Articles

Dominant Social Status and Contraceptive Hormone Treatment Inhibit Atherogenesis in Premenopausal Monkeys

Jay R. Kaplan, Michael R. Adams, Mary S. Anthony, Timothy M. Morgan, Stephen B. Manuck, Thomas B. Clarkson
https://doi.org/10.1161/01.ATV.15.12.2094
Arteriosclerosis, Thrombosis, and Vascular Biology. 1995;15:2094-2100
Originally published December 1, 1995

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Abstract

Abstract The stress of social subordination is associated with exacerbation of coronary artery atherosclerosis in premenopausal cynomolgus monkeys, possibly as a result of the ovarian dysfunction that reliably accompanies subordinate social status. The primary objective of the current study was to determine whether treatment with an oral contraceptive (OC) provides relative protection from development of atherosclerotic plaques, especially among animals made vulnerable to atherosclerosis by social subordination. In the present study, 193 adult female monkeys (Macaca fascicularis) were placed in social groups of 5 or 6 animals each. Half of the animals were then fed an atherogenic diet to which had been added a triphasic OC, while the remainder received only the atherogenic diet. At the end of 26 months, atherosclerosis was measured in an iliac artery biopsy taken from each monkey. The results demonstrated that among untreated animals subordinate individuals developed significantly more atherosclerosis than did their dominant counterparts (P<.01); however, OC treatment inhibited atherosclerosis in subordinate animals (P<.05) and eliminated the difference between dominant and subordinate animals that was observed in the untreated condition. Subordinate social status and OC treatment were both associated with reduced plasma concentrations of HDL cholesterol (P<.01 for both), and subordinate monkeys also had elevations in LDL cholesterol plus VLDL cholesterol (P<.01). Nonetheless, the interaction between social status and OC treatment remained significant even after covariance adjustment for variation in plasma lipids. Taken together, these results suggest that social subordination worsens, whereas OC treatment inhibits, atherosclerosis, and that these effects are independent of concomitant variability in plasma lipids.

  • Reprint requests to Jay R. Kaplan, PhD, Department of Comparative Medicine, Bowman Gray School of Medicine of Wake Forest University, Medical Center Blvd, Winston-Salem, NC 27157-1040. E-mail jkaplan@cpm.bgsm.wfu.edu.

  • Received April 4, 1995.
  • Accepted August 16, 1995.

Considerable evidence supports the hypothesis that ovarian hormones suppress the development of atherosclerosis and CHD in women. For example, surgical or natural menopause is associated with an increased incidence of CHD, an outcome that can be reduced significantly by hormone replacement therapy.1 2 Similarly, investigations using nonhuman primates have shown that ovariectomy (surgical menopause) exacerbates coronary artery atherosclerosis, whereas parenteral estrogen replacement, with or without progesterone, inhibits it.3 4 If ovarian hormones are indeed cardioprotective, then sustained or extreme interruptions of normal ovarian function, which may occur in up to 18% of women less than 40 years old,5 6 7 8 could also place affected premenopausal individuals at risk for developing premature atherosclerosis and CHD. For example, one study links a lifelong history of menstrual irregularity in women to an increased incidence of acute myocardial infarction.9 Furthermore, investigations with premenopausal monkeys have established that the stress associated with chronically low social status (subordination to dominant animals within social groups) is associated with an increased frequency of anovulatory and luteal phase–deficient menstrual cycles10 11 and with exacerbation of coronary artery atherosclerosis.12 13 The current study was an intervention trial designed to explore the potential public health implications of the foregoing association among psychosocial stress, ovarian hormones, and atherosclerosis. The three major objectives of this investigation were to (1) confirm that low social status enhances the vulnerability of female monkeys to atherosclerosis; (2) evaluate the hypothesis that contraceptive hormone treatment provides relative protection from development of atherosclerosis, especially in those animals at increased risk of ovarian impairment by subordinate social status; and (3) determine the effects on plasma lipids of a contraceptive regimen containing low doses of ethinyl estradiol and levonorgestrel and to establish whether any such effects influenced atherogenesis.

