N.D.M. Lehner

Comparative primate atherosclerosis: I. Tissue Cholesterol Concentration and Pathologic Anatomy

Abstract Stump-tailed macaques ( Macaca arctoides ), African green monkeys ( Cercopithecus aethiops ), squirrel monkeys ( Saimiri sciureus ), and woolly monkeys ( Lagothrix lagothricha ) were fed control, solid atherogenic (1 mg cholesterol/cal) or liquid diets containing 0, 0.5, or 1 mg cholesterol/cal. Stump-tailed macaques fed the solid atherogenic diet had the highest tissue and serum cholesterol concentration (about 700 mg/dl) and the most extensive atherosclerosis. These monkeys appeared to respond differently to diets containing 1 mg cholesterol/cal. Those animals fed the liquid diet had higher liver cholesterol concentration but lower serum cholesterol concentration than animals fed the solid diet. African green monkeys fed the solid atherogenic diet had serum cholesterol concentrations of about 450 mg/dl. A greater percentage of the abdominal aorta was covered by plaque than the thoracic aorta. Coronary artery atherosclerosis was focal with the largest plaques being found in the left main coronary artery. The microscopic appearance of these plaques was similar to that of plaques from people. Squirrel monkeys fed the atherogenic diet were the most variable group. The average serum cholesterol concentration averaged about 450 mg/dl (range: 291 to 716). The percentage of aorta covered by plaque ranged from 0 to 55% with more thoracic than abdominal aortic atherosclerosis. There were findings consistent with hemorrhage in plaques from two animals. These monkeys, like stump-tailed macaques but unlike African green monkeys had relatively high liver cholesterol concentrations. Woolly monkeys appeared to develop atherosclerosis when fed 1 mg cholesterol/cal but did not have greatly elevated serum cholesterol concentrations.

A study of atherosclerosis regression in Macaca mulatta: I. Design of experiment and lesion induction

Abstract Rhesus monkeys were fed an atherogenic diet containing 40% of calories from lard and 1.0 mg/Cal cholesterol for either 19 months (Colony I) or 38 months (Colony II). At the end of the induction period the animals from each colony were divided into three groups (A, B, C) on the basis of total plasma cholesterol concentration during the induction period. Group A animals were killed at the end of the induction period for baseline observation of the extent and severity of atherosclerosis. Group B from each colony was fed a diet which maintained total mean plasma cholesterol concentrations between 280 to 320 mg/dl comparable to human beings with modest hyperlipoproteinemia. Group C from each colony was fed a diet which maintained total mean plasma cholesterol concentrations between 180 to 220 mg/dl, comparable to people who had modest hyperlipoproteinemia but were able to reduce plasma cholesterol concentrations by approximately 100 mg/dl using diet or drugs. Each group was further divided into two subgroups (B1, B2, and C1, C2). Animals from subgroup 1 were fed these diets for 24 months and animals from subgroup 2 were fed the same diet for 48 months. This report describes the clinical history, chemical analyses of arteries and the morphological extent and severity of atherosclerosis in arteries from animals of both colonies at the end of the induction period.

Ketamine-HCI as a Suitable Anesthetic for Endocrine, Metabolic, and Cardiovascular Studies in Macaca fascicularis Monkeys

Abstract Ketamine-HCl has been reported, depending on experimental conditions and dosage given, to have significant cardiovascular and endocrine effects in some species. However, previous studies in primates have inadequately distinguished between animal handling and ketamine effects. We, therefore, examined the effects of various doses of ketamine (0, 5, 10, 15, and 20 mg/kg) on mean arterial blood pressure and on plasma insulin, glucose, and cortisol concentrations in 10 chronically cannulated Macaca fascicularis monkeys which had been acclimated to restraining chairs. Each monkey received three different doses of ketamine according to a balanced incomplete block design. Ketamine anesthesia produced no significant changes in plasma insulin, glucose, or cortisol concentrations nor did it affect mean arterial blood pressure. In addition, the effects of ketamine-HCl on endocrine responses to insulin-induced hypoglycemia were examined in four animals according to a cross-over design. These animals also had chronically maintained cannulas and had been acclimated to restraining chairs. Plasma glucose concentrations, as well as plasma ACTH, growth hormone, and Cortisol responses in ketamine-anesthetized animals receiving an insulin challenge were no different from those in unanesthetized control animals. Thus, our studies indicate that ketamine-HCl does not perturb these particular hormonal systems in M. fascicularis monkeys.

