Hormonal Therapy Increases Arterial Compliance in ... - JACC

350

JACC Vol. 30, No. 2 August 1997:350 – 6

ATHEROSCLEROSIS

Hormonal Therapy Increases Arterial Compliance in Postmenopausal Women CHAKRAVARTHI RAJKUMAR, MD, BRONWYN A. KINGWELL, PHD, JAMES D. CAMERON, MD, MENGSC,* TAMARA WADDELL, BSC, RISHI MEHRA, NICHOLAS CHRISTOPHIDIS, MBBS, PHD, PAUL A. KOMESAROFF, MBBS, PHD, BARRY MCGRATH, MD,† GARRY L. JENNINGS, MD, KRISHNANKUTTY SUDHIR, MD, PHD, ANTHONY M. DART, BCHBM, DPHIL Melbourne, Australia

Objectives. This study investigated the effects of hormonal therapy on large arterial properties. Background. Arterial stiffness is an emerging risk marker for coronary heart disease and is potentially modifiable. Postmenopausal use of hormonal therapy is associated with a lower risk of coronary heart disease. Methods. Total systemic arterial compliance (SAC) and pulse wave velocity (PWV) were determined in 26 premenopausal and 52 postmenopausal women, 26 of whom were taking hormonal therapy. Results. Arterial compliance was greater in the premenopausal group (mean 6 SEM 0.57 6 0.04 arbitrary compliance units [ACU]) than in the postmenopausal group not taking hormonal therapy (0.26 6 0.02 ACU, p 5 0.001). Postmenopausal women taking hormonal therapy had a significantly increased total SAC compared with women not taking hormonal therapy (0.43 6 0.02 vs. 0.26 6 0.02 ACU, p 5 0.001). PWV in the aortofemoral region

in the premenopausal women was 6.0 6 0.2 vs. 8.9 6 0.3 m/s (p < 0.001) in untreated postmenopausal women. However, postmenopausal women taking hormonal therapy had a significantly lower PWV than those not taking hormonal therapy (7.9 6 0.2 vs. 8.9 6 0.3 m/s, p 5 0.01). Eleven postmenopausal women had their hormone replacement therapy withdrawn for 4 weeks, resulting in a significant decrease in SAC and a significant increase in aortofemoral PWV. Conclusions. The increased SAC and decreased PWV in women receiving hormonal therapy suggest that such therapy may decrease stiffness of the aorta and large arteries in postmenopausal women, with potential benefit for age-related cardiovascular disorders. The reduction of arterial compliance with age appears to be altered with hormonal therapy. (J Am Coll Cardiol 1997;30:350 – 6) ©1997 by the American College of Cardiology

Postmenopausal use of estrogen-containing hormonal therapy appears to be beneficial in reducing the incidence of coronary heart disease (1,2) and possibly stroke (3). It has recently been shown (4,5) that postmenopausal women taking estrogens also have significantly lower death rates than age-matched control subjects who did not. The precise mechanisms responsible for this beneficial effect are not established. Estrogens appear to have beneficial effects on lipoprotein profiles (6); however it has been estimated that estrogen-induced changes in lipoprotein levels account for only 25% to 50% of the observed risk

reduction (7), suggesting that additional factors may be involved. There is evidence that estrogens exert a direct effect on the vasculature (8 –10). Receptors for sex steroids have been identified in the human aorta, left atrial appendage, internal carotid, internal mammary and coronary arteries and saphenous veins (11–13). A number of studies have shown increased stiffness of the proximal arterial circulation in subjects with coronary heart disease (14 –19), although the presence of increased cholesterol levels itself appears not to necessarily result in this increase (14,20 –22). The possibility has been raised that proximal aortic stiffness is a modifiable target for therapy (23). Prospective studies (24) have established that pulse pressure, a surrogate measure of arterial stiffness, is an independent predictor of future coronary events. Arterial stiffness, determined by pulse wave velocity, increases with age in both men and women, but a study in 600 subjects (25) has indicated that, whereas arterial compliance in premenopausal women is greater than that in men of equivalent age, this difference is lost in the postmenopausal years, with evidence of a rapid decline in compliance in the perimenopausal period. A recent

From the Baker Medical Research Institute; *Biomedical Engineering, Latrobe University; and †Department of Medicine, Monash Medical Centre, Melbourne, Australia. Dr. Rajkumar is the recipient of a postgraduate medical research scholarship from the National Health and Medical Research Council, Canberra, Australia. This work was supported by the Alfred Healthcare Group, Melbourne, Australia and by an Institute Grant to the Baker Medical Research Institute from the National Health and Medical Research Council of Australia. Manuscript received June 28, 1996; revised manuscript received April 18, 1997, accepted April 25, 1997. Address for correspondence: Dr. Anthony M. Dart, Alfred and Baker Medical Unit, Baker Medical Research Institute, Commercial Road, Prahran, 3181 Australia. ©1997 by the American College of Cardiology Published by Elsevier Science Inc.