Methods

Animals

The study animals were 213 female cynomolgus monkeys (Macaca fascicularis) imported from Indonesia as fully mature, premenopausal adults (average age, 5.7 years, estimated from dentition). Premenopausal females of this species are susceptible to diet-induced atherosclerosis and have previously proven useful in studies of behavioral and hormonal influences on coronary artery atherosclerosis.14

Experimental Design and Procedures

All animals were housed in single cages for 6 weeks of a 3-month quarantine period and then placed in social groups of 5 or 6 animals each, with all groups housed in pens measuring approximately 2×3×3 m3 with outdoor exposure. The monkeys were fed a commercial chow diet (Purina 5045, Ralston Purina) throughout the quarantine and pre-experimental periods. The animals were assigned by social group to either an OC or control condition. A blocked randomization procedure with two groups per block was used to ensure equivalence in the numbers, characteristics, and experimental timing of animals assigned to the two conditions. Experimenters as well as study technicians were blinded to the treatment assignments. After their entry into the experiment, the monkeys consumed a moderately atherogenic diet containing 43% of calories from fat and 0.28 mg cholesterol/kcal (127 mg cholesterol/100 g dry weight constituents) (Table 1⇓). Animals were fed twice daily (at 7 am and 2 pm) and received an amount of food sufficient to provide 120 kcal·kg body wt−1·d−1. The duration of this part of the experiment was 26 months, at the end of which time a biopsy sample was removed from the right iliac artery of each monkey for determination of atherosclerosis. Atherosclerosis measured in this sample was used as the primary dependent measure in the current investigation. All monkeys exposed to the biopsy procedure also underwent bilateral oophorectomy. These animals were then assigned to a second experiment designed to evaluate the effects of postmenopausal treatments on coronary artery atherosclerosis.

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Table 1.

Experimental Diet

There were 213 animals in the study when it began; data from 193 monkeys are reported here. Twenty monkeys died, representing a mortality rate of 3.7% per annum of experimental exposure. Notably, 16 of the deaths (generally of monkeys that had low body weight and enteric disorders) occurred among animals treated with an OC, and the remaining 4 were in monkeys from the control group (χ2=5.91, P<.02). There is no ready explanation for the disparate number of deaths in the two groups.

All procedures involving animals were conducted in compliance with state and federal laws, standards of the US Department of Health and Human Services, and guidelines established by our Institutional Animal Care and Use Committee. For routine sample collection, animals were anesthetized with ketamine hydrochloride (15 mg/kg) administered intramuscularly. Ketamine hydrochloride (15 mg/kg) and butorphanol (0.025 mg/kg) were used for surgical anesthesia when ovariectomies were done.

Pharmacological Manipulation

The OC chosen for use in this experiment contained a combination of ethinyl estradiol and levonorgestrel (Triphasil, Wyeth-Ayerst). The drug was finely ground and administered in the diet on a 28-day cycle. Different proportions of estrogen and progestin were provided for the initial 21 days of each cycle, and a placebo was given for the final 7 days (Table 2⇓). As shown in Table 2⇓, the OC was included in the diet on a caloric rather than a body weight basis. This was done because of the relatively high metabolic rate of cynomolgus monkeys. Females of this species have a caloric intake nearly one fifth that of human females despite weighing only one twentieth as much. The drug was administered throughout the 26-month period of the experiment. To determine the effectiveness of hormone delivery, plasma ethinyl estradiol and levonorgestrel were sampled 10 times over the course of the study, on day 21 of the pill cycle. These samples revealed that blood concentrations averaged 60.7 pg/mL ethinyl estradiol and 1.27 ng/mL levonorgestrel, comparable to those observed in women using the same compound. Animals in the untreated group received only the atherogenic diet, were fed at the same time of day, and lived in adjacent pens.

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Table 2.

