12 
CARDIAC MODELS 
ture (8 to 24 years). The animals receiving the 
control diets without added cholesterol had 
somewhat higher serum cholesterol concentra- 
tions when the dietary fat was saturated (coco- 
nut oil) than when unsaturated (corn oil). 
When cholesterol was added to the diet, those 
receiving coconut fat had a more pronounced 
hypercholesterolemia. These authors found a 
high correlation between the mean serum cho- 
lesterol concentration and the amount of aortic 
fatty streaking (r = 0.70). Serum cholesterol 
concentration correlated to a lesser degree with 
coronary artery lesions and not at all with cere- 
bral artery lesions. Microscopically, the lesions 
observed in chimpanzees fed the atherogenic 
diet bore a striking resemblance to early human 
atherosclerosis, particularly in the cerebral ar- 
teries. By comparison with other nonhuman 
primate models of atherosclerosis, however, 
these animals are expensive, not plentiful, and 
their advantage over other nonhuman primates 
in atherosclerosis research has not been estab- 
lished. 
BABOON (Papio species) 
Several investigators have studied naturally 
occurring atherosclerosis of baboons.^" The 
study by McGill et al.^^ is the most complete of 
these reports, being based on autopsy studies of 
163 baboons trapped in their natural habitat in 
Kenya, British East Africa. In that study, ap- 
proximately three-fourths of the adult animals 
had some degree of aortic fatty streaking after 
gross staining with Sudan IV; however, no le- 
sions were found in the coronary arteries that 
resembled early human coronary artery ather- 
osclerosis. Coronary artery intimal lipid accu- 
mulation was seen rarely and was not asso- 
ciated with significant reduction in lumen size. 
Lipid metabolism of baboons has been studied 
extensively. A review of some of the current re- 
search in this area has been presented by How- 
ard and Kritchevsky.^^ Of particular interest is 
the fact that plasma cholesterol concentration 
decreases with age in baboons; whereas, in- 
creases with age are seen in most other animals. 
To better evaluate baboons as animal models 
Strong et al.^^ ^ged a factorial design with two 
levels of added cholesterol (0.5 and 0.01%) fed 
with diets either high or low in protein and con- 
taining either a saturated or unsaturated fat. 
They concluded that only the level of dietary 
cholesterol affected the serum cholesterol con- 
centration and this change was extraordinarily 
small. When the highest level of cholesterol was 
fed (0,5% ) , the mean increase for 12 months of 
study was 12 to 18 mg%. The authors concluded 
"the magnitude of the increase suggests that 
the baboon, like man, is only moderately suscep- 
tible to dietary cholesterol as measured by 
serum cholesterol changes." Somewhat higher 
serum cholesterol concentrations have been re- 
ported by Gresham and Howard for baboons 
fed high butter and egg yolk diets.^^ 
The effect of age on cholesterol metabolism of 
baboons has been studied by Eggen and 
Strong. 1* Young and old male baboons were 
used in this study. The fractional contribution 
of dietary cholesterol was greater in young than 
in old baboons. The most striking age difference 
concerned the suppression of endogenous cho- 
lesterol synthesis by dietary cholesterol with 
young baboons having a greater suppression 
than old animals. 
In response to cholesterol-containing diets, 
baboons developed fatty streaks in the aortic in- 
tima, although these lesions were usually 
small. Coronary artery atherosclerosis is al- 
most never seen even after prolonged periods of 
feeding dietary cholesterol. It seems unlikely 
that baboons will become an important primate 
model for studies on lesions. However, baboons 
will probably be of particular advantage as ani- 
mal models for the study of whole body choles- 
terol metabolism since their ability to absorb 
cholesterol is about the same as that of man. 
RHESUS MONKEYS (Macaca mulatta) 
Rhesus monkeys are important animal mod- 
els of atherosclerosis, particularly for studies 
on lesions because of the severe and extensive 
atherosclerosis they develop when fed diets con- 
taining added cholesterol. We find that the ex- 
tent and complexity of diet-induced atheroscle- 
rosis in this species exceeds that of any other 
species of nonhuman primates that we have 
studied so far and, additionally, one of few in 
which myocardial infarction occurs with rea- 
sonable frequency. They not only develop exten- 