0735-1097/97/$17.00 PII S0735-1097(97)00191-5


RAJKUMAR ET AL. HORMONAL THERAPY AND ARTERIAL COMPLIANCE

Abbreviations and Acronyms ACU 5 arbitrary compliance units AFU 5 arbitrary flow units ARU 5 arbitrary resistance units PWV 5 pulse wave velocity SAC 5 systemic arterial compliance

study in hypertensive women (26) has also found a rapid change in compliance perimenopausally and has related this to changes in endogenous estrogen levels. Loss of hormonal modulation is a likely explanation for reduced arterial compliance, or increased stiffness, in postmenopausal women. The consequent change in left ventricular afterload and diastolic perfusion may be causally related to the production of myocardial ischemia (27,28). Long-term treatment with estrogencontaining hormonal therapy may be expected to partially reverse the increase in arterial stiffness and thereby contribute to a lower incidence of myocardial ischemia, and preliminary reports (29) have indicated a reduction in stiffness with shortterm intravenous administration of estrogenic compounds in a group of six postmenopausal women. However, long-term postmenopausal use of the synthetic steroid Tibolone, structurally related to norethisterone, was found (30) not to alter aortic compliance, as derived from aortic pulse wave velocity. We examined the capacity of long-term treatment with estrogen-containing therapy to influence the age-related decrease in compliance in postmenopausal women. We compared measures of arterial stiffness in postmenopausal women receiving hormonal therapy with those of untreated women of comparable age as well as a group of younger women. In addition, 11 postmenopausal women were restudied after a 1-month cessation of hormonal therapy. Measures were made of regional as well as integrated or total arterial compliance.

Methods Subjects. Participants in the study were recruited by advertisements placed in clinical and nonclinical areas of our own and associated institutions. Twenty-six postmenopausal women (mean age 6 SEM 58 6 1 years, range 50 to 71) treated continuously with estrogen for at least 1.5 years (mean duration of therapy 6.7 6 1.0 years) were studied and compared with 26 age-matched women (59 6 1 years old, range 50 to 70) not taking such therapy. All women in the hormonal therapy group had commenced taking hormones after cessation of regular menstrual cycles, whereas those not taking such therapy had a minimum of 2 years since the last menstrual cycle and were free of vasomotor symptoms associated with the onset of menopause. Twenty-six young women (mean age 23 6 1 years, range 17 to 35) were also studied. All studies were performed in the first week after menstruation. No subjects in either the premenopausal or postmenopausal groups had any known cardiovascular disease; none were taking medication (other than hormonal therapy in the

351

postmenopausal group); none smoked cigarettes; and all were healthy, as determined from clinical history and full physical examination. None of the subjects were participating in physical fitness programs, and all gave informed consent for participation in the study, which was performed with the approval of the Alfred Hospital Ethics Committee. Protocol. Height and weight were determined for all participants. Blood pressure and heart rate measurements were made at the time of arterial compliance assessments. Blood samples were taken for measurement of total plasma, high and low density lipoprotein cholesterol and triglyceride levels. Hormonal therapy was withdrawn in 11 of the postmenopausal women for 1 month, at which time arterial compliance measurements were repeated. Systemic arterial compliance. Systemic arterial compliance (SAC) was determined using calculations based on the area method, as described by Liu et al. (31). In our laboratory (32,33), SAC is estimated by the measurement of aortic flow using continuous wave Doppler velocimetry with simultaneous carotid pressure waveform recording. The formula used for calculation is SAC 5 Ad/@R~Ps 2 Pd!#,

