Comparison of arterial wall mechanics in normotensive and spontaneously hypertensive ROBERT H. COX Bockus Research Institute, University of Pennsylvania,

the Graduate Philadelphia,

Hospital, and Department Pennsylvania 19146

Cox, ROBERT H. Comparison of arterial wall mechanics in normotensive and spontaneously hypertensive rats. Am. J. Physiol. 237(2): H159-H167, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 6(2): H159-H167, 1979.-Segments of carotid artery from Wistar (NW), Kyoto Wistar (WKY), and spontaneously hypertensive rats (SHR) were used to compare mechanical properties and connective tissue composition. Pressure-diameter measurements were made under conditions of active (5 pg/ml norepinephrine) and passive (0 mM Ca2’ and 2 mM EGTA) smooth muscle. Systolic blood pressures averaged NW, 121 k 3; WKY, 124 k 4; and SHR, 187 t 5 mmHg. Passive mechanics were stiffest in SHR and most compliant in NW arteries. No differences in collagen-elastin ratio were found but collagen + elastin was lowest in SHR and highest in NW carotids. These results are not consistent with current concepts of the contribution of connective tissue elements to passive mechanics. Maximum stress development was NW, 561 t 49; WKY, 735 k 50; and SHR, 944 k 79 X lo3 dyn/cm2. Diameter reductions to NE at 100 mmHg were NW, 17.6 t 2.4%; WKY, 16.7 * 2.0%; and SHR, 24.8 k 2.4%. The former suggests different contractile protein contents or more efficient intercellular force coupling in SHR. The latter suggests a more effective contractile apparatus as a result of stiffer passive muscle elements and/or a relatively larger wall thickness. vascular smooth muscle; connective tissue; norepinephrine; arterial stiffness; active force development; constriction responses THERE ISGENERALAGREEMENT thatchangesinarterial wall mechanical properties are associated with established hypertension (19, 33). However, studies to date have not determined unequivocally if these changes are related to the etiology of hypertension or simply represent the response of the arterial wall to elevated arterial pressure or a combination of the two (22,23,25). Previous studies suggest that an increase occurs in the stiffness of the arterial wall associated with the development of sustained hypertension (1-4, 7, 16, 21, 24). For the most part, however, these previous studies were performed with methodologies that. raise doubts concerning the conclusions. These objections are based upon a) the type of preparation employed, i.e., strips or rings of blood vessels, and/or b) the presence of unquantified tone of the smooth muscle in these vessels. Because of these deficiencies, more recent studies have been undertaken to quantitate the changes in passive arterial wall mechanical properties that occur with the development of experimental hypertension (5, 11). The results of these studies suggest that (at least) the initial 0363-6135/79/0000-0000$01.25

Copyright

0 1979 the American

Physiological

rats

of Physiology,

response of the arterial wall to elevated pressure is to produce a more compliant blood vessel with a reduction in the ratio of collagen to elastin content (5, 11). This response may be viewed as a form of “structural autoregulation” by which the arterial wail attempts to increase compliance in order to normalize some parameter such as wall stress or wall strain as arterial pressure increases. It was the objective of the experiments reported herein to determine- the differences in active and passive mechanical properties of carotid arteries from normotensive and spontaneously hypertensive rats of the Kyoto Wistar strain. In addition, these changes were correlated with alterations in the composition of these vessels. METHODS