Administration Schedule of Triphasil for 28 Days

Determination of Social Status

We have reported previously that the stress of social subordination among premenopausal monkeys fed an atherogenic diet results in ovarian dysfunction, estrogen deficiency, a reduced ratio of TPC to HDLC, and exacerbated coronary artery atherosclerosis.10 11 12 13 14 For cynomolgus monkeys, a series of specific facial expressions, postures, and vocalizations indicate the occurrence of a fight, and an animal’s relative social status may be based on these. Typically, one animal in a fight signals aggression and the other signals submission, particularly in stable social groupings.15 This highly asymmetric pattern allows fight outcomes to be judged in terms of clear winners and losers.16 17 The female in each group that defeats all others (as evidenced by her ability to elicit consistently submissive responses) is designated as the first-ranking monkey. The female that defeats all but the first-ranking monkey is designated as the second-ranking monkey, and so forth. In general, dominance relationships within small groups are transitive; that is, if monkey 1 is dominant to monkey 2 and monkey 2 is dominant to monkey 3, then monkey 1 is usually dominant to monkey 3 also.13 Dominance was determined weekly for all groups on the basis of data collected by technicians who recorded all fights observed during a 30-minute period. Social status was stable during both the experimental and pre-experimental periods of the present study. The average correlation between ranks measured in consecutive months was greater than .90, whereas ranks aggregated over the entire experimental period correlated with those determined pre-experimentally at r=.96. The aggregated experimental period rankings were used in all analyses, with animals above the median considered dominant and those below the median subordinate. That is, overall median rank was used as the cutpoint for determining dominant versus subordinate status.

Clinical Evaluations

During the pretreatment and treatment periods, several determinations of plasma lipids and body weight were made. All sample collections were made after an 18-hour fast while the animals were anesthetized. TPC and plasma HDLC concentrations were determined once pre-experimentally and nine times during the study by use of previously published methods.18 19 LDLC and VLDLC were calculated together as the difference between TPC and HDLC. Blood pressure was measured indirectly with a Dinamap portable adult/pediatric and neonatal vital signs monitor (model 8100), which uses an oscillometric technique. Measurements were taken once before the experiment and three times during the study. Previous work at our center has shown that indirect measurements of blood pressure recorded in animals under ketamine anesthesia correlate well with blood pressure measurements obtained from the same animals when they are fully conscious.20 Animals were weighed every other month during both the pre-experimental and experimental phases of the study. Finally, blood samples collected from a small subset of untreated animals (n=22) three times per week for 10 weeks were evaluated for plasma progesterone concentrations.10 The peak progesterone concentrations from the luteal phase of each cycle were identified; these peaks were averaged to provide a mean progesterone value for each monkey. The data then were assessed with respect to the dominance status of the animals. All hormone assays were done at Dr Mark Wilson’s Comparative Endocrinology Laboratory of the Yerkes Regional Primate Center (Atlanta, Ga).

Iliac Artery as a ‘Surrogate’ for the Coronary Artery

The study animals were not necropsied at the termination of the current experiment to allow their use in a subsequent trial investigating postmenopausal as well as premenopausal influences on atherosclerosis. Because biopsy of the coronary arteries was not feasible, an alternative artery was chosen to provide information on the atherogenicity of the premenopausal treatments. Ideally, such an artery would react to an atherogenic stimulus in a manner similar to that of the coronary arteries. We have termed these alternative arteries “surrogate” or indicator arteries. We considered the common carotid and iliac arteries for this purpose, because removal of either does not affect tissue perfusion distally. A series of comparisons in which data from 116 reproductively intact and ovariectomized female monkeys were used revealed that the average correlation between atherosclerosis extent in the common iliac artery and the coronary arteries (mean extent in left circumflex, left anterior descending, and right arteries) was .71. In the same sample, the average correlation between atherosclerosis in the common carotid and coronary arteries was .60. On the basis of these data, we concluded that the common iliac artery was a reasonable surrogate artery for the determination of atherosclerosis.

Determination of Atherosclerosis

To obtain biopsy material, we first anesthetized the animals with ketamine and butorphanol as described above. Then a longitudinal incision was made, the viscera were retracted to expose the aorta, and a 3-cm section of the left common iliac artery was removed after ligation distally and proximally. The section was perfusion fixed with 10% neutral buffered formalin at a pressure of 100 mm Hg for 10 minutes. The iliac artery section was then placed in a vial of 10% neutral buffered formalin with a suture indicating the distal end. To evaluate the extent of iliac atherosclerosis, we cut five standard blocks (each approximately 3 mm in length) perpendicularly to the long axis of the artery. The tissue blocks were dehydrated by increasing concentrations of ethanol and embedded in paraffin. Two 5-μm sections were cut from each block, and one was stained with Verhoeff–van Gieson’s stain and the other with hematoxylin and eosin. We determined intimal area from the Verhoeff–van Gieson–stained sections by digitizing the area between the internal elastic lamina and the luminal surface of the artery. The mean of the five sections was used to represent each animal.

