Monkey studies offer clues to heart disease

Studies of monkeys are helping researchers discover hidden mechanisms at the core of women?s risk for heart disease.

By Beth Azar
Monitor staff

The 75 female monkeys in the experiment appeared to react identically when put alone into a small empty room for half an hour. Each monkey initially froze in place?wary of the new environment?but soon relaxed and began to explore the confines of the room, a toy on the floor or an object flashed on a video screen placed at the back of the room.

Hidden from the passive observer, however, were some rather dramatic differences between the monkeys. Readings from the heart-rate jackets that each monkey wore indicated that some of the monkeys showed extreme heart rate increases upon entering the new environment and a precipitous drop as they grew comfortable and began to move about. Others had a healthy heart profile, with a lower peak heart rate and less change from beginning to end.

In addition, monkeys with the healthy profiles were those who held a dominant rank in their home troupe while the ones with the sensitive hearts were more submissive, holding socially subordinate ranks. That was a significant finding since research shows that subordinate female monkeys are at increased risk for atherosclerosis?the buildup of fatty plaques in arteries that eventually leads to heart disease. Atherosclerosis is the leading cause of heart disease in humans.

This study is part of an experimental program run by psychologist Carol Shively, PhD, at the Wake Forest School of Medicine in Winston?Salem, N.C., that is designed to examine connections between behavior, social stress and health in women. The monkeys represent a model for premenopausal women?a group largely ignored in heart disease research because they?ve been assumed to be protected from cardiac damage by high levels of the sex hormone estrogen.

But research by Shively and her Wake Forest colleague Jay Kaplan, PhD, funded in part by the National Heart, Lung and Blood Institute, indicates complacency may not be warranted. They are finding subtle physiological differences, such as abnormal heart fluctuations and irregular ovarian function, that can easily go unnoticed but that put female monkeys on a trajectory toward heart disease. And these same mechanisms may well be in effect in women, representing hidden risks for developing heart disease.

Stressing the ovaries

Kaplan does not dispute that premenopausal women are far more resistant to developing atherosclerosis than are men of the same age. He finds the same thing in his premenopausal monkeys. (Of course, in the wild neither male nor female monkeys develop atherosclerosis because they eat an extremely low-fat diet. The Wake Forest researchers control for this by feeding their monkeys a diet designed to mimic that of the average American. 'Our kitchen smells like McDonalds,' says Kaplan.)

But although the average female monkey is protected from atherosclerosis, those under stress lose their natural protection?developing a rate of atherosclerosis equal to that of male monkeys.

Kaplan and Shively use the monkeys? natural process of social hierarchy as a model of social stress. Female cynomolgus macaques normally live in large hierarchical groups. So when researchers house two or more monkeys together, a natural pecking order quickly develops. Some monkeys become dominant over the others?being first to gain access to food and sometimes bullying the subordinate monkeys. In the wild, this social hierarchy is likely not stressful to the animals, because subordinates can escape from dominant females, says Kaplan, an anthropologist by training. But in captivity, where there is no place to hide, being socially subordinate can cause stress.

It also interferes with monkeys? menstrual cycles, Kaplan finds. Subordinate females have fewer ovulatory cycles and tend to show abnormalities in the luteal phase of their cycles?the phase just after ovulation, when estrogen and progesterone levels are high. If Kaplan gives subordinate monkeys estrogen, he can subvert the increased risk of atherosclerosis, indicating that the modified ovarian function may be responsible, at least in part, for the build up of arterial plaques.

He and his colleagues are now testing whether soy protein?which contains naturally occurring phytoestrogens?offers a safe way of providing protection without the risks associated with other forms of estrogen. Preliminary studies show that phyto-estrogens act on heart tissue, thereby preventing heart disease, while avoiding breast and uterine tissue, thereby reducing the risk of cancer found with other types of estrogens.

The link between ovarian function and atherosclerosis may be significant for a larger proportion of women than most would think, says Kaplan. Although only 5 percent to 15 percent of women show clinical signs of ovarian dysfunction, as many as 30 percent of women have what?s considered 'normal' fluctuations in the luteal phase of their menstrual cycle. And even these subclinical impairments are associated with increased risk of atherosclerosis in female monkeys, he says.

Hearts a-flutter

The monkey research offers hints that premenopausal women may have several 'hidden' factors at work behind the scenes that put them at risk of atherosclerosis, says psychologist Steve Manuck, PhD, who collaborates with Kaplan. As Kaplan finds, the chronic stress of social subordination can cause ovarian deficiency in female monkeys. But it may also heighten an animal?s reaction to acute stress, and Manuck and Kaplan find that animals that react the strongest to stress are the most likely to develop atherosclerosis.

Shively has the first evidence that, in fact, subordinate female monkeys do react differently to new experiences than dominant females. She used a version of the standard 'open field' test traditionally used with mice and rats to measure emotionality. Each monkey?outfitted in a fancy heart-rate jacket?had half an hour to explore an abandoned room as researchers watched their heart rate and videotaped their every move. And, although subordinate and dominant monkeys looked the same, their heart profiles were vastly different. The subordinate animals showed dramatic fluctuations in heart rate?a profile known to damage artery walls, which may be a precursor to atherosclerosis, says Shively.

'If you ignored behavior in these monkeys and just looked at the physiological data you would say ?Wow, they are much more afraid and aroused,?' says Shively of her study, which was published last month in Biological Psychiatry (Vol. 44, p. 88?97).

Interestingly, in a subgroup of monkeys taking oral contraceptives, the heart-rate differences between dominant and subordinate animals disappeared?the dominant animals took on the same oscillating heart profile as the subordinate animals.

'The oral contraceptives were obliterating what was a good cardiovascular profile,' says Shively.

What this all means in terms of risk for women is still unclear, but it?s obvious to Shively that the link between behavior, environment, hormones and heart disease is extremely complex and that premenopausal women may have more hidden risks for heart disease than they think.

Social-status incongruity

Complicating matters is a finding by Shively that landing in a social status that doesn?t match one?s natural or historical social status may also put female monkeys at risk. Such 'social-status incongruity' occurs in humans when people lose their jobs and suddenly find themselves plummeting from a relatively high socioeconomic status to near poverty.

To mimic this in monkeys, Shively artificially manipulated social status by housing naturally subordinate animals together and naturally dominant animals together. This forced some normally subordinate animals into a dominant position and some normally dominant animals into a subordinate role.

The animals that switched status?taking on an unfamiliar social role?had increased risk of atherosclerosis, Shively found. But those who stayed the same status had no increased risk, including subordinates who stayed subordinate. These findings suggest that there is a complex interaction between inherent characteristics that result in naturally occurring social status, be it subordinate or dominant, and environmental factors that may or may not stress the animal.

This finding meshes with the social psychology literature on person-environment incongruity, also called social-status incongruity, says Shively. This literature finds that people who are in incongruous environments or social situations have worse health outcomes.

Solving the human dilemma

Finding connections between stress, behavior and the human condition is what drew both Shively and Kaplan to the study of heart disease in monkeys. Kaplan had a broad interest in populations under stress and the effect it has on physiology and behavior. And Shively is particularly interested in the link between social stress and disease susceptibility.

'Most of us are looking for answers to the human dilemma,' says Shively.

And they?re learning that they have to look carefully because those answers might be masked behind profiles that appear normal, as with Shively?s monkeys who behaved like ordinary monkeys but who harbored a hidden clue.