THOMAS B. CLARKSON 
13 
sive aortic atherosclerosis but extensive coro- 
nary, cerebral, renal and femoral artery 
atherosclerosis as well. The anatomic distribu- 
tion of the coronary artery lesions of these ani- 
mals is another of their advantages ; the lesions 
are more significant in the epicardially distribut- 
ed proximal branches of the coronary arteries 
than in the smaller intramyocardial arteries as is 
the case v^ith many of the other primate models. 
Naturally occurring atherosclerosis of rhesus 
monkeys is uncommon. Chawla et al.^® studied 
the prevalence of arterial lesions in 150 rhesus 
monkeys trapped in India. Among those ani- 
mals only four had fatty streaks of the aorta 
and one had atherosclerosis of the aorta. The in- 
frequent occurrence of atherosclerosis among 
rhesus monkeys fed control diets is in keeping 
with our own observations and those of others. 
A particularly exciting aspect of research on 
rhesus monkeys as animal models of atheroscle- 
rosis has been the finding of type II hy- 
perlipoproteinemia.^''!^ The first of these re- 
ports by Greenberg and Moon^^ was that of a 
single male rhesus monkey. While it appeared 
that the plasma lipid defect in that monkey was 
a primary disorder and did fit the type II phen- 
otype, the animal was apparently sterile and the 
familial character of the disease could not be 
proven. The report by Morris and Fitch^'^ 
leaves little doubt that the two animals studied 
by them had primary type II hyperlipopro- 
teinemia. These two animals had plasma choles- 
terol concentrations averaging 450 to 500 mg%, 
more than 90% of which was transported by 
low density lipoprotein, and were not hypertrig- 
lyceridemic. Studies are underway in their labo- 
ratory to establish if the defect observed in 
these two monkeys is familial. The intense need 
for more and better models of primary type II 
hyperlipoproteinemia suggests the importance 
of screening the large number of rhesus mon- 
keys being imported into this country yearly. 
There has been a considerable advancement 
in our understanding of lipid metabolism of 
rhesus monkeys during the past decade. Rhesus 
monkeys, like several other nonhuman pri- 
mates, vary in the concentration of free and es- 
terified cholesterol in the plasma not only from 
day to day but from week to week.20.21 Alexan- 
der and KopelolT22 measured the weekly varia- 
tion in plasma cholesterol concentration in a 
group of 53 rhesus monkeys and found that the 
fluctuations tended to disappear when values 
were recalculated and expressed on the basis of 
dry serum solids. Their findings indicated that 
the variability observed was in most cases not 
due to changes in cholesterol metabolism but 
rather to changes in the water content of 
serum. 
Another source of variation in plasma choles- 
terol concentration in rhesus monkeys is the oc- 
currence of pregnancy. Wolf and co-workers^^ 
recorded the changes in plasma cholesterol con- 
centration of 22 pregnant rhesus monkeys dur- 
ing the course of the pregnancy and for three 
weeks postpartum. During pregnancy there was 
more than a 50% decrease in the total plasma 
cholesterol concentration which occurred by the 
12th week and continued throughout preg- 
nancy. After parturition there was a gradual 
increase and the animals reached control con- 
centration of plasma cholesterol (about 160 
nig% ) by the third postpartum week. 
Rhesus monkeys appear to be useful models 
for studies on the mechanism of the effects of 
saturated and unsaturated fat diets on plasma 
cholesterol homeostasis. Effects of saturated 
and unsaturated fat diet on most laboratory an- 
imals are difficult to demonstrate unless con- 
siderable amounts of dietary cholesterol are 
also included. Emerson and her co-workers^* 
have examined the effect of saturated and unsat- 
urated fat diet on lipid metabolism of rhesus 
monkeys. She and her group demonstrated 
significant increases in the serum cholesterol 
concentration of animals fed butterf at and a sig- 
nificant lowering effect due to safflower oil. Fur- 
ther, they showed that small amounts of dietary 
safflower oil prevented the hypercholesterolemic 
effect of butter in these animals. As little as 
1.8% safflower oil in the diet was capable of 
preventing the approximate 60 mg% increase 
due to the addition of 20 % butterf at. 
Wissler and his co-workers^^ studied the 
blood lipids of rhesus monkeys fed either "an 
average American diet" or a "prudent Ameri- 
can diet." After 105 weeks, the animals fed the 
"average American diet" had a mean plasma 
cholesterol concentration of 358 mg%, while 
those on the "prudent diet" was about 250 