Statistical Analysis

To reduce skewness and equalize group variances, all atherosclerosis data underwent square root transformation of the form X′= 214 Math before analysis.21 ANOVA and ANCOVA were used for initial statistical analysis. Where appropriate, a priori comparisons by t tests22 were used to test the significance of hypothesized group contrasts.

Results

Iliac Artery Atherosclerosis

An approximately equivalent number of animals could be categorized as either dominant or subordinate in each experimental condition. The mean intimal areas of dominant and subordinate animals in the treated and untreated conditions are depicted in Fig 1⇓. Actual values, standard errors, and the numbers of animals in each condition are included in Table 3⇓. For determination of whether OC treatment or social status influenced extent of iliac artery atherosclerosis, the intimal area measurements were subjected to a 2×2 (experimental conditionuntreated,treated×statussubordinate,dominant) ANOVA. The ANOVA showed a main effect for status (F1,189=5.57, P<.02), as well as a status×condition interaction (F1,189=6.18, P=.01). Because of these effects, t tests were used for evaluation of preselected orthogonal contrasts between groups. The particular contrasts chosen reflected three predictions: (1) untreated dominant monkeys would be protected compared with subordinates; (2) OC-treated subordinates would be protected compared with subordinate monkeys in the untreated condition; and (3) atherosclerosis in OC-treated dominants and subordinates would not differ. These predictions were all substantiated. The first contrast revealed that untreated dominant monkeys had significantly less atherosclerosis than did their untreated subordinate counterparts (P<.01). The second comparison demonstrated that treated subordinates had significantly less atherosclerosis than did untreated subordinate monkeys (P<.05). The third comparison demonstrated that there was no difference in atherosclerosis between dominant and subordinate animals in the treated condition (P=NS).

Figure 1.
Figure 1.

Bar graph shows iliac artery intimal area (IA, in mm2) by condition and status. The significant main effect for status (P=.02) and the condition×status interaction (P=.01) were supplemented by post hoc contrasts showing that dominant and subordinate monkeys differed significantly in the untreated but not the treated condition and that the treated animals, regardless of status, differed from untreated subordinates.

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Table 3.

Iliac Artery Atherosclerosis Extent in Dominant and Subordinate Monkeys by Experimental Group

For illustrative purposes, we plotted the mean atherosclerosis score (in mm2) for animals grouped on the basis of average rank into one of five categories, from high to low (Fig 2⇓). This figure demonstrates that the relationship we have already reported for dominant and subordinate animals in the control condition appears to be linear across the range of social status. In contrast, there is no apparent association between social rank and atherosclerosis among OC-treated animals.

Figure 2.
Figure 2.

Graph shows iliac artery intimal area (IA, in mm2) plotted against the average social rank of animals in each of the treatment conditions (control, Triphasil). Average rank is aggregated into five categories from high (1) to low (5).

Clinical Observations

Parallel to the atherosclerosis data, the plasma lipid determinations obtained during the experiment were subjected to a series of 2×2 (conditionuntreated,treated×statussubordinate,dominant) ANOVAs. The relevant data are shown in Table 4⇓. The analysis for HDLC revealed main effects for both condition (F1,189=30.62, P<.01) and status (F1,189=5.61, P<.02), indicating that HDLC was suppressed in treated compared with untreated animals and in subordinates compared with dominants. The condition×status interaction approached significance (F1,189=3.44, P<.07). There was a significant status effect for LDLC+VLDLC (F1,189=8.95, P<.01), indicating that LDLC+VLDLC was lower, overall, in dominant monkeys than in subordinate monkeys. Neither the condition effect nor the condition×status interaction was significant for LDLC+VLDLC. Across all animals, both HDLC and LDLC+VLDLC were significantly associated with atherosclerosis extent (r=−.39 for HDLC and r=.51 for LDLC+VLDLC, P<.01 for both) as well as within treatment groups (treated, r=−.40 for HDLC and r=.51 for LDLC+VLDLC; untreated, r=−.47 for HDLC and r=.53 for LDLC+VLDLC; P<.01 for all).

