Effect of Sympathectomy on Mechanical Properties of Common Carotid and Femoral Arteries

Methods

We studied 12-week-old Wistar-Kyoto rats (n=19) with a body weight of 294.0±8.0 g (mean±SEM). Nine rats were sympathectomized by pretreatment with 6-hydroxydopamine (100 mg/kg body wt IP, twice over 5 to 6 days),¹⁴ by a method that allows almost complete sympathectomy to be obtained without affecting the animal’s body weight and behavior. The remaining 10 rats were pretreated with vehicle alone and served as controls. Each rat was anesthetized with sodium pentobarbital (40 mg/kg body wt IP). Polyethylene catheters were inserted into the common carotid and femoral arteries for continuous blood pressure measurement by a Statham P23D pressure transducer (Oxnard). The blood pressure signal was visualized on a Grass 7D recorder and recorded continuously by a personal computer. The heart rate was also derived continuously as the reciprocal of the interval between two consecutive systolic peaks. After verification of the effectiveness of sympathectomy by the observation of a >80% attenuation of the pressor and tachycardiac response to the injection of tyramine (100 mg/kg body wt IV),¹⁴ diameters of the carotid and femoral arteries were measured by an ultrasound method described in detail and validated in a previous report.¹⁵ Briefly, an ultrasonic 10-MHz transducer (NIUS 01 System, Asulab)¹⁶ was positioned over the common carotid and femoral arteries contralateral to the cannulated vessel to track the movements of the anterior and posterior arterial walls by the highest radiofrequency reflection peaks (ie, the peaks that have been shown to arise from the inner border of the walls¹⁷ ). Arterial diameter could thus be obtained continuously over the systolic and the diastolic periods. High-quality signals could be obtained transcutaneously for the common carotid artery and, after a superficial skin incision (not severing the muscle fasciae), for the femoral artery. In both vessels, sound transmission was optimized by the interposition of an ultrasound gel, which prevented tissue displacement and vessel deformation by the probe. Care was taken to position the probe perpendicular to the longitudinal axis of the vessel on its greatest cross-sectional dimension and at a site corresponding to the tip of the catheter inserted contralaterally. Echoes from the proximal and distal arterial walls were recorded continuously at 300 Hz by the same computer that was used to store the blood pressure signals. The computer receiving the blood pressure and arterial diameter signals was programmed to calculate the diameter-pressure curves of the vessels for both the increasing blood pressure values from diastole to systole and for the decreasing blood pressure values from systole to diastole, according to its fitting with the arctangent model of Langewouters¹⁸ and expressed by the formula S=α{π/2+tan⁻¹[(P−β)/γ]}, where S is the cross-sectional area, P is blood pressure, and α, β, and γ are three optimal parameters that describe the spatial position of the diameter/pressure curve from minimal to maximal existing blood pressure values. From this formula, compliance (C=ΔS/ΔP) was calculated as follows: The values were normalized for arterial diameter to obtain distensibility values, which are expressed as distensibility-pressure curves.¹⁸ The area under the curve that relates arterial distensibility to blood pressure was normalized for pulse pressure and referred to as the “distensibility index.”¹⁹

The Peterson arterial elastic modulus (Ep, a pressure-independent index of arterial mechanical properties) was calculated for the diameter-pressure relationship according to the following formula²⁰ : Ep=pBP/[(Ds−Dd)/Dd], where pBP is pulse blood pressure, Ds is systolic diameter, and Dd is diastolic diameter.

The carotid and femoral arteries were studied by a single operator, whose possible reading bias was avoided because the arterial diameter values were available only after completion of the probe positioning and echo selection procedures. The sequence of the carotid and femoral artery studies was randomized, and both studies were completed in a single experimental session. It was shown previously that the within-operator coefficient of variation of two arterial diameter measurements, obtained on the same vessel in two different experimental sessions, is <8%.