Preparations. These experiments were performed using the following groups of rats: normal Wistar (NW), Kyoto Wistar (WKY), and spontaneous hypertensive rats (SHR) derived from the Kyoto controls. There were 15 animals studied in each group. The animals were all 10 wk of age when obtained from our supplier (Charles River Breeding Laboratories). They were maintained in our facility for a period of 5 wk, housed two animals per cage, and given food and water ad lib. During this period, weekly measurements of weight, blood pressure, and heart rate were obtained. Values of systolic blood pressure were estimated in these animals using the tail-cuff occlusion method (Narco Biosystems, Programmed Electro-Sphygmomanometer). At the time of their study the animals were anesthetized with ether. The chest was opened, and the animals were exsanguinated. The two carotid arteries were rapidly removed from the animals, trimmed of loose connective tissue, and placed in a buffered physiologic salt solution (PSS). The PSS was aerated with 95% 0205% CO2 mixture and maintained at 37OC. The PSS had the following composition in millimoles per liter: 116.5 NaCl, 22.5 NaHCOB, 1.2 NaH2P04, 2.4 NazSOd, 4.5 KCl, 1.2 MgS04, 2.5 CaClz, and 5.6 dextrose. The solutions consistently had a measured Pea in excess of 500 mmHg and pH of about 7.4 t 0.02. One of the two arteries was mounted in the experimental apparatus, which has been described in detail previously (8,10). The vessels were mounted on two modified 23-g hypodermic needles in the horizontal position. One needle was connected to an isometric force transducer for the measurement of axial wall force. The other needle Society

H159

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H160 was connected to a manifold on a movable slide assembly used for positioning the vessel in the bath as well as for adjusting its initial length. Pressure used to inflate the segment was also introduced through the manifold. The transmural pressure was measured using a transducer (Microdot MS-lo) coupled to the manifold through a sideport. The external diameter of the segment was measured using a cantilever diameter transducer pivoted from above (26). Procedures. After the segment was mounted in the experimental apparatus, the length of the segment was adjusted to its in vivo value; pressure was elevated to and maintained at about 150 mmHg for a period of 60 min. Then, continuous inflation/deflation cycles were performed between 0 and 250 mmHg at a rate of 1 mmHg/ s. When reproducible closed loop curves were obtained, they were recorded on magnetic tape (Sangamo model 3500). Transmural pressure was then adjusted to 50 mmHg and the diameter was allowed to equilibrate. A value of 50 mmHg was selected for several reasons. This pressure produced an initial muscle length close to L,,,, the length for maximum active responses. This pressure produced a smaller wall thickness, which allowed for more rapid diffusion of norepinephrine. Past experience has shown that reproducible responses could be obtained with this methodology. Next, maximal constriction responses were produced at this pressure, which allowed for a rapid initial assessment of the “state of smooth muscle” in the sample. Sufficient norepinephrine was added to the bath to produce a final concentration of 5 pg/ml. That this represented a maximal response to norepinephrine was tested by adding additional quantities of drug, which were uniformly without effect. After 1 min, pressure was lowered to about 3-5 mmHg and maintained until perceptual changes in diameter ceased (AD > 5 pm/mm). Then, the response to inflation at a rate of 0.2 mmHg/s was obtained up to 250 mmHg. No deflation response was recorded under activated conditions. Subsequently the bath was drained, rinsed, and refilled with a calcium and glucose-free PSS containing 2 mM EGTA (ethylene glycol-bis(P-aminoethyl ether)N,N’-tetraacetic acid). Pressure was maintained at 150 mmHg for 30 min in the presence of this solution. At the end of this period, continuous inflation/deflation responses were repeated at a rate of 1 mmHg/s and recorded on tape after reproducible results have been obtained. It should be noted that no significant difference in the pressure-diameter data were found under control or passive conditions at inflation rates of 0.2 and 1 mmHg/s. Accordingly, the faster rate was used because the time involved in data acquisition was considerably shorter. It should be noted that the faster inflation rate could not be used with activated smooth muscle, as an overestimation of active responses would be produced (9). Base lines and calibrations were also recorded on the magnetic tape. When all data had been recorded on a given vessel, it was removed from the bath, its unstressed length measured, and its wet weight determined with an analytical balance. Data analysis. At the end of the experiment on a given vessel, the recorded data were replayed from magnetic tane on an X-Y nlotter (Hewlett-Packard model 7046A).