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Table 4.

Plasma Lipids by Condition and Status

These data indicate that both OC treatment and status influenced the major lipid risk factors for atherosclerosis and that these factors were themselves associated significantly with atherosclerosis extent. Because the effects of OC treatment and status on atherosclerosis may have been mediated, in part, by plasma lipids, the atherosclerosis data were subjected to a 2×2 (conditionuntreated,treated×statussubordinate,dominant) ANCOVA, with HDLC and LDLC+VLDLC entered as covariates. This analysis yielded a significant condition effect (F1,187=4.24, P<.05) as well as the previously observed condition×status interaction (F1,187=4.25, P<.05) (Fig 2⇑). The latter outcome suggests that the interactive influence of social status and OC treatment on atherosclerosis could not be accounted for by concomitant variability in serum lipid concentrations among these experimental animals. The significant condition effect indicated that OC treatment is generally protective, but only after adjustment for the potential adverse effects of such treatment on plasma lipids (Fig 3⇓).

Figure 3.
Figure 3.

Bar graph shows iliac artery intimal area (IA, in mm2) by condition and status, adjusted for variability in plasma lipids. A significant condition×status interaction persisted (P<.05), indicating that the protection of untreated dominant and treated subordinate animals was largely independent of concomitant variability in plasma lipids.

Systolic and diastolic blood pressures were measured before and during the experiment (Table 5⇓). Conditionuntreated,treated×statussubordinate,dominant ANOVAs indicated that pretreatment systolic and diastolic blood pressure measurements were higher among dominant animals than among their subordinate counterparts (systolic, F1,189=10.38, P<.01; diastolic, F1,189=9.75, P<.01). There were no significant differences pre-experimentally by condition, nor was there a significant condition×status interaction. Accordingly, systolic and diastolic values obtained during the experiment were subjected to conditionuntreated,treated×statussubordinate,dominant ANCOVAs, with the pre-experimental values used as covariates. Adjustment for pre-experimental values completely eliminated all significant effects (adjusted values in Table 5⇓). Notably, none of these blood pressure measurements were correlated significantly with atherosclerosis (r<.10 for all, P=NS).

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Table 5.

Systolic and Diastolic Blood Pressure by Condition and Status

Body weight was also measured before and during the experiment. A conditionuntreated,treated×statussubordinate,dominant ANCOVA, with the pre-experimental values used as covariates, was applied to these data. This analysis revealed that no significant effects of either OC treatment or status were manifested during the experiment (control subordinate, 2.99±0.03 kg; control dominant, 3.00±0.02 kg; treated subordinate, 3.06±0.03 kg; treated dominant, 3.01±0.03 kg [mean±SEM]). However, a subsidiary analysis comparing pre-experimental and experimental body weights across groups indicated that animals, in general, gained weight (pre-experimental, 2.98±0.03 kg; experimental, 3.02±0.03 kg; P<.05).

Finally, peak luteal phase plasma progesterone concentrations were assessed in a subset of 22 animals from the untreated group. These data confirmed that, as in previous investigations, there was a significant inverse relationship in this experiment between ovarian function and social status. Specifically, mean peak progesterone concentrations averaged 10.36±1.28 ng/mL in 11 dominant monkeys and 6.05±1.46 ng/mL in 11 subordinate monkeys (t21=2.22, P<.04).

Discussion

Female cynomolgus macaques fed atherogenic diets have been shown to be suitable models for the development of coronary artery atherosclerosis in terms of male/female differences, reproductive hormone influences, and behavioral effects.3 4 The present study extends earlier results by demonstrating that social factors and OC treatment interact to influence atherogenesis in premenopausal animals. Specifically, the stress of low social status was associated with worsened atherosclerosis among untreated monkeys; however, treatment with an OC inhibited the development of atherosclerosis in subordinate animals and eliminated these differences between dominant and subordinate monkeys. Thus, equivalent protection from atherosclerosis was observed among untreated dominant females and all treated animals. Notably, atherosclerotic plaque development was inhibited in treated monkeys despite the profound reduction in HDLC that accompanied OC treatment. To the extent that these results represent concomitant events in women, they highlight the potential importance of behavioral stressors in the development of atherosclerosis during the premenopausal period. They also illuminate the possible antiatherogenic effects of OC treatment among premenopausal women with estrogen deficiency.

