**2. Myogenic mechanism**

The myogenic mechanism which was first described by Bayliss is an intrinsic property of the vascular smooth muscle to respond to changes in intravascular pressure which is independent of other mechanisms of tone regulation including neural, metabolic, and hormonal influences [19]. The intrinsic nature of the myogenic response is supported by its existence in arteries and arterioles that have been sympathetically denervated and had their endothelium removed [20] thus leaving only the vessel itself to initiate and execute the response. The prototypical response of the vascular smooth muscle in response to an increase in intraluminal pressure is initial distension quickly followed by a constriction. The opposite can be said in situations of decreased intraluminal pressure; a fall in intraluminal pressure results in vessel collapse followed by dilation [21]. The myogenic response has several physiological roles including the establishment and maintenance of basal vascular tone (some degree of constriction) so that resistance may be increased or decreased by metabolic vasoconstrictors or vasodilators respectively, in order to regulate tissue perfusion. The establishment and maintenance of this partially constricted state

**23**

*Cerebral Vascular Tone Regulation: Integration and Impact of Disease*

in a pressurized vessel is referred to as myogenic tone. Additionally, the myogenic tone has a role in flow and pressure regulation. It functions by constricting to drop the pressure that reaches the downstream capillaries and protect them from edema or vascular remodeling associated with hypertension in the capillaries [22–25]. Equally important as protecting from hypertension is the ability of the vasculature to promote flow during low pressure by dilating. This ability to alter diameter over a range of pressures is referred to as myogenic reactivity. Beyond the local implications for the regulation of vascular tone, resistance to flow also has implications for systemic blood pressure since mean arterial pressure is the product of total peripheral resistance and cardiac output. This relationship illustrates system-wide implications of accurate vascular tone regulation. While the myogenic response is present throughout the body in a variety of vessels (arterioles, veins, lymphatic vessels) [26] the aforementioned functions of the myogenic response are particularly important in the cerebral circulation due to the catastrophic outcomes associated with under or over perfusion including unconsciousness and edema respectively. It is therefore not surprising that the most prominent myogenic response is found in the cerebral circulation with arterioles (resistance vessels) having the most pronounced response [1]. It should also be noted that the large arteries feeding the brain have a greater contribution to regulating vascular resistance in the cerebral circulation compared to other vascular beds, again providing evidence for the importance of

A phase model of arterial myogenic behavior is commonly used to describe the response over a range of pressure. In the first phase, there is an initial development of myogenic tone at approximately 40–60 mmHg with increasing pressure up to that point causing passive distension. There is then a phase of myogenic reactivity in the pressure range of 60–140 mmHg and finally a phase of forced dilation at

Myogenic tone develops at approximately 40–60 mmHg and is characterized by an increase in intracellular Ca2+, of about 200%, followed by a reduction in lumen diameter. This pressure causes cellular deformation, depolarization of the vascular smooth muscle cells (VSMC), and a significant increase in wall tension. Wall tension appears to be the controlled parameter in the myogenic response which is altered during an increase or decrease in transmural pressure. The myogenic response adjusts the diameter of the vessel in an attempt to restore or limit the change in basal wall tension through a negative feedback mechanism [28, 29]. The suggestion that wall tension is the controlled parameter is supported by its correlation with changes in cell calcium and myosin light-chain phosphorylation,

The mechanism of the myogenic response is attributed to stretch-activated ion channels: including L-type calcium channels, voltage-activated calcium channels and calcium activated potassium channels along with enzymatic mechanisms. Specifically, increased intraluminal pressure causes depolarization of the VSMC membrane and calcium influx by the opening of voltage-gated calcium channels (VGCCs), with the most prominent involvement being from CaV1.2. This influx of Ca2+ leads to increased myosin light-chain (MLC) phosphorylation which promotes increased actin/myosin interaction followed by cross-bridge cycling and cell shortening (vasoconstriction) [7, 24, 32–34]. The importance of Ca2+ influx in the generation of myogenic tone supported by its complete abolishment under Ca2+ free conditions [32, 35]; this is also a technique frequently used to study the passive mechanical characteristics of vessels to determine the degree of vascular

transmural pressures greater than approximately 140 mmHg [27].

a relationship that is not seen with vessel diameter [30, 31].

*DOI: http://dx.doi.org/10.5772/intechopen.90404*

regulating tone in the cerebral circulation.