where Ad is the area under the diastolic portion of the blood pressure waveform (from end-systole to end-diastole), R the total peripheral resistance, Ps the end-systolic blood pressure and Pd the end-diastolic blood pressure. Continuous ascending aortic flow velocity was measured using a hand-held Doppler flow velocimeter (MDI MultiDoplex, Huntleigh Technology, Cardiff, UK) placed in the suprasternal notch. This device provides an analog signal proportional to the instantaneous frequency determined by the number of detected zero crossings per unit time of the backscattered Doppler signal, which can be related by the Doppler equation to flow in the ascending aorta (34). This technique provides an average (approximately root-mean square) value for flow, which differs from the method used in clinical applications estimating maximal flow. Because the derived numerical value for flow determined using zerocrossing analysis will be less than that obtained invasively, we chose to report our results in arbitrary compliance units (ACU [dimensionally equal to ml/mm Hg]), arbitrary flow units (AFU [dimensionally equivalent to liters/min]) and arbitrary resistance units (ARU). Volume flow was calculated as the product of average systolic flow and aortic root area measured by two-dimensional echocardiography (Hewlett-Packard Sonos 1500). Aortic root pressure was estimated by applanation tonometry of the right carotid artery using a noninvasive Miller Mikro-Tip pressure transducer (model SPT-301, Miller Instruments). The pressures obtained by tonometry were calibrated against brachial diastolic and mean pressure measurements determined oscillometrically and made simultaneously using a Dinamap vital signs monitor (model 1846SX, Critikon) at the brachial artery (average of five blood pressure recordings per person). Briefly, the brachial mean arterial pressure was

352


assigned the area under the carotid waveform, and the brachial diastolic pressure was assigned the automatically determined end-diastolic point on the carotid waveform. This allowed calculation of carotid systolic pressure and, thus, estimation of central pulse pressure. Comparison between carotid waveforms recorded by tonometry and direct intraaortic waveforms recorded with a microtipped manometer has demonstrated the similarity between these waveforms, with the detectable difference between them being a smaller carotid than aortic augmentation index, a difference that would not affect the calculation of SAC, which is dependent only on the diastolic component of the wave (35). In accord with this, there was a close correspondence between SAC calculated from direct intraaortic recordings and carotid tonometry (32). Both flow and pressure signals were digitized at 200 Hz. Analysis was performed using custom-written software (Pascal Turbo, Borland International, Inc.). The value of SAC was determined for each patient from 10 representative flow and pressure waveforms. Pulse wave velocity. Pulse wave velocity (PWV), a measure of arterial stiffness, was determined centrally from the carotid to femoral artery and distally from the femoral to dorsalis pedis artery. PWV over the trunk and leg was determined by simultaneous applanation tonometry of the carotid and femoral and femoral and dorsalis pedis arteries, respectively. Data were digitized (2,000 Hz/channel) and acquired using customwritten software (J.D. Cameron). The time of travel between applanation points was calculated from the upstroke of the systolic waveform detected as the maximum of the second derivative. Distances from the carotid sampling site to the manubrium sternum, manubrium sternum to the femoral artery (AF), and femoral artery to the dorsalis pedis (FD) were measured. The central PWV (aortofemoral) was calculated as

Table 1. Group Characteristics Postmenopausal Women*

Age (yr) LDL-C (mmol/liter) HDL-C (mmol/liter) Triglycerides (mmol/liter) SBP (mm Hg) MAP (mm Hg) DBP (mm Hg) BMI (kg/m2) HR (beats/min)

Premenopausal Women (n 5 26)

No Hormonal Therapy (n 5 26)

Hormonal Therapy (n 5 26)

23 6 1 2.7 6 0.1 1.4 6 0.1 1.1 6 0.2 106 6 2 77 6 2 63 6 2 22.7 6 0.7 64 6 2

59 6 1† 3.2 6 0.2 1.4 6 0.1 1.3 6 0.1 126 6 3† 92 6 2† 71 6 1† 27.0 6 1.4† 67 6 1

58 6 1‡ 3.3 6 0.3‡ 1.3 6 0.1 1.5 6 0.1‡ 119 6 3‡ 88 6 2‡ 69 6 2 26.7 6 0.9‡ 68 6 2

*p 5 NS for all comparisons between the postmenopausal groups. †Premenopausal versus postmenopausal women not receiving hormonal therapy. ‡Premenopausal women versus postmenopausal women receiving hormonal therapy. Data presented are mean value 6 SEM. All pressures are peripheral brachial pressures. BMI 5 body mass index; DBP 5 diastolic blood pressure; HDL-C 5 high density lipoprotein cholesterol; HR 5 heart rate; LDL-C 5 low density lipoprotein cholesterol; MAP 5 mean arterial blood pressure; SBP 5 systolic blood pressure.