R. H. COX

Values of external diameter were obtained from these records in pressure steps of 10 mmHg from 0 to 250 mmHg. Values of pressure and diameter were used to compute values of tangential wall stress using the following equation (10) a P a9 = b-a (1) where a and b are internal and external radii, respectively. Values of b were obtained from external diameter, and values of a were obtained using values of b, segment length, and wet weight. This analysis assumes that the deformation of these arterial segments is isovolumic and yields values of stress averaged over the entire wall thickness. These data were obtained under all three conditions for inflation responses only, i.e., control, norepinephrine, and passive (2 mM EGTA). Values of incremental isotropic elastic modulus were computed from pressure-diameter data using the following equation (10) E =

2a2b AP b2 - a2 Ab

(2)

In addition, theoretical values of characteristic impedance were also computed from these data using the following equation (10) &=-$

j/p

(3)

where p is the density of blood taken as 1.06 g/cm3. Values of various parameters were averaged for all experiments in each of the animal groups for each of the three conditions at specific pressure levels from 0 to 250 mmHg in steps of 10 mmHg. These values were compared for statistical significance using the double-ended Student t test. The effects of activation of smooth muscle were assessed from two points of view using concepts of classical muscle mechanics. In the first, values of active stress response were computed as the increase in tangential wall stress at a given muscle length (i.e., diameter) from the passive and active pressure-diameter data, and using Eq 1. This was done at a number of different values of muscle length and is essentially equivalent to isometric active stress (force/area) development (9). In the second method, values of active diameter response were computed from differences in values of midwall diameter (i.e., (a + b)/2) at a given value of transmural pressure from pressure-diameter data under active and passive conditions. These diameter differences were normalized by dividing by the value of the passive diameter at each pressure level. These responses are essentially equivalent to isobaric constriction responses (9). The vessel segment used for mechanical studies and the remaining carotid from the same animal were used for the determination of connective tissue content. Collagen and elastin were determined in these vessels by a modified version of the method of Neuman and Logan (18, 27). The segments were weighed before and after oven drying at 90°C for 20 h. Values of wet and dry weight were used to compute water content of the seg-

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ARTERIAL

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ments. Values of water content were determined using freshly dissected and PSS incubated segments from the same animal. The ratio of water content in the incubated segment to that in the fresh segment was averaged for 12 animals and was 1.09 t 0.05, a value not statistically significantly different from 1.00. Collagen and elastin fractions were separated using heat and pressure, and their contents were determined using the respective hydroxyproline content and appropriate factors.

250

1

NOR#AL

RESULTS

Passive mechanics. A summary of some hemodynamic parameters from the various animal groups is given in Table 1. Compared to control animals, the SHR had lower body weights and higher heart rates and systolic blood pressure levels. These results have been reported by others working with this animal model (17,28,32). In addition, differences in geometric properties of the carotid arteries were also found between the spontaneously hypertensive and control rats. Values of internal radius at 100mmHg were significantly smaller in the SHR when compared to both control groups. Values of radius-towall-thickness ratio at 100 &mHg under passive conditions were also smaller than those of the two control groups, primarily as a result of an increased wall thickness. Examples of on-line curves under the various conditions for animals in each of the three groups are shown in Fig. 1. As is apparent from these graphs, substantial differences exist in the pressure-diameter relations of arteries from normotensive and hypertensive animals under the various conditions. Quantitative aspects of these differences are presented in the subsequent paragraphs of this section. Figure 2 contains a summary of pressure/external diameter relations under passive conditions for arteries from the three animal groups. For values of transmural pressure above 70 mmHg the external diameter is significantly smaller in the SHR compared to the WKY and normal Wistar rats. Compared with WKY carotids, differences are significant at the 0.05 level for 50 < P < 60, at the 0.0025 level for 60 < P < 80 and 120 < P < 200, and at the 0.005 level for 80 < P < 120. Compared to NW carotids, these differences are significant at the 0.05 level for 70 < P < 80 and 120 < P < 140, at the 0.025 level for 80 < P < 100 and at the 0.01 level for 100 < P < 120. Therefore, the reason for the smaller internal radius

