SHAPE Society

1. SHEAR STRESS ISSHEAR STRESS IS
DIFFERENTIALLYDIFFERENTIALLY
REGULATED AMONGREGULATED AMONG
INBRED RAT STRAINSINBRED RAT STRAINS
Jamila Ibrahim PhD, Jody K. Miyashiro PhD,Jamila Ibrahim PhD, Jody K. Miyashiro PhD,
Bradford C. Berk MD, PhDBradford C. Berk MD, PhD
Circ Res, May 2003Circ Res, May 2003

2. Importance of Vascular RemodellingImportance of Vascular Remodelling
(1)(1)
 Vascular remodelling is considered to play a central role in theVascular remodelling is considered to play a central role in the
aetiology of major clinical disorders such as atherosclerosis,aetiology of major clinical disorders such as atherosclerosis,
restenosis and hypertensionrestenosis and hypertension
 Remodelling is an important determinant of blood flow inRemodelling is an important determinant of blood flow in
vascular disease. At sites of atherosclerotic plaque formation,vascular disease. At sites of atherosclerotic plaque formation,
outward remodelling restricts while inward remodellingoutward remodelling restricts while inward remodelling
accelerates luminal narrowing thereby compromising blood flowaccelerates luminal narrowing thereby compromising blood flow
 Remodelling at sites of atherosclerosis and plaque accumulationRemodelling at sites of atherosclerosis and plaque accumulation
is critically important clinically and is likely to occur by some ofis critically important clinically and is likely to occur by some of
the same pathways as flow-induced remodellingthe same pathways as flow-induced remodelling

3. Regulation of Shear StressRegulation of Shear Stress
 Blood flow through the vasculature generates shearBlood flow through the vasculature generates shear
stress, a tangential dragging force exerted by flow onstress, a tangential dragging force exerted by flow on
the endothelial surface of blood vesselsthe endothelial surface of blood vessels
 In accordance with Poisseulle’s Law, blood vesselsIn accordance with Poisseulle’s Law, blood vessels
subjected to abnormal flow and shear stress willsubjected to abnormal flow and shear stress will
respond to normalise this stress by regulating vascularrespond to normalise this stress by regulating vascular
diameter, by a process known as vascular remodelingdiameter, by a process known as vascular remodeling
 Increased shear stress is normalised by outwardIncreased shear stress is normalised by outward
vascular remodelling and reduced shear stress by inwardvascular remodelling and reduced shear stress by inward
vascular remodellingvascular remodelling

4. Vascular remodelling at sites ofVascular remodelling at sites of
plaque formationplaque formation
Reduced Flow Increased Flow
Plaque formationPlaque free vessel
Cross-section of
Blood Vessel

5. Importance of Vascular RemodellingImportance of Vascular Remodelling
(2)(2)
 Shear stress regulation by vascular remodelling isShear stress regulation by vascular remodelling is
likely to be complex and polygenic and thereforelikely to be complex and polygenic and therefore
efforts to identify the key regulatory genesefforts to identify the key regulatory genes
require appropriate animal modelsrequire appropriate animal models
 The ability of vessels to compensate for plaqueThe ability of vessels to compensate for plaque
burden, a process that requires outwardburden, a process that requires outward
remodelling, may critically depend on specificremodelling, may critically depend on specific
genesgenes

6. Experimental model (1)Experimental model (1)
 We have developed a rat model involvingWe have developed a rat model involving
ligation of the external and internal carotidligation of the external and internal carotid
arteries, as depicted in the schematic belowarteries, as depicted in the schematic below
Left Common
Carotid Artery
External
Carotid Artery
Internal
Carotid Artery
Occipital
Artery
Superior
Thyroid Artery

7. Experimental model (2)Experimental model (2)
 This model is well tolerated by rats, allowing aThis model is well tolerated by rats, allowing a
surgical success rate of 100%surgical success rate of 100%
 Flow is reduced in the left common carotidFlow is reduced in the left common carotid
artery by ~90% of baseline flow, and flow in theartery by ~90% of baseline flow, and flow in the
right common carotid artery is ~150% ofright common carotid artery is ~150% of
baseline flow following arterial ligationbaseline flow following arterial ligation
 We have examined the influence of 28days ofWe have examined the influence of 28days of
chronic flow alteration by arterial ligation onchronic flow alteration by arterial ligation on
vessel diameter, blood flow, and shear stressvessel diameter, blood flow, and shear stress

