressure perfusion and a sudden change in flow with constant-flow perfusion. 3. The rate of change of the pressure-flow ratio was quantified by t50, the time required to establish half of the completed response. For a pressure decrease t50 was 4.9 +/- 0.2 s (n = 35) (mean +/- S.E.M., n = number of individual measurements), 11.3 +/- 1.2 s (n = 25) for a flow decrease, 14.5 +/- 1.6 (n = 34) for a pressure increase and 25.1 +/- 2.3 (n = 19) for a flow increase. 4. No effect of the level of flow or pressure on t50 was found for a decrease in perfusion. Furthermore, with a flow increase, the t50 value did not depend on the level of flow, which is in agreement with the outcome of earlier experiments where the response to a change in heart rate was measured. With a pressure increase, the mean t50 value of the pressure-flow ratio was lower at high perfusion pressure but the difference with low perfusion pressure was not significant (P = 0.11). 5. The t50 value in the cases of an increase in pressure and flow are similar to those found for a change of heart rate in an earlier study. 6. Unlike step changes of metabolic rate, some of the measured responses to mechanical step changes were not predicted by the oxygen hypothesis. It is suggested that the increased rate of coronary adjustment induced by the reduction of coronary perfusion is due to arteriolar smooth muscle mechanics which apparently differ in strength depending on the direction of change of the arteriolar dimensions. 7. This suggestion is strengthened by the results of experiments in which smooth muscle responses were abolished with adenosine.

A mathematical model of cerebral blood flow chemical regulation--Part II: Reactivity of cerebral vascular bed

In the present paper an original mathematical model of the chemical oxygen-dependent cerebral blood flow (CBF) regulation in the rat is proposed. Taking into account recent experimental works, the model assumes that oxygen acts on cerebral vessels through an indirect mechanism, mediated by the release of two metabolic substances (adenosine and H+) from tissue, and that any change in perivascular concentration of these substances affects the diameter of both the medium and small pial arteries as well as of intracerebral arterioles. The model is composed of several submodels, each closely related to a different physiological event. mathematical equations, which describe the reaction of the vasoactive portion of the cerebral vascular bed, are reported in detail and justified. The model permits the simulation of the role played by chemical factors in the control of CBF under many different physiological and pathological conditions in an attempt to clarify their relevance. Several events associated with an alteration in oxygen supply to tissue (auto-regulation to changes in arterial and venous pressure, reactive hyperemia following on cerebral ischemia, arterial hypoxia) have been simulated with the model. The results suggest that chemical factors, adenosine and H+, play a significant but not exclusive role in the regulation of the cerebral vascular bed. The action of other mechanisms (which are probably neurogenic) must be hypothesized to explain completely the CBF changes occurring in vivo.

A mathematical model of cerebral blood flow chemical regulation--Part I: Diffusion processes

This paper proposes a mathematical model which describes the production and diffusion of vasoactive chemical factors involved in oxygen-dependent cerebral blood flow (CBF) regulation in the rat. Partial differential equations describing the relations between input and output variables have been replaced with simpler ordinary differential equations by using mathematical approximations of the hyperbolic functions in the Laplace transform domain. This model is composed of two submodels. In the first, oxygen transport from capillary blood to cerebral tissue is analyzed to link changes in mean tissue oxygen pressure with CBF and arterial oxygen concentration changes. The second submodel presents equations describing the production of vasoactive metabolites by cerebral parenchyma, due to a lack of oxygen, and their diffusion towards pial perivascular space. These equations have been used to simulate the time dynamics of mean tissue PO2, perivascular adenosine concentration, and perivascular pH to changes in CBF. The present simulation points out that the time delay introduced by diffusion processes is negligible if compared with the other time constants of the system under study. In a subsequent work the same equations will be included in a model of the cerebral vascular bed to clarify the metabolite role in CBF regulation.

Integrated myogenic and metabolic control of vascular tone in skeletal muscle during autoregulation of blood flow

The hypothesis that autoregulation of blood flow in cat skeletal muscle is due to metabolic factors related to blood flow that interact with force-sensitive myogenic mechanisms is tested by means of a mathematical model. The vascular bed is assumed to consist of the reactive series-coupled proximal arterial and microvascular sections and the passive large vein section. Myogenic mechanisms are described by a slowly adapting force-receptor that determines the activation level of smooth muscle. The contractile machinery is represented by our recently developed mathematical model of smooth muscle mechanics. The metabolic control is described by the vasodilator theory whereby changes in the interstitial concentration of a dilator substance(s) alter the excitation-contraction coupling by shifting the [Ca2+]-force relationship. Experimental results indicate that the model correctly simulates vascular resistance responses to a variety of pressure stimuli, as well as autoregulation of blood flow and capillary pressure. Our results show that autoregulation of blood flow requires myogenic mechanisms which act synergistically with metabolic factors related to blood flow. It is also shown that the autoregulation of capillary pressure can be attributed to passive pressure-induced changes of postcapillary resistance, and that in autoregulation of blood flow the concentration of the mediating vasodilator metabolite(s) is a controlled variable being kept virtually constant in the range where blood flow is autoregulated.