1. Summary of some hemodynamic in the various animal groups

TABLE

Group

Normal

n

BW

15

305

wistar Kyoto Wi&U SHR

t,9 15 15

269 *13 245* *5

g

se

HR,,

mmHg

min

121 t_3 124 *4 187* *5

368 t,13 361 It15 401* *12

quantities a, m m

0.574 *0.019 0.603 kO.018 0.508” *0.012

a/h

16.9 a.4 17.3 u.3 10.3 20.7

BW, body weight; SP, systolic pressure; HR, heart rate; a, internal radius at 100 mmHg; a/h, radius-wall thickness ratio at 100 mmHg; n, number of animals per group. *Statistically significant compared to Kyoto Wistars (P < 0.05).

r

-

r,,lI,rlrl l

.06

.08

.I0

.I2

08

EXTERNAL

.I0

.I2

.I4

DIAMETER

.06

.08

.I0

.I2

(cm)

1. On-line records showing the carotid artery pressure-diameter responses to continuous inflation for typical animal in each group. Data are given for passive (continuous), control (long-dashed) and active conditions (short-dashed). FIG.

0 NW A WKY 0 SHR

‘5 5 5 E

t 0 1 0

-FOCI /

k-

MA

OtO8

EXTERNAL

I

I

0.10

OJ2

DIAMETER

J

0.14

km)

2. Summary of passive transmural pressure-external diameter relations for arteries from each of the animal groups. Symbols represent means and horizontal bars represent *SE. Continuous curve connects data points for the WKY group. FIG.

found in the SHR is not completely due to an increased wall thickness. In part, this difference is also the result of a smaller external diameter. The variation of average tangential wall stress with normalized wall diameter is summarized in Fig. 3 for the three animal groups. Wall diameter was normalized by dividing by the value of passive diameter at zero pressure under passive conditions, i.e., Do. In the low stress or strain range, values of these stress/strain curves for the SHR and WKY carotids were not significantly different. Only at values of wall stress above 2 x lo6 dyn/cm2 were values of normalized wall diameter (equal to 1 + strain)

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H162

R. H. COX

termined by the content of the connective tissue elements collagen and elastin (5,11,18). A summary of connective tissue content of these blood vessels is given in Table 2. The total amount of connective tissue (collagen + elastin) on a wet weight basis was significantly higher in SHR than in WKY carotids as a result of wall hypertrophy. However, the ratio of collagen to elastin content was not significantly different. The total connective tissue content (collagen + elastin) expressed as a percentage of dry weight was not significantly different. The results given in Table 2 suggest that no significant difference should exist in passive mechanical properties of carotids from these two animal groups. However, significant differences were found at stresses above 2 x lo6 dyn/cm2 or strains above 0.7 with the carotids from the SHR being somewhat stiffer than those from the WKY rats. It should be noted that these values of stress and strain correspond to values within the physiologic range. Figure 4 shows a summary of values of incremental elastic modulus under passive conditions as a function of 2. Summary of water and connective content of carotid arteries

tissue

TABLE

I

Group

NORMALIZED

EXTERNAL

DIAMETER,

D/D,

H20,

wistar

I

I

W/L I c IE % w/cm

71.4 *0.7 71.2 a.4 70.8 kO.8

2.9 *0.2 2.5 kO.2 3.2*-f-t ztO.l

0.35 kO.03 0.28 *O.Ol 0.32t zko.02

0.23 to.01 0.20 to.01 0.22 *0.01

I

I 1

C

E %drywt

30.1 43.3 zt1.8 k1.8 36.9** 28.3 k2.1 *1.5 35.3* * 25.3* *1.3 * 1.0

I 1

C+E

1

C/E

73.4 1.51 *0.11 k2.4 64.3*** 1.34 k1.6 *o. 10 61.4** * 1.46 a.5 kO.12

FIG. 3. Summary of variation of average values of tangential wall stress with normalized external diameter for each animal group under passive conditions. Symbols are as in Fig. 2.

Kyoto Wistar SHR

significantly smaller in the carotids from the SHR. Under all circumstances stress/strain curves for the carotids from normal Wistar rats were significantly shifted to the right relative to carotids from the other two groups. It is generally thought that values of passive wall properties (i.e., stress/strain relations) are primarily de-

C, collagen; E, elastin; W/L, sample weight per unit length; HzO, water content as percentage of wet weight. *Statistically significant compared with Wistar at P < 0.05 level. **Statistically significant compared with Wistar at P < 0.01 level. ***Statistically significant compared with Wistar at P < 0.005 level. jStatistically significant compared with Kyoto Wistar at P < 0.05 level. ttStatistically significant compared with Kyoto Wistar at P < 0.01 level.