Figure 1. SAC and PWV in premenopausal women (PM) and postmenopausal women with (1HRT) and without hormonal therapy (2HRT). PWV A-F 5 PWV over the aortofemoral region; PWV F-D 5 PWV over the femorodorsalis pedis region. Data shown are mean value 6 SEM. *Significant differences (p , 0.05).

AF 2 CA divided by the time interval between carotid and femoral pulses (36). PWV in the leg was calculated as the distance traversed (FD) divided by time taken. Statistics. Data were tabulated and analyzed using SPSS (6.1 Windows). Results are given as mean value 6 SEM. Group comparisons were made by one-way analysis of variance and the Scheffe´ test. Multiple regression, by stepped backward and forward entry, was undertaken to investigate determinants of SAC. Variables were entered if the respective F values had a p value , 0.05 and were removed if they had a p value . 0.1. Paired t tests were used to compare variables in the 11 postmenopausal women withdrawn from therapy for 1 month. The level of significance used was p , 0.05.

Results There were no significant differences in age; body mass index; total serum cholesterol, high and low density lipoprotein cholesterol and triglyceride levels; heart rate; or blood pressure between postmenopausal women receiving and those not receiving hormonal therapy (Table 1). In postmenopausal women, SAC was 0.26 6 0.02 ACU in untreated women compared with 0.43 6 0.02 ACU in women receiving treatment (p , 0.001) (Fig. 1). Mean PWV over the aortofemoral region



Table 2. Hemodynamic Variables Postmenopausal Women

CO (AFU) Central PP (mm Hg) Total PR (ARU)

Premenopausal Women (n 5 26)

No Hormonal Therapy (n 5 26)


1.10 6 0.08 43 6 1 14 6 1

0.97 6 0.09 56 6 4* 22 6 2*

1.09 6 0.08 45 6 3† 17 6 1

*Premenopausal versus postmenopausal women not receiving hormonal therapy. †Hormonal therapy versus no hormonal therapy. Data presented are mean value 6 SEM. AFU 5 arbitrary flow units; ARU 5 arbitrary resistance units; CO 5 cardiac output; PP 5 pulse pressure; PR 5 peripheral resistance.

was significantly lower in women taking hormonal therapy (7.9 6 0.2 vs. 8.9 6 0.3 m/s, p 5 0.01) (Fig. 1). Consistent with these findings was the lower central pulse pressure in the group taking hormonal therapy (Table 2). However, there were no differences in either cardiac output or total peripheral resistance between the two postmenopausal groups (Table 2). PWV in the leg was lower in postmenopausal women with than in those without hormonal therapy, but the difference failed to achieve statistical significance (8.1 6 0.3 vs. 8.8 6 0.3 m/s) (Fig. 1). PWV is related to SAC (PWV } 1/=SAC), and we found that PWV in the aortofemoral region correlated better with 1/=SAC (r 5 0.48, p 5 0.001) than did PWV in the leg (r 5 0.28, p 5 0.02). Premenopausal women had significantly lower body mass index; blood pressure; and total serum cholesterol, low density cholesterol and triglyceride levels than postmenopausal women, but there was no difference in heart rate. SAC in premenopausal women (0.57 6 0.04 ACU) was significantly higher than in either of the postmenopausal groups. Similarly, PWV over the trunk (6.0 6 0.2 m/s) was significantly lower than in either of the postmenopausal groups. Central pulse pressure and total peripheral resistance in the premenopausal women were not different from that in the postmenopausal women taking hormonal therapy but were significantly lower than that in the postmenopausal women not taking therapy (Table 2). Cardiac output (Table 2) and PWV over the leg (7.7 6 0.3 m/s) (Fig. 1) were not significantly different from that in the postmenopausal women. Women receiving hormonal therapy were then classified into those receiving an estrogen only and those receiving both an estrogen and a progestogen. Women receiving estrogen only had an arterial compliance of 0.37 6 0.04 ACU compared with 0.26 6 0.02 ACU in postmenopausal women not receiving hormonal therapy (p 5 0.01) and 0.57 6 0.04 ACU (p , 0.01) in premenopausal women. Women receiving a combination of estrogen and progestogen had an arterial compliance of 0.46 6 0.03 ACU compared with 0.26 6 0.02 ACU in postmenopausal women not receiving hormonal therapy (p , 0.001) and not significantly different from that of premenopausal women (0.57 6 0.04 ACU). SAC in women receiving estrogen only was not significantly different from those receiving a combination