Autoregulation of capillary pressure and filtration in cat skeletal muscle in states of normal and reduced vascular tone

The controversial hypothesis that capillary pressure (Pc) is autoregulated in response to arterial pressure (PA) alterations was tested in sympathectomized cat skeletal muscle by studying the relation between Pc and PA under conditions of well preserved vascular tone and reactivity, during papaverine-induced maximal vasodilatation (passive vascular bed), and during impaired vascular reactivity caused by preparatory surgery, or by low dose isoproterenol administration. The latter states resembled such under which Pc autoregulation unintentionally seems to have been studied previously. Capillary pressure was assessed with the Pcvenule method for continuous direct pressure recordings from capillaries/postcapillary venules (Mellander et al. 1987) and simultaneously derived from observed net transvascular fluid flux divided by CFC. Resistances in the whole vascular bed and in its pre- and postcapillary segments (Ra and Rv) were determined from recordings of blood flow, PA, Pc, and PV. During preserved vascular reactivity, Pc was found to be virtually constant, that is, almost perfectly autoregulated, over the PA range from 50 to 180 mmHg, whereas in the passive vascular bed there was a direct linear relation between Pc and PA (y = 0.137x + 11.69; r = 0.97). The delta Pc/delta PA ratio was about 1/70 in the normal reactive, and 1/7 in the passive, vascular bed, implying an increase in Pc by 1 mmHg for every 70 mmHg and every 7 mmHg increase in PA, respectively. Capillary pressure autoregulation was explained by precise adjustments of Ra/Rv in relation to PA elicited by myogenic and metabolic regulatory mechanisms. This protective reaction against plasma loss during increased PA was abolished during maximal vasodilation, and was much impaired by surgical trauma, partly via a beta-adrenergic inhibitory effect, and by isoproterenol, in turn causing gross transcapillary fluid fluxes. The latter findings might explain failing Pc autoregulation in some previous studies undertaken under seemingly similar conditions.

The interaction between noradrenaline activation and distension activation of the rabbit ear artery

Excised, pressurized segments of the rabbit ear artery have been used to examine the interaction between the transmural pressure and constriction of arteries by extraluminal noradrenaline. The bath temperature was kept at 32-33 degrees C to suppress instability and spontaneity of constriction. Fast, reproducible jumps in pressure were obtained by using a microcomputer to control an electropneumatic transducer. The arteries did not react actively to transmural pressure changes unless already activated by noradrenaline. Active arteries responded to distension by a pressure jump with a reproducible compensatory constriction which was unaffected by tetrodotoxin. The counteraction of distension was due primarily to enhanced activation of the muscle. Distension activation decreased with increasing constriction. Utilization of the force-generating capacity of the arteries either remained unchanged at 20-30% or, in one experiment, increased slightly when constriction against a transmural pressure of 60 mmHg was increased from 20 to 75% of maximal by raising the noradrenaline concentration. When the transmural pressure was 90 mmHg, the 35-55% utilization of the force-generating capacity either remained constant or fell as constriction was increased. Most of the force-generating capacity was available for counteracting the distension of moderately constricted arteries (less than 40% of maximal) produced by a 60-90 mmHg jump. More than 78% of the maximum capacity was used in attempting to overcome the distension when it was maintained by computer-controlled increases in transmural pressure. The moderate constrictions were produced with noradrenaline concentrations which were 500-10,000 times lower than that used to maximally activate the arteries. The rabbit ear artery possesses a powerful, distension-sensitive mechanism that acts to minimize diameter changes initiated by transmural pressure jumps. The diameter of the active artery was determined by a combination of noradrenaline activation and distension activation for constrictions up to at least 75% of maximal. It is concluded that the interaction between noradrenaline activation and distension activation helps the muscle to fulfil the special functional requirements imposed by the geometry of tubes. This type of myogenic control system may be particularly well developed in those muscular blood vessels exposed to pulsatile internal pressures.

An evaluation of the metabolic interaction with myogenic vascular reactivity during blood flow autoregulation

An attempt was made to evaluate the possible metabolic interaction with myogenic vascular reactivity during autoregulation of blood flow in sympathectomized cat skeletal muscle. This was done by studying the extent to which a purely myogenic response, elicited by a standardized 2 s vascular transmural pressure impulse stimulus was altered when mean arterial inflow pressure was varied in the range from 160 down to 40 mmHg. The observations were made during the steady state blood flows encountered at the different pressure levels. The data were corrected for the effects of physical factors inherent in altered basal vascular tone and intravascular pressure with the aid of a mathematical model for purely myogenic responses. The results demonstrated a flow dependent decline in myogenic vascular reactivity during reduction of arterial pressure, even in the range where blood flow was autoregulated quite effectively. This suggested a significant metabolic interaction with myogenic reactivity, an interpretation corroborated by a similar decline in myogenic reactivity found during more defined activation of the vascular metabolic control system by graded light muscle exercise. The fact that a significant metabolic interaction was revealed even at such minute flow changes that occur in the autoregulatory range indicates a high 'gain' in the metabolic feedback interacting, directly or indirectly, with myogenic mechanisms in local vascular regulation.

Active reactions of the rabbit ear artery to distension

Changes in the external diameter of active arteries, excised from the rabbit ear, were recorded following jumps in pressure within the arteries. The arteries were either spontaneously active or were constricted with noradrenaline. Active arteries dilated when the transmural pressure was jumped from 60 to 100 mmHg, but the dilatation was largely, sometimes completely, overcome by compensatory constriction within 1-2 min. Varying the constriction from 15 to 80% of the maximal constriction had no effect on the ability of the arteries to counteract distension. An average of 90 +/- 2% of the distension was overcome in 2 min and this was achieved against increases in stress (force/wall cross-sectional area) on the muscle of not less than 74%. Jumps in pressure rarely enhanced constriction and then only when constriction was slight (less than 15% of maximal). Restoring the transmural pressure to 60 from 100 mmHg produced a transient constriction when the initial constriction was less than 50% of the maximal constriction. The sequence of counteraction of distension and transient constriction on reversing the pressure jump was reproducible for many hours. Increasing constriction of the arteries first decreased and then, at maximal constriction, suppressed all transient changes in diameter. Smaller jumps in pressure produced less dilatation which was more readily prevented by increasing constriction. These results show that the wall of the ear artery possesses a pressure-sensitive, negative feed-back mechanism which minimized changes in diameter following jumps in pressure.