0

100 TRANSYURAL

PRESSURE

hnHg)

NORMALIZED

FIG. 4. Left: variation of passive incremental isotropic elastic modulus with transmural incremental elastic modulus on normalized external diameter. Symbols are as in Fig. 2.

EXTERNAL

DIAMETER,

D/D,

pressure. Right: dependence of passive values of

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ARTERIAL

MECHANICS

IN

GENETIC

H163

HYPERTENSION

transmural pressure (left panel) and normalized external diameter (right panel). Values of incremental modulus for carotids from the SHR were significantly stiffer than those of the two groups of control animals at all values of normalized diameter. These results agree qualitatively with those presented earlier in Fig. 3, but again are somewhat unanticipated from the results of connective tissue composition shown in Table 2. Although the average value of collagen-to-elastin ratio in carotids from normal Wistars was iarger than the ratio in WKY carotids, values of passive stiffness were larger for the latter. In addition, total connective tissue content (collagen + elastin) was significantly larger in the normal Wistars compared to WKYs. It seems, therefore, that collagen and elastin content per se cannot be invoked to explain differences in passive mechanical properties between carotids from WKY and normal Wistar rats. Although presentation of incremental elastic modulus data as a function of wall strain is more appropriate from the mechanical point of view, its variation with transmural pressure is probably more important from the physiological point of view. Values of incremental elastic modulus were largest at specific values of arterial pressure in the carotids from normal Wistar rats and smallest in the carotids from the SHR. For values of pressure from 80 to 160 mmHg the differences between incremental modulus of SHR and WKY carotids were statistically significant. Although from the point of view of continuum mechanics the carotids of the SHR were stiffer than those of the WKY, from the physiological point of view they appeared to be more compliant within the physiologic range of pressure. Active mechanics. Activation of vascular smooth muscle results in reductions in values of external diameter at specific values of transmural pressure relative to control conditions. Also, inactivation of intrinsic tone in vascular

smooth muscle results in an increase in values of diameter at a given pressure level in these arteries. The magnitude of these effects varies substantially with transmural pressure and animal group. The effects of activation on the variation of incremental elastic modulus with transmural pressure is shown in Fig. 5. In general, the effect of activation of vascular smooth muscle is to reduce values of incremental elastic modulus at all values of transmural pressure. There are only minor quantitative differences in this reduction of incremental modulus for arteries from the three groups of animals. For example, at 100 mmHg activation of vascular smooth muscle produces the following percent reductions in incremental elastic modulus: -54 t 4% in SHR, -60 t 5% in WKY, and -58 =t 6% in normal Wistars. At 150 mmHg these values are -68 t 8% in SHR, -62 t 8% in WKY, and -55 =t 7% in NW. When the effects of vascular smooth muscle are assessed in this manner it appears that no significant difference exists in the contribution of vascular smooth muscle to carotid artery mechanical properties in normotensive and hypertensive rats. Theoretical values of characteristic impedance for the three animal groups under active and passive conditions are summarized in Fig. 6. Characteristic impedance determines the relation between pulsatile pressure and flow in a blood vessel, and is determined by its mechanical and geometric properties. Characteristic impedance exhibits a U-shaped variation with transmural pressure under both active and passive conditions, possessing clear minimum values. Under passive conditions, the minimum value of characteristic impedance is significantly larger in the carotids from the SHR and occurs at a higher value of transmural pressure than in the WKY. In all three animal groups, the magnitude of the minimum value of characteristic impedance was not signifi-

IO0

lot

I SO

SO

-0

100

200

- 0

TRANSMURAL FJC. 5. Effects Symbols represent

of activation mean values

tO0

200

PRESSURE

of smooth muscle on variation of incremental and vertical bars represent &SE.

modulus

-0

loo

200

(mmHg) with

transmural

pressure

for the three

animal groups.