**2.1 Myogenic tone**

#### *Cerebral Vascular Tone Regulation: Integration and Impact of Disease DOI: http://dx.doi.org/10.5772/intechopen.90404*

in a pressurized vessel is referred to as myogenic tone. Additionally, the myogenic tone has a role in flow and pressure regulation. It functions by constricting to drop the pressure that reaches the downstream capillaries and protect them from edema or vascular remodeling associated with hypertension in the capillaries [22–25]. Equally important as protecting from hypertension is the ability of the vasculature to promote flow during low pressure by dilating. This ability to alter diameter over a range of pressures is referred to as myogenic reactivity. Beyond the local implications for the regulation of vascular tone, resistance to flow also has implications for systemic blood pressure since mean arterial pressure is the product of total peripheral resistance and cardiac output. This relationship illustrates system-wide implications of accurate vascular tone regulation. While the myogenic response is present throughout the body in a variety of vessels (arterioles, veins, lymphatic vessels) [26] the aforementioned functions of the myogenic response are particularly important in the cerebral circulation due to the catastrophic outcomes associated with under or over perfusion including unconsciousness and edema respectively. It is therefore not surprising that the most prominent myogenic response is found in the cerebral circulation with arterioles (resistance vessels) having the most pronounced response [1]. It should also be noted that the large arteries feeding the brain have a greater contribution to regulating vascular resistance in the cerebral circulation compared to other vascular beds, again providing evidence for the importance of regulating tone in the cerebral circulation.

A phase model of arterial myogenic behavior is commonly used to describe the response over a range of pressure. In the first phase, there is an initial development of myogenic tone at approximately 40–60 mmHg with increasing pressure up to that point causing passive distension. There is then a phase of myogenic reactivity in the pressure range of 60–140 mmHg and finally a phase of forced dilation at transmural pressures greater than approximately 140 mmHg [27].

#### **2.1 Myogenic tone**

*Basic and Clinical Understanding of Microcirculation*

focus on the impact of MetS and its associated risk factors.

The myogenic mechanism which was first described by Bayliss is an intrinsic property of the vascular smooth muscle to respond to changes in intravascular pressure which is independent of other mechanisms of tone regulation including neural, metabolic, and hormonal influences [19]. The intrinsic nature of the myogenic response is supported by its existence in arteries and arterioles that have been sympathetically denervated and had their endothelium removed [20] thus leaving only the vessel itself to initiate and execute the response. The prototypical response of the vascular smooth muscle in response to an increase in intraluminal pressure is initial distension quickly followed by a constriction. The opposite can be said in situations of decreased intraluminal pressure; a fall in intraluminal pressure results in vessel collapse followed by dilation [21]. The myogenic response has several physiological roles including the establishment and maintenance of basal vascular tone (some degree of constriction) so that resistance may be increased or decreased by metabolic vasoconstrictors or vasodilators respectively, in order to regulate tissue perfusion. The establishment and maintenance of this partially constricted state

**2. Myogenic mechanism**

regulation of flow is done primary by altering the diameter of blood vessels, and thus the resistance to flow. The major mechanisms of local regulation of vascular tone intrinsic to the cerebral vasculature include myogenic, shear, and metabolic based regulation. Although each mechanism has a discrete effect on vascular tone the integration of the different contributors to determine an appropriate level of tone is much more difficult to discern, especially in the cerebral circulation. These complex interactions allow for highly accurate control of cerebral blood flow in addition to protecting vulnerable downstream capillaries from high pressures and flow rates that could otherwise lead to edema; but, they also introduce several potential areas for failure. The intimate interactions of the various mechanisms of regulation of flow mean that the failure of one mechanism has the potential to initiate a cascade of events that results in inappropriate regulation of flow. As such abnormal execution of vascular tone regulation may form the basis of vascular pathologies [7]. One of these pathologies with a significant vascular component associated with impaired cerebral vascular tone regulation is metabolic syndrome (MetS). The incidence and prevalence of MetS is growing in Western society [8–10] and is contributing to decreased quality of life and increased economic burden. Thus an understanding of how it alters the cerebral circulation is crucial. MetS is categorized by a collection of metabolic risk factors including obesity, hypertension, atherogenic dyslipidemia and impaired glycemic control creating a pro-oxidant pro-inflammatory environment that raises the risk of developing impaired vascular structures and function [11–14]. These impairments are particularly detrimental when they affect the cerebral circulation and lead to cerebrovascular pathologies such as stroke or transient ischemic attack (TIA) due to the detrimental consequences associated with such events. However, cognitive impairments are not limited to individuals that have experienced an acute ischemic event since even in their absence MetS is strongly associated with impaired cognitive function and decreased quality of life [15–18]. Therefore preventing their occurrence by protecting the cerebrovasculature from functional and structural decline is paramount. This chapter will present a description of the local mechanisms involved in the regulation of cerebral vascular tone, how they integrate with one another and how they can be compromised in disease. Although impairments to the regulation of cerebral vascular tone are not limited to conditions associated with MetS this discussion will