Premenopausal women usually are considered relatively protected from atherosclerosis and CHD.23 However, this protection diminishes after menopause, as the incidence of CHD in women ultimately becomes equivalent to that observed in men.24 The exacerbation of atherosclerosis observed in the present study among untreated, socially subordinate monkeys suggests that there also may be a subset of premenopausal women who, perhaps because of behavioral factors, experience an accelerated development of atherosclerosis. The existence of such a subset would help explain the occurrence of CHD among some premenopausal women, as well as the apparently rapid onset of CHD during the postmenopausal period. Consistent with this suggestion are the results of numerous studies in women relating an increase in CHD risk to low-status occupations (eg, clerical workers and video-display terminal operators) and an inability to express or discuss anger.25 The behavioral situation of such women may be characterized as a form of stress involving low control, high demands, and suppressed emotions.25 Significantly, low social status in confined monkeys also is often considered stressful to the animals, in part because it is accompanied by relative social withdrawal and isolation and a generally reduced freedom of movement and expression.13 26 27

The mechanism or mechanisms mediating an association between social stress and increased risk of CHD (in women) or atherosclerosis (in premenopausal monkeys) remain unknown. However, in previous investigations that focused on menstrual cyclicity in cynomolgus monkeys, we have observed that subordinate social status is reliably associated with a significant increase in anovulatory and luteal phase–deficient menstrual cycles.10 11 In addition, other investigators have noted reduced reproductive performance in subordinate monkeys.28 29 30 31 32 In the present study, data from a subset of untreated monkeys indicated that, as in earlier studies, subordinate monkeys experienced relative deficiencies in luteal phase progesterone activity. In turn, premenopausal monkeys with a high frequency of endocrinologically abnormal menstrual cycles develop significantly more coronary artery atherosclerosis than do monkeys with relatively normal ovarian endocrine function and as much coronary artery atherosclerosis as surgically ovariectomized animals.11 Hence, impaired ovarian function may have contributed to the atherosclerosis of untreated subordinate animals in the present study.

Consistent with the results of the present report, numerous studies of women have shown that emotional distress can result in ovarian dysfunction.33 34 35 36 37 38 39 40 Furthermore, one investigation has linked chronic menstrual irregularity to a significantly increased risk of premature CHD.9 Unfortunately, such data rarely are collected in conjunction with investigations of atherosclerosis and CHD in women. However, independently of such studies, it has been determined that estrogen deficiency associated with secondary amenorrhea is relatively common, affecting between 5% and 18% of women less than 40 years old.5 6 7 8 Up to half the incidence of premature ovarian failure is attributed to undetermined causes, which are often assumed to be of environmental or psychogenic origin (FHA).8 These data provide circumstantial evidence that a substantial subset of premenopausal women may experience prolonged periods of ovarian dysfunction, some of which is due to “stress” but all of which may contribute to atherogenesis. A related observation of potential importance is that, in the studies involving nonhuman primates, relatively modest impairment of ovarian function is associated with substantial exacerbation of atherosclerosis.11 It is reasonable to presume that such moderate ovarian abnormalities, in women, would be occult. If so, the percentage of premenopausal women at risk of accelerated atherosclerosis may be considerably larger than that indicated by the incidence of (diagnosed) FHA.11

It is perhaps worth noting that FHA in women is characterized by hypercortisolemia (presumably induced by stress-related release of corticotropin-releasing factor), further emphasizing the link between psychogenic stress and ovarian impairment.41 Women with FHA also exhibit increased cognitive dysfunction and psychiatric morbidity.42 Taken together, these findings indicate that FHA may be one of the manifestations of a multifaceted stress disorder. Subordinate social status may place female monkeys at high risk for an analogous stress disorder, because such animals tend to be hypercortisolemic10 27 and, as stated previously, often exhibit behavioral withdrawal and other signs of depression26 as well as ovarian impairment. Thus, although there may exist no precise human analogue for subordinate female monkeys, these animals appear to display behavioral and physiological characteristics similar to those observed in at least one subset of women, those with FHA.