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H164

R. H. COX

cantly different under active and passive conditions, but it occurred at a higher value of transmural pressure after activation. There appears to be a correlation between the value of transmural pressure at which this minimum occurs under active conditions and the systolic level of blood pressure in these three animal groups (compare Table 1 and the results in Fig. 6). Thus, the minimum value of characteristic impedance occurs at approximately 170 mmHg in the SHR, and at approximately 125 mmHg in the two control groups. These results support our hypothesis that prevailing levels of transmural pressure contribute to the determination of the variation of characteristic impedance with transmural pressure (13), and more specifically, to the value of transmural pressure at which the minimum in this relation occurs. Also the variation of characteristic impedance with pressure is much greater in the SHR than in the WKY. From this WKY

W/STAR

a 3

0

no

200

0

I

0

muscle

activation

on the variation

I

loo

TRANSMURAL FIG. 6. Effects of smooth Symbols are as in Fig. 5.

point of view, therefore, the smooth muscle appears to be more effective in controlling the elastic properties of the rat carotid artery. The results summarized in Fig. 7 show the variation of the active stress response with normalized external diameter for vessels from the three animal groups. Values of active stress response were obtained from the difference in values of pressure under active and passive conditions at a given value of diameter by use of Eq. 1. Maximum values of active stress response were largest in the carotids from the SHR compared to the two control groups (Table 3). In addition, the maximum value of active stress response for the WKY carotids was significantly larger than the value for the carotids from normal Wistar rats. Likewise, the value of strain at which the maximum occurred was smaller in the WKY than in the normal Wistar carotids. Thus, differences in the

200

PRESSURE of theoretical

values

0

I

0

1

loo

200

(mmtig) of characteristic

impedance

with

transmural

pressure.

FIG. 7. Summary of values of active stress response to maximal norepinephrine activation as a function of normalized external diameter for the three animal groups. Symbols are as given before.

NORMALIZED

EXTERNAL

DIAMETER,

D/D,

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ARTERIAL

MECHANICS

IN GENETIC

H165

HYPERTENSION

optimum muscle length for force development are similar to differences in passive pressure-diameter relations. These differences in active stress response were also associated with differences in active diameter responses as shown in Fig. 8. Maximum values of active diameter response were not (statistically) significantly different in the carotids from the three animal groups (Table 3). However, the maximum diameter response did occur at a significantly higher value of transmural pressures in ’ carotids from the SHR. Also, for the range of transmural pressures from about 100 to 200 mmHg, values of active diameter response were larger in the SHR carotids. These results indicate substantial differences in the contractility of vascular smooth muscle from the spontaneously hypertensive rats. DISCUSSION

The results of these experiments have documented a number of differences in mechanical and geometric properties of carotid arteries from normotensive and spontaneously hypertensive rats under conditions of both active and passive smooth muscle. Values of passive mechanical properties for carotids of the SHR were stiffer than those of the WKY, especially for values of stress above 2 x lo6

dyn/cm2 (Fig. 3) or for values of strain above 1.7 (Fig. 4). In previous studies from this laboratory (12, 13, 15) we found that the ratio of collagen to ekstin content correlated very closely with passive mechanical properties at a given arterial site. In the present study, no significant difference was found in collagen/elastin ratio for carotids from WKY and SHR, while total connective tissue (collagen + elastin) was lower in the SHR. Therefore, it was unexpected that mechanically the SHR carotids were stiffer than those of the WKY. In addition, no significant difference in collagen/elastin ratio was found between carotids from WKY and normal Wistars in spite of the fact that the former were stiffer than the latter. Such findings are not completely surprising as other factors in addition to total connective tissue content could theoretically contribute to the passive mechanical properties of arteries (14, 29, 34). We have recently demonstrated that the passive mechanical properties of arteries can be described on the basis of a model in which collagen and elastin fibers are assumed to be arranged in parallel with the fraction of the former supporting wall stress varying with transmural pressure or wall strain (14). Such a model can be formalized by the following equation E = E,WI, + f,E,W,