353

of an estrogen and a progestogen (0.37 6 0.04 vs. 0.46 6 0.03 ACU, p 5 0.07). Multiple regression analysis, including age, presence of any form of hormonal therapy, total cholesterol and triglyceride levels and heart rate as independent variables and SAC as the dependent variable in a stepwise model showed that age and hormonal therapy were the only two independent factors affecting SAC. These two factors accounted for 50% (r2 5 0.50) of the variance in SAC. Although compliance and mean arterial pressure were inversely related in a univariate analysis (r2 5 0.22, p , 0.001), neither mean, systolic nor diastolic blood pressures were significant independent factors when included in a multiple regression analysis. In the 11 women in whom hormonal therapy was withdrawn for 1 month, SAC was reduced from 0.43 6 0.04 to 0.33 6 0.02 ACU (p 5 0.008) (Fig. 2). PWV in the aortofemoral region increased from 8.2 6 0.4 to 8.8 6 0.5 m/s (p 5 0.046) (Fig. 2), and there was a trend toward elevation of central pulse pressure (Table 3). However, there were no changes in mean arterial, brachial systolic or diastolic pressures, total peripheral resistance or cardiac output (Table 3). Leg PWV was also elevated by the 4-week cessation of therapy, from 7.6 6 0.4 to 8.8 6 0.4 m/s (p 5 0.036) (Fig. 2).

Figure 2. SAC and PWV in 11 postmenopausal women receiving hormonal therapy (1HRT) and after 1-month cessation of hormonal therapy (2HRT4W). Other abbreviations as in Figure 1. Solid circles 5 mean value 6 SEM. *Significant differences (p , 0.05).

354


Table 3. Cessation of Hormonal Therapy Postmenopausal Women*

SBP (mm Hg) DBP (mm Hg) MAP (mm Hg) Central PP (mm Hg) CO (AFU) Total PR (ARU)


After 4-wk Cessation of Hormonal Therapy (n 5 11)

122 6 4 70 6 4 90 6 4 45 6 3 1.15 6 0.44 18 6 2

125 6 6 71 6 4 92 6 5 49 6 4 1.12 6 0.11 18 6 2

*p 5 NS for all comparisons. Data presented are mean value 6 SEM. All pressures are peripheral brachial pressures, unless otherwise stated. Abbreviations as in Tables 1 and 2.

Discussion The present study demonstrates that long-term hormonal therapy in postmenopausal women significantly reduces aortofemoral PWV and increases SAC compared with agematched control subjects not using such therapy. SAC was lower in the older women; however, this decline appeared to be less in women receiving hormonal therapy. Both PWV and SAC are dependent on arterial wall properties, with PWV increasing and SAC decreasing with increased arterial stiffness (37). We thus conclude that hormonal therapy reduces arterial stiffness and support this contention further by the reduction in SAC and elevation in aortofemoral PWV after a 4-week cessation of therapy. Factors influencing arterial compliance. Passive arterial properties are dependent on mean distending pressure (37), but there was no difference in mean arterial pressure between the postmenopausal women with and without hormonal therapy. However, differences in blood pressure may contribute to reduced compliance in older compared with younger women. Arterial compliance is affected by exercise training (32,33), but none of our subjects were involved in fitness programs, and mean heart rate, a crude measure of cardiovascular fitness, was similar in the premenopausal and postmenopausal women with and without hormonal therapy. Serum lipid levels affect endothelial function in resistance vessels, although the effect on conduit vessels is more controversial (14,20,38). However, measures of total, low and high density lipoprotein cholesterol were similar in both postmenopausal groups. Age, which is also a major determinant of arterial compliance (25,37), was not different between the two groups of postmenopausal women. Mechanisms. The mechanisms for the effects of sex hormones on arterial wall properties described here are not well understood. The effects appear to be estrogen mediated because subgroup analysis revealed an effect on arterial properties in women taking estrogen alone. As discussed, the higher arterial compliance of women receiving hormonal therapy than age-matched control subjects not taking therapy was independent of blood pressure. This difference therefore relates either