8. FIGURE 2C
%∆Shearstress
(NTG)
Right carotid arteries
(high flow)
Left carotid arteries
(low flow)
%∆Shearstress
(NTG)Percent change in shear stress following chronic
flow alteration
-80
-60
-40
-20
0
GH Fischer SHR-SP BN-100
-20
0
20
40
GH Fischer SHR-SP BN
80
60
-40
-60
GH = genetically hypertensive rats
SHR-SP = stroke-prone spontaneously hypertensive rats
BN = brown norway rats
NTG = nitroglycerin

9. FIGURE 3B
Left carotid arteries
(low flow)
%∆Outerdiameter
(NTG)
GH Fischer SHR-SP BN
-25
-20
-15
-10
-5
0
5
10
-30
-35
%∆Outerdiameter
(NTG)
SHR-SP BNGH Fischer
0
5
10
15
20
25
30
35
40
45
Right carotid arteries
(high flow)
Percent change in outer diameter following chronic
flow alteration
GH = genetically hypertensive rats
SHR-SP = stroke-prone spontaneously hypertensive rats
BN = brown norway rats
NTG = nitroglycerin

10. ∆ Flow, ml/min
∆ Flow, ml/min
SHR-SP
GH
∆Shearstress,dyne/cm2
FIGURE 4A
-20
-15
-10
-5
0
5
10
15
20
Control
+ NTG
SHR-SP
GH
∆Shearstress,dyne/cm2
-20
-15
-10
-5
0
5
10
15
20
1 3 5
2 4 6
2 4 6
1 3 5
Again, for a given change in flow,
GH and SHR-SP rats exhibit different
shear stress responses. GH rat vessels
are relatively more responsive to NTG
than SHR-SP vessels.
Relationships between flow and shear stress in RCA
(increased flow)
For a given change in flow, GH and
SHR-SP rats exhibit different shear
stress responses. GH rats are better
regulators of shear stress than SHR-SP.

11. FIGURE 4B
∆ Flow, ml/min
∆ Flow, ml/min
∆Outerdiameter,mm∆Outerdiameter,mm
Control
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6
SHR-SP
GH
+ NTG
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6
SHR-SP
GH
Relationships between flow and outer diameter in
RCA (increased flow)
The slopes of the lines do not differ from control
(upper) for GH and SHR-SP in the presence of
NTG, indicating that the vascular structural changes
are not dependent on differences in vascular tone
GH rat outer diameters are markedly more sensitive
to flow than SHR-SP outer diameters

12. Carotid artery endothelial cells shown en face following immunostaining for eNOS.
After 28 days of increased blood flow, there was an increase in eNOS expression in GH
carotid arteries compared with carotid arteries exposed to control flow (compare b
with f). In SHR-SP carotid arteries, however, increased flow produced a relatively
attenuated increase in eNOS expression compared with corresponding controls
(compare d with e). Moreover, GH RCA express greater eNOS compared with SHR-SP
RCA following chronic flow increase (b vs. d).

13. Major findings (1)Major findings (1)
 Shear stress regulation and vascular remodelling following chronicShear stress regulation and vascular remodelling following chronic
flow alteration vary considerably between rat strainsflow alteration vary considerably between rat strains
 We identified a rat strain with a poor ability to regulate shear stressWe identified a rat strain with a poor ability to regulate shear stress
(SHR-SP), and a strain with a good ability to regulate shear stress(SHR-SP), and a strain with a good ability to regulate shear stress
(GH)(GH)
 These findings suggest SHR-SP and GH rat strains may exhibit:These findings suggest SHR-SP and GH rat strains may exhibit:
 differential sensitivity to chronic blood flow alterationdifferential sensitivity to chronic blood flow alteration
 differences in signal transduction mechanisms activated by flowdifferences in signal transduction mechanisms activated by flow
 differences in the chronic vascular response to flow changes thatdifferences in the chronic vascular response to flow changes that
mediate remodelling of the blood vesselmediate remodelling of the blood vessel

14. Major findings (2)Major findings (2)
 Further work utilising these rat strains and experimental modelFurther work utilising these rat strains and experimental model
will permit identification of the key regulatory mechanisms andwill permit identification of the key regulatory mechanisms and
genes participating in flow mediated physiological processes suchgenes participating in flow mediated physiological processes such
as shear stress and vascular remodellingas shear stress and vascular remodelling
 Specifically, identification of genes that augment or restrictSpecifically, identification of genes that augment or restrict
remodelling is now possible. In the long term, this strategy mayremodelling is now possible. In the long term, this strategy may
determine novel therapeutic targets to augment remodelling anddetermine novel therapeutic targets to augment remodelling and
restore blood flow, and thus improve the management andrestore blood flow, and thus improve the management and
treatment of cardiovascular or cerebrovascular disease with atreatment of cardiovascular or cerebrovascular disease with a
critical vascular remodelling componentcritical vascular remodelling component