TABLE 3. Summary of maximum actiue responses to norepinephrine in various artery groups Aue, lo3 dyn/

cm2

D/Do

-AWD, CR

R =Hg

Normal

561*

WiStar Kyoto Wil3ta.r SHR

*49

1.89 *0.05 1.79 zto.05 1.67* *0.04

29.5 zt2.5 28.2 a.4 28.8 *1.4

67.0 *4.0 67.0 *3*0 90.0* *2.9

Group

735

so 944* *79

Auo, maximum stress response to norepinephrine; D/Do, wall strain at maximum stress response; AD/D, maximum diameter response to norepinephrine; P, pressure at maximum diameter response. *Statistically significant compared to Kyoto Wistars (P < 0.05)

TRANSMURAL

PRESSURE

(mmHg)

8. Variation of normalized active diameter kesponse with transmural pressure for the three animal groups. Symbols are as given in Fig. 7. FIG.

(4

This relation states that the elastic modulus of a blood vessel (E) is determined by the elastic moduli of elastin (E,) and collagen (E,), the relative amount of elastin and collagen in the blood vessel ( Wk and WC, respectively), and by the fraction of collagen fibers supporting wall stress at a given strain or pressure ( fc). Though there are obviously many assumptions and simplifications implicit in this relation, it has been shown to be useful in representing the passive mechanical properties of arteries from different anatomic sites in an animal (14). Based on these previous experiments, values for the elastic modulus of elastin were estimated to be 2.8 t 0.4 x lo6 dyn/cm2, and that of collagen to be 1.2 t 0.1 x 10' dyn/cm2. These values agree reasonably with data given in the literature (7) When this model was applied to the data in this paper, substantial differences were found to exist. Values of Ee were estimated by assuming that no collagen fibers support wall stress at very low values of strain (i.e., D/D0 + 1.0). For carotids from both the WKY and SHR, an estimated value for E, of 13.4 k 1.8 x lo6 dyn/cm2 was found, which was essentially the same for the two animal groups. For carotids from normal Wistars a value of 5.6 k 0.8 x lo6 dyn/cm2 was found for Ee. All of these values are larger than values previously obtained for arteries from the dog (14). There are at least several possible explanations for the differences. Firstly, the presence of any smooth muscle tone would cause estimated values of E, to be high. This is unlikely because in the presence of 2 mM EGTA, smooth muscle agonists (e.g., norepinephrine and potassium) have no effect on pressure-diameter relations. Alternatively, it is possible that collagen supports a portion of the wall stress even at very low values of the latter. The results shown in Fig. 4 indicate that values of

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H166 E remain low but relatively constant for values of strains up to 1.3. This suggests that very little recruitment of collagen fibers occurs at strains below this value. This, of course, does not rule out that some small fraction of collagen fibers does support wall stress at low values, but that this value remains constant up to strains of about 1.3, or that smooth muscle cells support some portion of wall load, e.g., intracellular intermediate filaments (31). In comparing values of E, for control carotids, a value of 1.1 t 0.2 X 10' dyn/cm2 was found for the normal Wistars and a value of 1.9 t 0.3 x 10' dyn/cm2 for the WKY. This suggests that either some of the collagen fibers do not contribute to supporting wall stress in the normal Wistar, even at very large strains, or that the collagen fibers and/or their matrix is mechanochemically different in the WKY carotids. Values of E, from the SHR carotids were estimated to be 1.3 t 0.1 x 10' dyn/ cm2, which is identical to the value obtained from the dog arteries but is lower than the one obtained from WKY carotids. However, it is probable that collagen fibers are still being recruited to support wall stress in the SHR carotids at pressures around 250 mmHg because of the higher operating pressures in these animals. These considerations can now be integrated to explain the differences in passive mechanical properties between carotids from SHR and WKY rats. These differences (Fig. 4) are probably the result of a greater rate of recruitment of collagen fibers with wall strain in the SHR. To some extent the smaller radius-wall thickness ratio in the SHR carotids mitigates these differences (i.e., Fig. 4) so that values of stress-strain relations are quite similar (Fig. 3). When values of elastic moduli for these arteries are represented as a function of transmural pressure, they are smaller for SHR than for WKY carotids because of this smaller radius-wall thickness ratio. However, the differences in computed values of Ee and E,, if real, suggest that intrinsic differences may exist in the mechanochemical properties of these connective tissue elements. Because both the collagen and elastin matrices are composed of different components or types (20, 31), it is possible that more subtle changes in the composition of the connective tissue matrix may exist in carotid arteries from the SHR. Significant differences were found in the variation of characteristic impedance with transmural pressure in carotid arteries from the SHR compared to the WKY. In general, the same sort of variation occurred for each group; that is, characteristic impedance displayed a minimum at a specific value of transmural pressure, with values of the former increasing above and below the latter. However, the value of characteristic impedance at the minimum was significantly larger in the SHR than in the WKY, as was the pressure at which this minimum occurred. These differences between WKY and SHR carotids were accentuated under conditions of smooth muscle activation. Thus, in the SHR carotids the minimum occurred at a transmural pressure of approximately 180 mmHg, but in the WKY carotids it occurred at approximately 120 mmHg. It is of some interest that the value of transmural pressure at which the minimum occurred in these two animal groups was similar to the