**22**

Myogenic tone develops at approximately 40–60 mmHg and is characterized by an increase in intracellular Ca2+, of about 200%, followed by a reduction in lumen diameter. This pressure causes cellular deformation, depolarization of the vascular smooth muscle cells (VSMC), and a significant increase in wall tension. Wall tension appears to be the controlled parameter in the myogenic response which is altered during an increase or decrease in transmural pressure. The myogenic response adjusts the diameter of the vessel in an attempt to restore or limit the change in basal wall tension through a negative feedback mechanism [28, 29]. The suggestion that wall tension is the controlled parameter is supported by its correlation with changes in cell calcium and myosin light-chain phosphorylation, a relationship that is not seen with vessel diameter [30, 31].

The mechanism of the myogenic response is attributed to stretch-activated ion channels: including L-type calcium channels, voltage-activated calcium channels and calcium activated potassium channels along with enzymatic mechanisms. Specifically, increased intraluminal pressure causes depolarization of the VSMC membrane and calcium influx by the opening of voltage-gated calcium channels (VGCCs), with the most prominent involvement being from CaV1.2. This influx of Ca2+ leads to increased myosin light-chain (MLC) phosphorylation which promotes increased actin/myosin interaction followed by cross-bridge cycling and cell shortening (vasoconstriction) [7, 24, 32–34]. The importance of Ca2+ influx in the generation of myogenic tone supported by its complete abolishment under Ca2+ free conditions [32, 35]; this is also a technique frequently used to study the passive mechanical characteristics of vessels to determine the degree of vascular

remodeling since the VMSC exhibit a passive response (no force production) its strain is only due to the applied stress applied and the composition of the vessel [36]. Under physiological conditions the magnitude of the constrictor response to increased intraluminal pressure is limited by calcium-activated potassium channels that carry hyperpolarizing current proportional to the intracellular calcium concentration [35]. This negative feedback mechanism is supported by enhanced myogenic constriction being observed following blockade of calcium-activated potassium channel by specific inhibitors of these channels [37–39]. Additionally, at this pressure of 40–60 mmHg there is an activation of enzymatic systems and a complex interaction between matrix metalloproteins, the extracellular matrix, integrins and the cytoskeleton [40–42] that contribute to the myogenic reactivity at higher intraluminal pressures within the range of 60–140 mmHg [27]. This myogenic tone phase can also be characterized as the lower limit of autoregulation, which has important physiological implications. Below this pressure blood flow becomes dependent on blood pressure since the vessel cannot further dilate and begins to collapse as the pressure drops below this point [43]. Having an appropriate lower limit becomes especially important in situations of cerebral ischemia to allow restoration of flow in the presence of hypotension.

#### **2.2 Myogenic reactivity**

In this range of intraluminal pressure of 60–140 mmHg where the myogenic tone has already been established increases in pressure generally result in mild constriction and decreased pressure leads to mild dilation. Just as previously discussed for the generation of myogenic tone, increased pressure within this range leads to stretch, depolarization, and constriction of the vascular smooth muscle. However, in the myogenic reactivity phase, there is little change in vessel diameter across the range of pressures along with relatively small increases in Ca2+ (<20%) despite sizable increases in force production [27]. Multiple studies suggest an increased sensitivity to Ca2+ compared to the previous phase in the development of myogenic tone [20, 27, 44–48]. Increased sensitivity to Ca2+ is achieved by inhibition of myosin light chain phosphatase (MLCP) which promotes the accumulation of phosphorylated LC20 without an accompanying increase in calcium-induced myosin light chain kinase activity [49]. The presence of a contractile mechanism that does not require large variation of calcium, such as altering Ca2+ sensitivity requires less storage and transmembrane shuttling and is therefore advantageous in terms of conserving Ca2+ [35].