Atherosclerosis in Cebus albifrons monkeys: III. Clinical and Pathologic Studies☆

Abstract “Young and adult Cebus albifrons monkeys were fed either a control or an atherogenic diet for periods of up to 2 years. Some of the monkeys were necropsied at the end of 1 year. Coronary artery atherosclerosis was evaluated by determining the percentage of diseased arteries and the percentage apparent luminal stenosis in five-step cryostat sections from each of three blocks of ventricular myocardium. Arbitrary scores from 0 to 4 were made on gross stained carotid and renal arteries. Sudan IV and hematoxylin-stained cryostat sections were prepared from portions of basilar, middle cerebral, and posterior cerebral arteries. At 1 year serum cholesterol and age but not sex were related to the extent and prevalence of coronary artery disease. Atherosclerosis was slight in young test monkeys but relatively severe in adult test monkeys. Observations on the adult monkeys after 2 years were not much different from those made at 1 year. Young test female monkeys had five times as many coronary artery lesions as young test males after the 2-year period which was not true at 1 year. Single coronary artery lesions were found in one control adult male and one young control female. Carotid artery atherosclerosis was more prevalent among males than females. One adult control male had a small carotid artery lesion. One adult male test monkey had cutaneous and tendinous xanthomas. No lesion was found in the cerebral arteries of any animal. Sections of the tongue, uterus, kidney, and liver of the second-year test animals were examined for arterial lesions. Lesions, very similar to those of the small coronary arteries, were found in the intramuscular arteries of the tongues. Intrarenal arterial lesions were present in some of the cholesterol-fed monkeys of both age and sex groups. The group with the most pronounced coronary artery atherosclesosis, the adult test group, had the greatest frequency and magnitude of S-T segment deviation in the terminal electrocardiogram. One monkey of this group had discordant T waves in all leads.

The effect of insulin deficiency, hypothyroidism, and hypertension on atherosclerosis in the squirrel monkey☆

Abstract Squirrel monkeys with induced insulin deficiency, hypothroidism, and hypertension, as well as controls were fed a diet containing 1 mg of cholesterol per calorie for over 3 years. The hypothyroid and insulin-deficient monkeys had significantly greater concentrations of serum cholesterol and β-lipoprotein than did the controls, while the controls and hypertensive monkeys did not differ in these regards. The insulin-deficient, hypothyroid, and hypertensive groups all had more extensive coronary arterial and aortic atherosclerosis than did the controls. Atherosclerosis was especially severe in the insulin-deficient monkeys. The level of systolic blood pressure, the concentration of serum cholesterol, and the concentration of serum β-lipoprotein were significantly and positively correlated with coronary arterial and aortic atherosclerosis. The rate of disappearance of intravenously administered glucose was inversely related to serum cholesterol concentration and the indices of atherosclerosis. The squirrel monkey may be a good animal in which to study the mechanisms by which these disorders affect atherosclerosis, since these syndromes appear to affect atherosclerosis in this animal similarly as in man.

Ascorbic acid deficiency in the squirrel monkey.

Evidence that primates require exogenous ascorbic acid has not been established for all species. The general statement that monkeys require exogenous vitamin C is based in large part on evidence for such a need in the rhesus monkey, Macaca mulatto, (1). A requirement for exogenous ascorbic acid has also been reported for Cebus fatuellus (2), Macaca cyclopsis swinhoi (1), and Cercopithecus aethiops (3). Elliot et al. (4) reported that liver slices of the prosimian primates, Tupaia glis and Nycticebus coucang, can synthesize ascorbic acid from gulonolactone in vitro, and presumably do not require an exogenous source of this vitamin. Chatterjee et al. (5), however, have noted that D-glucurono reductase, which converts D-glucuronolactone to L-gulonolactone, is specifically absent in those species which cannot synthesize L-ascorbic acid. If these prosimians can make the conversion of D-glucuronolactone to L-gulonolactone they presumably would not require an exogenous source of this vitamin. The present study was done to determine whether the squirrel monkey, Saimiri sciureus, requires an exogenous source of ascorbic acid. Materials and Methods. Six juvenile Brazilian squirrel monkeys, obtained from Leticia, Colombia were used in this study. The monkeys were fed a diet devoid of ascorbic acid (Table I). After 3 months on the ascorbic acid free regimen, 3 of the animals were supplemented with ascorbic acid at a level of 10 mg/kg of body weight per day. The ascorbic acid was given intramuscularly as an aqueous solution, prepared just prior to administration. The remainder of the animals were continued on the ascorbic acid free regimen for the 4-month period of study. Body weighty packed cell volume, and serum ascorbic acid were examined at monthly intervals. Serum ascorbic acid was determined by titration of the serum with dichlorophenolindophenol (6).

Atherosclerosis in Cebus monkeys: II. Arterial metabolism☆

Abstract Some aspects of arterial metabolism have been studied in young and adult male and female Cebus albifrons which had been maintained on either a control diet or a similar diet that included 0.5% cholesterol. Using a perfusion technique, it has been possible to show that isolated segments of the aorta, as well as the coronary arteries, are capable of synthesizing fatty acids from acetate-1- 14 C. The rate of such synthesis appears to be more rapid in the areas of the arterial tree that show the greatest extent of atherosclerotic involvement—i.e., coronary arteries > thoracic aorta > abdominal aorta. The newly synthesized fatty acids are found primarily in the phospholipid and triglyceride fractions. Using mevalonic acid-2- 14 C in perfusion studies, the synthesis of labeled squalene has also been established for both thoracic and abdominal aorta and for the carotid arteries. In addition to this sterol precursor, unidentified digitonin-precipitable substances are synthesized, but which, to a large extent, do not appear to be cholesterol. Somewhat less than 3% of this material was purified as the dibromide and may indeed represent cholesterol. The aortas of Cebus monkeys contain cholesterol and cholesterol esters in increased amounts as atherosclerotic involvement becomes more extensive, with the free cholesterol form predominating over the ester form in a ratio of about 3:1 (in contrast to the 1:3 ratio found in the plasma of these animals). When 14 C-cholesterol is administered to these animals intravenously, after 37 days the isotope is found in the artery in a free:ester ratio of 3:1, resembling the actual concentration of these forms in the artery, and unlike the distribution in blood. These findings suggest that the artery, in the early stages of atherosclerosis seen here, is selective in either admission of or the retention of certain forms of cholesterol.