R. H. COX

normal operating value of arterial blood pressure in these animals. This finding supports the hypothesis that the normal operating value of transmural pressure in the living animal determines (by way of mechanical and geometric properties) the variation of characteristic impedance with transmural pressure. A corollary to this hypothesis is that changes in arterial blood pressure in an animal will be associated with changes in the variation of characteristic impedance with transmural pressure, and especially the value of pressure at which the minimum in this relationship occurs. In general, activation of smooth muscle in carotids from the SHR produced larger effects compared to carotids from the control animals. Values of active stress response were larger in the SHR than in controls at almost all values of wall strain. The maximum value of active stress response was significantly larger in the SHR than in the WKY and occurred at a smaller value of normalized external diameter. In addition, the maximum active stress response for WKY carotids was larger than that of carotids from normal Wistar rats and occurred at a smaller value of normalized diameter. The active stress response was computed on the basis of total cross-sectional area of the blood vessel wall. Therefore, a relative increase in the amount of wall composed of smooth muscle could account for the increase in maximum stress response. This possibility is supported by the fact that total connective tissue content as a percentage of arterial wall volume was decreased in SHR carotids. By implication the relative content of muscle cells may be increased. Such an explanation could be the reason for the increase in active stress response. Alternatively, one could speculate that other factors could have contributed to the increase in active stress response, such as an increased stiffness of passive tissue elements, closer coupling of individual smooth muscle cells, etc. However, in the absence of any specific evidence the simpler explanation of increased smooth muscle mass remains the most likely explanation. It was also found that values of active diameter response were significantly larger in the carotids from the SHR. This could be the result of an increase in shortening capacity of the contractile system in SHR smooth muscle and/or differences in the dynamic characteristics of muscle load changes associated with this shortening. It could also be the result of the larger active stress-diameter relationship in SHR carotid arteries. The relative contribution of these various factors to active responses in SHR smooth muscle remains for future evaluation. In any regard, the results of these experiments suggest that the contractility of smooth muscle in the carotid artery of the SHR is increased compared to control animals. This conclusion is supported by the results of recent experiments on small mesenteric arteries from the SHR (25) A’word of caution should be injected at this point. The results of the studies reported herein document differences in. the mechanical properties and composition of carotid arteries from SHR and Kyoto Wistar rats. These results should not be extrapolated to human (essential) hypertension. It is not clear at this time if these dser-

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ARTERIAL

MECHANICS

IN

GENETIC

HI67

HYPERTENSION

ences simply represent differences in the properties of arteries from different strains of rats or differences related to the cause or the result of the hypertension in the SHR.

This work was supported in part by a Grant-in-Aid from the Southeastern Pennsylvania Heart Association and by Public Health Service Grant HL-17840 and HL-23348. Received

23 June

1978; accepted

in final

form

2 April

1979.

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