There are several proposed mechanisms that regulate Ca2+ sensitivity within this phase including, activation of protein kinase C (PKC), RhoA/Rho kinase pathways, and reactive oxygen species (ROS) [27, 35, 44, 46, 50]. The following studies provide evidence for the aforementioned mechanisms of enhanced Ca2+ sensitivity in this phase of the myogenic response through the utilization of specific inhibitors or transgenic animal models. Inhibition of PKC stops myogenic vasoconstriction in middle cerebral arteries with no impact on pressure-induced Ca2+ elevation [46]. Direct assessment of Ca2+ sensitivity by measuring the Ca2+-tone relationship has consistency found decreased sensitivity during Rho kinase inhibition [20, 45]. ROK has also been reported to trigger smooth muscle depolarization during myogenic constriction and limit the extent of depolarization by opening delayed rectifier potassium channels [51, 52]. Arteries from transgenic animals missing NADPH oxidase function show an absence of myogenic activity [53], while mice deficient in superoxide dismutase, an endogenous antioxidant enzyme that catalyzes the breakdown of superoxide radical to H2O2, acquired enhanced myogenic reactivity [54]. Additional mechanisms that contribute to the myogenic reactivity phase independent of Ca2+ sensitivity include actin cytoskeleton reorganization and thin filament regulation [55, 56].

**25**

*Cerebral Vascular Tone Regulation: Integration and Impact of Disease*

Although the prototypical response of increased intraluminal pressure is a constriction, at excessively high pressures, beyond the autoregulatory range of approximately 140 mmHg, forced dilation often occurs [57]. This process results in a loss of myogenic tone, and thus results in an increase in vessel diameter, rapid increase in wall tension and significant elevation in Ca2+ (>50%) [27]. Although the name implies a degree of passiveness in the process, forced dilation is likely an active vasodilation involving KCa channels, nitric oxide (NO) and or ROS, which are activated to protect the arterial wall from damage [44, 57]. If the pressure is reduced to within the myogenic reactivity range reestablishment of tone and

An increase in flow leads to an increase in a frictional force known as shear that is detected by the endothelial cells lining the vessel lumen as blood moves through a vessel. As such, the magnitude of the shear force is proportional to blood flow. Shear catalyzes physiologically important responses in the cerebral vasculature such as encouraging reperfusion after ischemia, aiding in the hyperemic response to increased metabolic demand, and perhaps protection of downstream capillaries from edema and structural damage. Flow has been found to induce both constrictor and dilator pathways that act on the VSMC and result in a final level of tone taking into account the opposing processes. It is generally accepted that in the peripheral circulation flow leads to dilation; however, in the cerebral circulation the response is more controversial with both constriction and dilation being reported. These opposing observations may be because of a variety of factors including the area of the brain studied, the preparation used or because of interactions with other mechanisms affecting cerebral vascular tone. Within the cerebral circulation, the vertebrobasilar systems appear to elicit flow-mediated dilation, as measured in rats and mice [58] and humans [59]. The increase in flow is sensed by the endothelium, which initiates a negative feedback mechanism in an attempt to decrease the shear stress by dilating. Shearinduced dilation is largely endothelium-dependent and is at least partially mediated by NO [60]. The production of NO is controlled by the enzyme nitric oxide synthase (NOS), particularly endothelial NOS (eNOS), which catalyzes the formation of NO from L-arginine and is itself dependent on phosphorylation by Akt [61, 62]. The NO formed by eNOS then diffuses into the vascular smooth muscle where it activates soluble guanylyl cyclase (sGC) and increases cyclic guanine monophosphate (cGMP) and activating protein kinase G [63, 64]. Activation of protein kinase G opens large-

(BKCa) channels reducing intracellular Ca2+ which

leads to relaxation of the vascular smooth muscle and thus vasodilation [65]. In addition to NO, eNOS can also lead to the formation of H2O2 which may also mediate flowinduced dilation [18]. During enzymatic cycling, eNOS produces oxygen radicals [66, 67] which, in the presence of sufficient antioxidants, are converted into H2O2 that may then, like NO, activate sGC and lead to dilation [68]. Other endothelium-dependent dilators such as prostaglandins do not appear to be involved in shear-induced dilation since the COX inhibitor indomethacin had no effect on the response to flow [60].

Constriction of cerebral vessels has also been reported in cats [69], rats [70] and human isolated cerebral arteries [71]. Constriction appears to predominate within the carotid circulatory area [58] especially when studies using *ex vivo* isolated vessels. In contrast to the dilator response, flow-induced constriction occurs independent of the endothelium [72, 73]. Although the mechanics for constriction

*DOI: http://dx.doi.org/10.5772/intechopen.90404*

**2.3 Forced dilation**

reduction in Ca2+ is observed.

conductance Ca2+-activated K+

**3. Response to flow (shear stress)**

## **2.3 Forced dilation**

*Basic and Clinical Understanding of Microcirculation*

restoration of flow in the presence of hypotension.