In the present study, treatment with an OC resulted in a significant inhibition of atherogenesis. Such inhibition is consistent with the hypothesis that impaired ovarian function potentiated atherogenesis in the stressed subordinates; the outcome also could imply an offsetting beneficial effect of ovarian hormone treatment, regardless of the mechanism responsible for the worsened atherosclerosis of the subordinate monkeys. In the case of women, controversy exists regarding the effects of combination OC use on risk of CHD and atherosclerosis. For example, there is a well-known increase in CHD risk associated with current OC use.43 However, present evidence indicates that this increase in risk is largely confined to users of older, high-dose OC formulations who are also cigarette smokers.43 44 45 46 47 Furthermore, the fact that the increase in risk disappears after cessation of OC use suggests the involvement of a nonatherogenic mechanism, such as thrombosis or vasospasm. More importantly, some studies provide evidence of decreased risk for CHD among OC users,47 and at least one angiographic investigation has shown that OC users have less atherosclerosis than nonusers.48 This latter evidence, together with the results of the present study, suggest that combination OC use may be antiatherogenic. The results of the present study tempt speculation that this effect is manifested largely among those females made vulnerable to atherosclerosis by psychosocial stress or other contributors to ovarian impairment.

The substantial reduction in HDLC associated with combination OC use is often identified as a potential contributor to increased risk of CHD.1 3 The present study provides no evidence that an OC-induced reduction in HDLC, even if large, can offset the antiatherogenic effects of hormone treatment. Although not demonstrated in the present study, considerable evidence is provided by previous research that the estrogenic component of the hormone treatment probably exerts a beneficial effect by acting directly on the artery wall to prevent LDL uptake and maintain normal vasomotor activity.49 50 The finding that the conjoint effects of social status and OC treatment remained even after removal of plasma lipid effects by covariance analysis indicates that both the exacerbation of atherosclerosis among untreated subordinates and the inhibition of lesion development among the rest of the monkeys were independent of variability in plasma lipids.

A potential limiting factor of the current study was the reliance on iliac artery atherosclerosis as a surrogate for atherogenesis in the coronary arteries. However, studies in other animal models (eg, pigeons and rabbits) have relied almost exclusively on noncoronary preparations, whereas the carotid artery is commonly used as a surrogate for the coronary arteries in studies of humans.51 52 Indeed, our preliminary data indicated that in the cynomolgus monkey atherosclerosis in the iliac artery corresponds to that in the coronary arteries at least as well as does atherosclerosis in the carotid artery.

Despite the foregoing limitation, we believe the present study demonstrates clearly that the stress of social subordination exacerbates atherosclerosis in premenopausal monkeys, possibly through a mechanism involving impaired ovarian function. Notably, this exacerbation of atherosclerosis can be inhibited by dominant social status and either inhibited or offset by OC treatment. These results provide additional evidence that estrogen treatment may be antiatherogenic in women. Such evidence is particularly important in the absence of clinical trials investigating the effects of estrogen treatment on atherosclerosis and CHD.53

Selected Abbreviations and Acronyms

CHD=coronary heart disease
FHA=functional hypothalamic amenorrhea
HDLC=HDL cholesterol
LDL-C=LDL cholesterol
OC=oral contraceptive
TPC=total plasma cholesterol
VLDLC=VLDL cholesterol

Acknowledgments

This study was supported in part by grant 1P01-HL-45666, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md.

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  • Dominant Social Status and Contraceptive Hormone Treatment Inhibit Atherogenesis in Premenopausal Monkeys
    Jay R. Kaplan, Michael R. Adams, Mary S. Anthony, Timothy M. Morgan, Stephen B. Manuck and Thomas B. Clarkson
    Arteriosclerosis, Thrombosis, and Vascular Biology. 1995;15:2094-2100, originally published December 1, 1995
    https://doi.org/10.1161/01.ATV.15.12.2094
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