The protocol and data analysis were as follows: (1) after the measuring devices were positioned, a 10- to 20-minute period of stabilization was allowed; (2) blood pressure and arterial diameter data were acquired over 30 periods of 4 seconds each, with each 4-second period being separated from the following one by an interval of 30 seconds; (3) in individual animals, the data were averaged first within each 4-second period and then for all thirty 4-second periods; (4) averages of data from intact and sympathectomized animals were calculated; and (5) the statistical significance of the differences (sympathectomy versus intact) in systolic blood pressure, diastolic blood pressure, heart rate, arterial diameter, arterial distensibility index, and elastic modulus was assessed by the two-tailed Student t test for unpaired observations. The same assessments were performed separately for the carotid artery and femoral artery data. A value of P<.05 was considered significant.

Results

Systolicbloodpressure(102.1±2.3versus116.5±3.7 mm Hg, mean±SEM), diastolic blood pressure (76.3±2.9 versus 80.5±3.5 mm Hg), mean arterial pressure (84.9±2.6 versus 92.5±3.5 mm Hg), and heart rate (225.2±19.0 versus 262.1±7.0 bpm) were somewhat lower in sympathectomized animals than in control vehicle-treated animals. The difference attained statistical significance (P<.05) for systolic blood pressure only.

Sympathectomy was associated with prominent changes in arterial mechanical properties. In the common carotid artery (Fig 1⇓, left panels), the diameter increased progressively from diastolic to systolic blood pressure with values that were only slightly and nonsignificantly different in sympathectomized animals compared with control animals. In contrast, distensibility decreased progressively from diastole to systole, but over this entire pressure range, the values were significantly greater in sympathectomized than in control animals, which was demonstrated by a significant and pronounced increase (+59%) of the distensibility index (Fig 1⇓, right panels). Similar data were obtained in the femoral artery, in which, however, the effect of sympathectomy was so pronounced that it induced a small (+18%) but significant increase in mean diameter (Fig 2⇓).

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Figure 1.

Diameter-pressure curves, distensibility-pressure curves, mean diameter, and distensibility (Dist.) index of the common carotid artery in intact (vehicle) and sympathectomized rats. Values are mean±SEM.

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Figure 2.

Diameter-pressure curves, distensibility-pressure curves, mean diameter, and distensibility (Dist.) index of the femoral artery in the animals of Fig 1⇑. Values are mean±SEM.

Compared with controls, the arterial elastic modulus of sympathectomized rats was significantly reduced in the common carotid artery and even more reduced in the femoral artery (Table⇓).

Discussion

In our rats, sympathectomy was associated with a marked upward shift of the distensibility-pressure curve in both the carotid artery and the femoral artery. It was similarly associated with a marked reduction of the stiffness of both vessels as evaluated by calculation of the elastic modulus. These findings indicate that the sympathetic nervous system exerts a marked tonic restraint on arterial distensibility and that this restraint involves both large arteries with a predominant elastic structure and arteries with a predominant muscle structure, thus providing direct unequivocal evidence of the contribution of this factor in the modulation of the mechanical properties of large arteries.

Our findings have a firm methodological basis because the technique we used is characterized by high sensitivity, high reproducibility,^{6 7 9 15 16} and, most important, the ability to assess arterial distensibility throughout the systolic-diastolic pressure range instead of at only one or a few pressure values, as was true with techniques used in the past.²¹ Although our methods offer a clear advantage also for studies on arterial distensibility in humans, the advantages are also evident in our animals because of (1) the use of direct blood pressure recording from a large artery and (2) the fact that the intra-arterial blood pressure recording was obtained at virtually exactly symmetrical sites of the contralateral arteries from which the diameter was measured. This represents an ideal situation that avoids the inconveniences that characterize arterial distensibility studies in humans, in whom blood pressure is measured normally indirectly and at sites not corresponding to those at which arterial diameter is measured; possible errors introduced by the suboptimal conditions of human studies (inaccurate blood pressure values, blood pressure damping or overshoot, temporal hysteresis of the diameter-pressure curves, etc), could be avoided with our experimental design.