to a change in the loading conditions on the elastin and collagen fibers or to structural changes in the vessel wall. Estrogen may alter vessel wall loading conditions through vasodilation by mechanisms that include potentiation of endothelium-dependent vasodilation mediated by nitric oxide (8 –10,39 – 42). Dilation transfers vascular wall stress from the collagen fibers to the more distensible elastin fibers, resulting in a more compliant vasculature. Dilation may profoundly influence proximal arterial compliance through direct effects on the aorta and through the vasa vasorum as well (43). The time course of the reduction in arterial compliance after withdrawal of hormonal therapy for 4 weeks would be consistent with a vasodilator mechanism. Although changes in vascular loading conditions may underlie the short-term effects of estrogen on arterial compliance, longer term effects may be mediated by structural vascular changes. Previous studies in experimental animals have shown that estrogens can modulate the relative proportions of collagen and elastin (44) and smooth muscle cell numbers (45,46). Peripheral compliance. Although SAC was significantly higher, and PWV over the trunk significantly lower, in premenopausal women, there was no significant difference between younger and older women in PWV over the leg. This finding is consistent with previous evidence that aging-related changes in the arterial circulation are more marked proximally than distally (37,38,47). Similarly, the effects of hormonal therapy in postmenopausal women were significant for PWV over the trunk but not for PWV over the leg. The functional compliance of the arterial circulation is predominantly contained within the proximal arterial circulation (48), and in keeping with previous findings (49) we found that 1/=SAC, which is related to the Bramwell-Hill equation (49), where Co 5 =V dP/r dV (where Co 5 PWV; P 5 pressure; V 5 volume; r 5 density), correlated better with PWV over the trunk than over the leg. It would thus appear that the effects of hormonal therapy are principally on the larger conduit vessels, which are also the principal site for age-related changes. However, previous studies (50,51) have shown effects of hormonal therapy on carotid distensibility so that medium-sized conduit vessels are also affected. Compliance and cardiac function. Cardiac ventricular function is dependent on the characteristics of the arterial circulation, and aspects of ventricular function are different with estrogen therapy (52). It is not possible at present to discriminate between the relative contribution of direct effects on the heart and those secondary to changes in arterial function. In addition, changes in the stiffness of the proximal circulation may also affect cardiac function indirectly through changes in the operation of the baroreflex (33). Conclusions. The present study demonstrates that hormonal therapy after menopause exerts a profound effect on the properties of the proximal arterial circulation. This mechanism may be important for the cardioprotective effect of hormonal



therapy and warrants further investigation through a prospective, placebo-controlled trial. We thank Elizabeth Dewar, BSc, Leonie Johnston, SRN, CNN, SCM and Kaye Varcoe for technical assistance.