**2.2 Myogenic reactivity**

remodeling since the VMSC exhibit a passive response (no force production) its strain is only due to the applied stress applied and the composition of the vessel [36]. Under physiological conditions the magnitude of the constrictor response to increased intraluminal pressure is limited by calcium-activated potassium channels that carry hyperpolarizing current proportional to the intracellular calcium concentration [35]. This negative feedback mechanism is supported by enhanced myogenic constriction being observed following blockade of calcium-activated potassium channel by specific inhibitors of these channels [37–39]. Additionally, at this pressure of 40–60 mmHg there is an activation of enzymatic systems and a complex interaction between matrix metalloproteins, the extracellular matrix, integrins and the cytoskeleton [40–42] that contribute to the myogenic reactivity at higher intraluminal pressures within the range of 60–140 mmHg [27]. This myogenic tone phase can also be characterized as the lower limit of autoregulation, which has important physiological implications. Below this pressure blood flow becomes dependent on blood pressure since the vessel cannot further dilate and begins to collapse as the pressure drops below this point [43]. Having an appropriate lower limit becomes especially important in situations of cerebral ischemia to allow

In this range of intraluminal pressure of 60–140 mmHg where the myogenic tone has already been established increases in pressure generally result in mild constriction and decreased pressure leads to mild dilation. Just as previously discussed for the generation of myogenic tone, increased pressure within this range leads to stretch, depolarization, and constriction of the vascular smooth muscle. However, in the myogenic reactivity phase, there is little change in vessel diameter across the range of pressures along with relatively small increases in Ca2+ (<20%) despite sizable increases in force production [27]. Multiple studies suggest an increased sensitivity to Ca2+ compared to the previous phase in the development of myogenic tone [20, 27, 44–48]. Increased sensitivity to Ca2+ is achieved by inhibition of myosin light chain phosphatase (MLCP) which promotes the accumulation of phosphorylated LC20 without an accompanying increase in calcium-induced myosin light chain kinase activity [49]. The presence of a contractile mechanism that does not require large variation of calcium, such as altering Ca2+ sensitivity requires less storage and transmembrane shuttling and is therefore advantageous in terms of conserving Ca2+ [35]. There are several proposed mechanisms that regulate Ca2+ sensitivity within this phase including, activation of protein kinase C (PKC), RhoA/Rho kinase pathways, and reactive oxygen species (ROS) [27, 35, 44, 46, 50]. The following studies provide evidence for the aforementioned mechanisms of enhanced Ca2+ sensitivity in this phase of the myogenic response through the utilization of specific inhibitors or transgenic animal models. Inhibition of PKC stops myogenic vasoconstriction in middle cerebral arteries with no impact on pressure-induced Ca2+ elevation [46]. Direct assessment of Ca2+ sensitivity by measuring the Ca2+-tone relationship has consistency found decreased sensitivity during Rho kinase inhibition [20, 45]. ROK has also been reported to trigger smooth muscle depolarization during myogenic constriction and limit the extent of depolarization by opening delayed rectifier potassium channels [51, 52]. Arteries from transgenic animals missing NADPH oxidase function show an absence of myogenic activity [53], while mice deficient in superoxide dismutase, an endogenous antioxidant enzyme that catalyzes the breakdown of superoxide radical to H2O2, acquired enhanced myogenic reactivity [54]. Additional mechanisms that contribute to the myogenic reactivity phase independent of Ca2+ sensitivity include

actin cytoskeleton reorganization and thin filament regulation [55, 56].

**24**

Although the prototypical response of increased intraluminal pressure is a constriction, at excessively high pressures, beyond the autoregulatory range of approximately 140 mmHg, forced dilation often occurs [57]. This process results in a loss of myogenic tone, and thus results in an increase in vessel diameter, rapid increase in wall tension and significant elevation in Ca2+ (>50%) [27]. Although the name implies a degree of passiveness in the process, forced dilation is likely an active vasodilation involving KCa channels, nitric oxide (NO) and or ROS, which are activated to protect the arterial wall from damage [44, 57]. If the pressure is reduced to within the myogenic reactivity range reestablishment of tone and reduction in Ca2+ is observed.
