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

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 largeconductance Ca2+-activated K+ (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

in response to increased flow is not fully understood it appears to be mediated in part by a combination of integrin signaling, free radicals, and tyrosine kinase. Giving integrin-binding peptides, scavenging ROS by superoxide dismutase (SOD) or inhibiting tyrosine [69] all significantly reduced or eliminated constriction in response to flow. Just as in flow-induced dilation shear stress appears to be the initiating parameter in flow induced constriction whose force can be transduced by integrin signaling [69]. Additionally, Koller and Toth found that flow-induced constriction of cerebral vessels were blocked by inhibiting the synthesis of 20-HETE, thromboxane A2 receptor, COX activity, and scavenging ROS. From that, they proposed that increase flow activates an AA cascade with metabolism by CYP450 4A enzymes resulting in the production of 20-HETE and ROS which contribute to the constriction via thromboxane A2/prostaglandin H2 receptors [71].

The differences in observed responses may also be dependent on whether the study was conducted *in vivo* or *ex vivo*. *Ex vivo* studies typically use a physiologic salt solution that has a much lower viscosity than blood and may be inadequate to stimulate the endothelium to produce endothelium-dependent dilation [74]. Consequently, the *ex vivo* responses to intraluminal shear have been shown to be endothelium-independent [69, 72]. Furthermore, *ex vivo* study generally lack pulsatile flow pumps but those that have been able to employ peristaltic or piston pump have indicated a role for shear-mediated regulation [68, 75, 76]. In contrast, in vivo study has produced more consistent shear-mediated dilation [62, 77]. Raignault et al. showed in a mouse model that the cerebrovascular endothelium optimally integrates shear stress to eNOS-mediated dilation under physiological pulse pressures, a phenomenon that was not seen in static flow conditions [76]. It appears that cerebral arteries are more responsive to a pulsatile environment with a viscose fluid (blood) as is experienced *in vivo* and consequently able to dilate in response to shear stress through endothelium-dependent production of NO [68, 76, 78].

Although flow induced constriction is not usually seen *in vivo* at high flow rates it may play a protective role in the cerebral circulation. Just as low flow situations are dangerous, especially in the brain, so too are instances of extreme flow; therefore it is important to have multiple regulatory mechanisms that are able to attenuate an increase in flow as a protective mechanism for the cerebral circulation from high volumes [1, 79, 80]. It is advantageous for the response to flow to adapt in accordance with other inputs on the regulation of vascular tone. This may be the reason why characterizing the response to flow in the cerebral circulation has been variable. One variable that may alter the response to flow is the absolute rate of flow. In isolated branches of MCA from rats vessels dilated with flow up to 10 μL/min but their diameter constricted back to baseline at higher levels of flow [81]. Shimoda et al. also found a biphasic flow-dependent response in anterior and middle cerebral arteries of neonatal pigs; flow rates between 0.077 and 0.212 mL/min constricted vessels while dilation occurred at higher flow rates up to 1.6 mL/min [82].

#### **3.1 Flow and pressure**

In a physiological setting multiple inputs are being processed by the cerebral vasculature leading to the generation of a certain level of tone. Pressure and shear stress exerted by flowing blood are two mechanical stimuli that have been described to play a major role in the regulation of vascular tone [83, 84]. It is therefore important to consider their interaction when determining the resultant effect on vascular tone. At high pressures (around >80 mmHg) cerebral vessels tend to constrict in response to flow [72, 85–87]. This biphasic response is further supported by findings from Garcia-Roldan and Bevan in isolated rabbit pial arterioles with flow rates from 0 to 20 μL/min at 90 mmHg but did not with the same flow at 60 mmHg [72].

**27**

**4. Metabolic control**

**Figure 1.**

adenosine, CO2, H+

vascular tone including adenosine, CO2, H+

, and K+

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

It is interesting that the tipping points for "high" and "low" pressures observed in previous studies are around 80 mmHg since, in resistance arterioles such as the MCA this pressure is commonly measured under physiological conditions. If 80 mmHg is indeed the point where higher pressures lead to constriction and lower to dilation in response to shear the resistance arterioles that are resting around this pressure would have the opportunity to tightly regulate the response to flow and shear thus allowing for precise control of cerebral blood flow. Please see **Figure 1** for a schematic representation of how flow and intravascular pressure can impact cerebral vascular tone.