References 1. Martin KA, Freeman MW. Postmenopausal hormone replacement therapy. N Engl J Med 1993;328:1115–7. 2. Stampfer MJ, Colditz GA, Willett WC, et al. Postmenopausal estrogen therapy and cardiovascular disease. N Engl J Med 1991;325:756 – 62. 3. Paganini-Hill A, Ross RA, Henderson BE. Postmenopausal oestrogen treatment and stroke: a prospective study. BMJ 1988;297:519 –22. 4. Frankel D. Postmenopausal oestrogen replacement affords cardioprotection. Lancet 1996;347:49. 5. Ettinger B, Friedman GD, Bush T, Quesenberry CP. Reduced mortality associated with long-term postmenopausal oestrogen therapy. Obstet Gynecol 1996;87:6 –12. 6. Wahl P, Walden C, Knopp R, et al. Effects of estrogen/progestin potency on lipid/lipoprotein cholesterol. N Engl J Med 1983;308:862–7. 7. Bush TR, Barrett-Connor E, Cowen LD, et al. Cardiovascular mortality and noncontraceptive use of estrogen in women: results from the Lipid Research Clinics Program Follow-up Study. Circulation 1987;75:1102–9. 8. Van Buren GA, Yang DS, Clark KE. Estrogen-induced uterine vasodilatation is antagonized by L-nitroarginine methyl ester, an inhibitor of nitric oxide synthesis. Am J Obstet Gynecol 1992;67:828 –33. 9. Sudhir K, Chou TM, Mullen WL, et al. Mechanisms of estrogen-induced vasodilation: in vivo studies in canine coronary conductance and resistance arteries. J Am Coll Cardiol 1995;26:807–14. 10. Volterrani M, Rosano G, Coats A, et al. Estrogen acutely increases peripheral blood flow in postmenopausal women. Am J Med 1995;99:119 – 22. 11. Ingegno MD, Money SR, Thelmo W, et al. Progesterone receptors in the human heart and great vessels. Lab Invest 1988;59:353– 6. 12. Karas RH, Patterson BL, Mendelsohn ME. Human vascular smooth muscle cells contain functional estrogen receptor. Circulation 1994;89:1943–50. 13. Losordo DW, Kearney M, Kim EA, Jekanowski J, Isner JM. Variable expression of the estrogen receptor in normal and atherosclerotic coronary arteries of premenopausal women. Circulation 1994;89:1501–10. 14. Dart AM, Lacombe F, Yeoh JK, Cameron J, Jennings G, Laufer E. Aortic distensibility in patients with isolated hypercholesterolaemia, coronary artery disease, or cardiac transplant. Lancet 1991;338:270 –3. 15. Hirai T, Sasayama S, Kawasaki T, Yagi S. Stiffness of systemic arteries in patients with myocardial infarction. Circulation 1989;80:78 – 86. 16. Lehmann E. Pulse wave velocity as a marker of vascular disease. Lancet 1996;348:744. 17. Mohiaddin RH, Underwood SR, Borgren HG, et al. Regional aortic compliance studied by magnetic resonance imaging: the effects of age, training, and coronary artery disease. Br Heart J 1989;62:90 – 6. 18. Stefanidis C, Stratos C, Boudolas H, Kouroukalis C, Toutouzas P. Distensibility of the ascending aorta: comparison of invasive and non-invasive techniques in healthy men and men with coronary artery disease. Eur Heart J 1989;11:990 – 6. 19. Dart AM, Cameron JD. Effects of coronary risk factors on aortic and arterial properties in man. In: Boudoulas H, Toutouzas PK, Wooley CF, editors. Functional Abnormalities of the Aorta. New York: Futura, 1996:353– 64. 20. Kupari M, Hekali P, Keto P, Poutanen VP, Tikkanen MJ, StanderskjoldNordenstam CG. Relation of aortic stiffness to factors modifying the risk of atherosclerosis in healthy people. Arterioscler Thromb 1994;14:386 –94. 21. Lehmann ED, Watts GF, Fatemi-Laground B, Gosling RG. Aortic compliance in young patients with heterozygous familial hypercholesterolaemia. Clin Sci 1992;83:717–21. 22. Cameron JD, Jennings GL, Dart AM. The relationship between arterial compliance, age, blood pressure and serum lipids. J Hypertens 1995;13: 1718 –23.