Several others points should be discussed. First, stable conditions were provided for multiple ultrasound readings by anesthetizing the animals with sodium pentobarbital, which might have reduced both sympathetic activity and vascular smooth muscle tone.²² It should, however, be emphasized that any direct anesthesia-induced relaxation of carotid and femoral vascular smooth muscle would have increased the distensibility of both intact and sympathectomized rats. Furthermore, a sodium pentobarbital–induced reduction of sympathetic activity would have minimized rather than increasing the between-group difference in distensibility we observed.

Second, sympathectomy was accompanied by a reduction in blood pressure, which might, given the shape of the distensibility-pressure relation (Figs 1⇑ and 2⇑), have increased arterial distensibility. In these experiments, however, the blood pressure reduction associated with sympathectomy was small. In addition, an accessory analysis of the data, in which the distensibility index was calculated only for the overlapping blood pressure range of intact and sympathectomized rats (“isobaric” distensibility index), showed this index to equal 3.6±0.5 mm/mm Hg×10⁻³ for the carotid artery and 1.3±0.1 mm/mm Hg×10⁻³ for the femoral artery in intact animals and to increase to 5.4±0.7 and 1.9±0.2 mm/mm Hg×10⁻³, respectively, in sympathectomized animals (P<.05 for both). Furthermore, a pressure-independent measure of arterial wall mechanical properties, such as the elastic modulus, showed the carotid and the femoral arteries to be less stiff in sympathectomized rats than in control rats. Thus, the possibility that the increase in arterial distensibility of sympathectomized rats was caused by the reduced blood pressure levels can be excluded.

Third, one may wonder why the increased distensibility in the carotid artery of sympathectomized rats was not associated with an increase (rather, a slight trend to a reduction occurred) in the mean diameter. An explanation of this finding may relate to the sympathectomy-induced reduction in blood pressure; the reduction in diameter may have been due to the reduced distending pressure offsetting the inherent tendency of the arterial wall to be more easily stretched. This may not have been seen in the femoral artery, in which the larger increase in distensibility after sympathectomy led to an increased diameter, despite the reduction in blood pressure.

Finally, a previous study on anesthetized Wistar rats¹¹ reported that short-term administration of guanethidine was accompanied by an increase in aortic diameter and compliance, which suggests that tonic sympathetic activity does affect arterial wall mechanics. However, when aortic compliance was normalized for diameter values (ie, when the data were expressed as distensibility), the results before and after short-term guanethidine were superimposable, therefore creating some uncertainty about the importance of sympathetic tone for arterial wall viscoelastic properties. Because our study allows for the conclusion that sympathetic tone is an important determinant of arterial distensibility, this uncertainty is relieved. The reasons for the different results obtained in the two studies may relate to differences in the rat strains, the vessels studied, and/or the methods used for removing sympathetic influences. It is more likely, however, that the peculiar features of our experimental design may account for the differing results: periarterial tissue was left intact, and no intra-arterial catheters were inserted in the vessel used for diameter measurement.

Our study did not address the factors by which sympathetic tone markedly limits large-artery distensibility. It is possible, however, that tonic contraction of the smooth muscle is involved, because a contracted muscle has a greater elastic modulus than a relaxed muscle.^{6 7 9 23} Because smooth muscle is abundant in the femoral artery, this explanation is compatible with the marked increase in femoral artery distensibility after sympathectomy. It is also compatible with the smaller but clear-cut increase in carotid artery distensibility after sympathectomy, because in the carotid artery and other elastic arteries, smooth muscle is less widely represented but still capable of modifying arterial mechanics.²⁴

In conclusion, our study provides an unequivocal demonstration that in the anesthetized rat the sympathetic nervous system tonically restrains large-artery distensibility and that the restraint involves both elastic and muscle-type vessels. We may speculate that diseases that increase sympathetic activity (eg, congestive heart failure)^{25 26} and therapeutic interventions that reduce it²⁷ may be accompanied by modifications not only in the arterioles but also in large conduit arteries, with potential implications for arterial impedance and cardiac load.