*A schematic representation of how intravascular pressure (myogenic), increased lumen flow rates (shear),* 

*tissue metabolism and neurovascular coupling can impact cerebral vascular tone regulation.*

The mechanical stimuli of pressure and flow are generally thought to be important in setting the basal vascular tone so that metabolic influences are able to cause dilation or constriction depending on the needs of the cerebral tissue [24]. Metabolic control of vascular resistance is of particular importance in the cerebral circulation since cerebral tissue is extremely intolerant to ischemia [88]. As such, the cerebral circulation has a precise and highly localized coupling between the metabolic requirements of cerebral tissue and the magnitude of blood flow by controlling vascular resistance. There are numerous vasoactive metabolites that contribute to the control of cerebral

, O2 and K+

VSM and dilation of cerebral resistance vessels. Each metabolite is associated with a cascade of events that ultimately either alters intracellular Ca2+ concentration or Ca2+ sensitivity of the VSM and results in a change in vessel diameter. Although the effects of each metabolite have been well characterized the relative importance of each along with its interaction with each other and other parameters of tone remains an area of

further investigation. The discussion below is summarized in **Figure 1**.

and decreased concentration of O2 result in relaxation of

. Increasing concentrations of

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

#### **Figure 1.**

*Basic and Clinical Understanding of Microcirculation*

in response to increased flow is not fully understood it appears to be mediated in part by a combination of integrin signaling, free radicals, and tyrosine kinase. Giving integrin-binding peptides, scavenging ROS by superoxide dismutase (SOD) or inhibiting tyrosine [69] all significantly reduced or eliminated constriction in response to flow. Just as in flow-induced dilation shear stress appears to be the initiating parameter in flow induced constriction whose force can be transduced by integrin signaling [69]. Additionally, Koller and Toth found that flow-induced constriction of cerebral vessels were blocked by inhibiting the synthesis of 20-HETE, thromboxane A2 receptor, COX activity, and scavenging ROS. From that, they proposed that increase flow activates an AA cascade with metabolism by CYP450 4A enzymes resulting in the production of 20-HETE and ROS which contribute to

the constriction via thromboxane A2/prostaglandin H2 receptors [71].

stress through endothelium-dependent production of NO [68, 76, 78].

it may play a protective role in the cerebral circulation. Just as low flow situations are dangerous, especially in the brain, so too are instances of extreme flow; therefore it is important to have multiple regulatory mechanisms that are able to attenuate an increase in flow as a protective mechanism for the cerebral circulation from high volumes [1, 79, 80]. It is advantageous for the response to flow to adapt in accordance with other inputs on the regulation of vascular tone. This may be the reason why characterizing the response to flow in the cerebral circulation has been variable. One variable that may alter the response to flow is the absolute rate of flow. In isolated branches of MCA from rats vessels dilated with flow up to 10 μL/min but their diameter constricted back to baseline at higher levels of flow [81]. Shimoda et al. also found a biphasic flow-dependent response in anterior and middle cerebral arteries of neonatal pigs; flow rates between 0.077 and 0.212 mL/min constricted

vessels while dilation occurred at higher flow rates up to 1.6 mL/min [82].

In a physiological setting multiple inputs are being processed by the cerebral vasculature leading to the generation of a certain level of tone. Pressure and shear stress exerted by flowing blood are two mechanical stimuli that have been described to play a major role in the regulation of vascular tone [83, 84]. It is therefore important to consider their interaction when determining the resultant effect on vascular tone. At high pressures (around >80 mmHg) cerebral vessels tend to constrict in response to flow [72, 85–87]. This biphasic response is further supported by findings from Garcia-Roldan and Bevan in isolated rabbit pial arterioles with flow rates from 0 to 20 μL/min at 90 mmHg but did not with the same flow at 60 mmHg [72].

Although flow induced constriction is not usually seen *in vivo* at high flow rates

The differences in observed responses may also be dependent on whether the study was conducted *in vivo* or *ex vivo*. *Ex vivo* studies typically use a physiologic salt solution that has a much lower viscosity than blood and may be inadequate to stimulate the endothelium to produce endothelium-dependent dilation [74]. Consequently, the *ex vivo* responses to intraluminal shear have been shown to be endothelium-independent [69, 72]. Furthermore, *ex vivo* study generally lack pulsatile flow pumps but those that have been able to employ peristaltic or piston pump have indicated a role for shear-mediated regulation [68, 75, 76]. In contrast, in vivo study has produced more consistent shear-mediated dilation [62, 77]. Raignault et al. showed in a mouse model that the cerebrovascular endothelium optimally integrates shear stress to eNOS-mediated dilation under physiological pulse pressures, a phenomenon that was not seen in static flow conditions [76]. It appears that cerebral arteries are more responsive to a pulsatile environment with a viscose fluid (blood) as is experienced *in vivo* and consequently able to dilate in response to shear

**26**

**3.1 Flow and pressure**

*A schematic representation of how intravascular pressure (myogenic), increased lumen flow rates (shear), tissue metabolism and neurovascular coupling can impact cerebral vascular tone regulation.*

It is interesting that the tipping points for "high" and "low" pressures observed in previous studies are around 80 mmHg since, in resistance arterioles such as the MCA this pressure is commonly measured under physiological conditions. If 80 mmHg is indeed the point where higher pressures lead to constriction and lower to dilation in response to shear the resistance arterioles that are resting around this pressure would have the opportunity to tightly regulate the response to flow and shear thus allowing for precise control of cerebral blood flow. Please see **Figure 1** for a schematic representation of how flow and intravascular pressure can impact cerebral vascular tone.