355

23. Hordes RJ, Lakatta EG, McNeil TC. Another modifiable risk factor for cardiovascular disease? Some evidence points to arterial stiffness. J Am Geriatr Soc 1995;43:581–2. 24. Madhavan S, Ooi WL, Cohen H, Alderman MH. Relation of pulse pressure and blood pressure reduction to the incidence of myocardial infarction. Hypertension 1994;3:395– 401. 25. Laogun AA, Gosling RG. In vivo arterial compliance in man. Clin Phys Physiol Meas 1982;3:201–12. 26. Karpanou EA, Vyssoulis GP, Papakyriakou SA, Toutouza MG, Toutouzas PK. Effects of menopause on aortic root function in hypertensive women. J Am Coll Cardiol 1996;28:1562– 6. 27. Watanabe H, Ohtsuka S, Kakihana M, Sugishita Y. Coronary circulation in dogs with an experimental decrease in aortic compliance. J Am Coll Cardiol 1993;21:1497–506. 28. Kass DA, Sacki A, Tunin RS, Recchia FA. Adverse influence of systemic vascular stiffening on cardiac dysfunction and adaptation to acute coronary occlusion. Circulation 1996;93:1533– 41. 29. Tsiamis E, Stefanadis C, Stratos C, et al. The effect of estrogen on the elastic properties of the aorta in post-menopausal women [abstract]. Circulation 1995;92 Suppl I:I-2615. 30. Lehmann ED, Hopkins KD, Parker JR, et al. Aortic distensibility in post-menopausal women receiving Tibolone. Br J Radiol 1994;76:701–5. 31. Liu Z, Brin KP, Yin F. Estimation of arterial compliance: an improved method and evaluation of current methods. Am J Physiol 1986;251:688 –700. 32. Cameron JD, Dart AM. Exercise training increases total systemic arterial compliance in humans. Am J Physiol 1994;266:H693–701. 33. Kingwell BA, Cameron J, Gillies KJ, Jennings GL, Dart AM. Arterial compliance may influence baroreflex function in athletes and hypertensives. Am J Physiol 1995;268:H411– 8. 34. Hattle L, Angleson B. Doppler Ultrasound in Cardiology, 2nd ed. Philadelphia: Lea & Febiger, 1985:32– 62. 35. Chen C-H, Ting C-T, Nussbacher A, et al. Validation of carotid artery tonometry as a means of estimating augmentation index of ascending aorta pressure. Hypertension 1996;27:168 –75. 36. Vaitkevicius PV, Fleg JL, Engel JH, et al. Effects of age and aerobic capacity on arterial stiffness in healthy adults. Circulation 1993;88:1456 – 62. 37. Nichols WW, O’Rourke MF, editors. McDonald’s Blood Flow in Arteries, Theoretical, Experimental, and Clinical Principles. London: Edward Arnold, 1990;398 – 420. 38. Dart AM, Xiu LQ. Determinants of arterial stiffness in Chinese migrants to Australia. Atherosclerosis 1995;117:263–72. 39. Lieberman EH, Gerhard MD, Uehata A, et al. Estrogen improves endothelium-dependent, flow-mediated vasodilation in postmenopausal women. Ann Intern Med 1994;121:936 – 41. 40. Gilligan DM, Badar DM, Panza JA, Quyyumi AA, Cannon RO. Effects of estrogen replacement therapy on peripheral vasomotor function in postmenopausal women. Am J Cardiol 1995;75:264 – 8. 41. Sudhir K, Jennings GL, Funder JW, Komesaroff PA. Estrogen enhances basal nitric oxide release in the forearm vasculature in perimenopausal women. Hypertension 1996;28:330 – 4. 42. Wellman GC, Brayden JE, Nelson MT. A proposed mechanism for the cardioprotective effect of oestrogen in women: enhanced endothelial nitric oxide release decreases coronary artery reactivity. Clin Exp Pharm Physiol 1996;23:260 – 6. 43. Stefanadis CI, Karayannacos PE, Boudoulas HK, et al. Medial necrosis and acute alterations in aortic distensibility following removal of vasa vasorum of canine ascending aorta. Cardiovasc Res 1993;27:951– 6. 44. Fischer G, Swain M. Effect of sex hormones on blood pressure and vascular connective tissue in castrated and noncastrated male rats. Am J Physiol 1977;232:H617–21. 45. Vargas R, Wroblewska B, Rego A, Hatch J, Ramwell PW. Oestradiol inhibits smooth muscle cell proliferation of pig coronary arteries. Br J Pharmacol 1993;109:612–7. 46. Nichols NR, Olsson CA, Funder JW. Steroid effects on protein synthesis in cultured smooth muscle cells from rat aorta. Endocrinology 1983;113:1096 – 101. 47. Benetos A, Laurent S, Hoeks AP, Boutouyrie PH, Safar ME. Arterial alterations with aging and high blood pressure: a noninvasive study of carotid and femoral arteries. Arterioscl Thromb 1993;13:90 –7.

356


48. Kelly RP, Tunin R, Kass DA. Effect of reduced aortic compliance on cardiac efficiency and contractile function if in situ canine left ventricle. Circ Res 1992;71:490 –502. 49. Dart AM, Qi XL, Cameron JD. In vivo determinants of arterial stiffness. Atherosclerosis 1996;125:145– 8. 50. Gangar KF, Vyas S, Whitehead M, Crook D, Meire H, Campbell S. Pulsatility index in internal carotid artery in relation to transdermal oestradiol and time since menopause. Lancet 1991;338:839 – 42.


51. Penotti M, Nencioni T, Gabrielli L, Farina M, Castiglioni E, Polvani F. Blood flow variations in internal carotid and middle cerebral arteries induced by postmenopausal hormone replacement therapy. Am J Ob Gynecol 1993;169:1226 –32. 52. Voutilainen S, Hippelainen M, Hulkko S, Karppinen K, Ventila M, Kupari M. Left ventricular diastolic function by Doppler echocardiography in relation to hormone replacement in healthy postmenopausal women. Am J Cardiol 1993;71:614 –7.