Atherosclerosis in Cebus albifrons monkeys: 1. Sterol metabolism

Summary Adult and young male and female Cebus albifrons monkeys exhibit differences in levels of serum cholesterol in response to cholesterol-containing diets; adult males had the highest levels, young males the lowest, and females of either age had values that were intermediate between the two groups of males. No differences were detectable among similar age and sex groups maintained on control diets to which no cholesterol was added. The kinetic characteristics of the disappearance of cholesterol- 14 C-containing lipoprotein were studied treating the system as a two-pool model. These studies indicated that adult males have a larger amount of cholesterol than do the other groups of monkeys in that pool which is represented by plasma plus those tissues that are in ready equilibrium with it. Similarly, adult males clear from their plasma each day a larger absolute amount of cholesterol, up to 200 mg/day, but this amount represents a smaller fraction of the total cholesterol present in this plasma-tissue pool. Using an isotopic method, the excretion of fecal bile acids and neutral sterols was studied. Results suggest that a large percentage of the sterol cleared from the plasma each day is excreted in the feces, and is made up of approximately equal amounts of bile acids and neutral sterols. Analysis of representative tissues of the animals for cholesterol, however, indicate that monkeys which showed the maximum elevation of serum cholesterol also had an expanded whole-body cholesterol pool. The distribution of cholesterol- 14 C in the tissues of the monkeys was studied after the semilogarithmic plasma die-away curve became linear with time. Results indicated a high degree of individuality among the different monkeys, and among different tissues from the same monkey.

ATHEROSCLEROSIS IN SOME SPECIES OF NEW WORLD MONKEYS

Nonhuman primates have recently gained considerable interest as animal models for the study of atherosclerosis. This interest has occurred because these animals share with man taxonomic classification as Primates and because the naturally occurring and induced lesions of atherosclerosis bear a striking resemblance to the lesions of young human beings. Because of the taxonomic classification of the nonhuman primates, it is generally considered that data obtained from experiments on their atherosclerosis might be more directly relevant to an understanding of the disease process in human beings. The extent to which this general notion is true must await the accumulation of larger amounts of data on primate atherosclerosis and on the testing in human beings of generalizations made from these data. Of the nonhuman primates, the Old World monkeys have been the most extensively used in atherosclerosis research. In recent years, however, investigations of the New World monkeys as experimental subjects in atherosclerosis has greatly increased. This increased attention has resulted because of their ready availability on this continent, their susceptibility to naturally occurring atherosclerosis and the ease by which the disease can be aggravated by diet, the rarity of tuberculosis among these animals, and the apparent lack of simian B-virus infections. The purpose of this paper is to review some of the characteristics of atherosclerosis in New World monkeys.

Effects of insulin deficiency, hypothyroidism, and hypertension on cholesterol metabolism in the squirrel monkey

Abstract Control, insulin-deficient, hypertensive, and hypothyroid squirrel monkeys were fed a diet containing 1 mg of cholesterol per calorie for 3 years. During this time, measurements were made of cholesterol absorption and serum cholesterol turnover. At the end of the study, measurements were made of lipid synthesis in the aorta, liver, and ileum in these monkeys as well as in monkeys fed a diet essentially devoid of cholesterol. About 70% of dietary cholesterol was absorbed, with no significant difference among the four groups. Hepatic cholesterol synthesis from acetate was almost completely inhibited in all the cholesterol-fed monkeys (control, hypertensive, hypothyroid, and insulin-deficient). Hepatic cholesterol synthesis from mevalonate by the control and hypertensive monkeys was also significantly less but still 25% as much as that of noncholesterol-fed monkeys. Cholesterol synthesis from mevalonate by hepatic tissue of the hypothyroid and insulin-deficient monkeys was even more greatly depressed, being significantly less than that of the control monkeys. Cholesterol synthesis from mevalonate by the distal ileums of the control, hypertensive, and noncholesterol-fed monkeys was about the same, but the monkeys not fed cholesterol had significantly greater synthesis from acetate. Incorporation of either acetate or mevalonate into cholesterol by the ileums of the hypothyroid and insulin-deficient monkeys was greatly reduced, being significantly less than that of the control monkeys. The incorporation of mevalonate into lipid fractions of the aorta followed a similar pattern, with markedly depressed synthesis in the aortas of the hypothyroid and insulin-deficient monkeys, and no significant difference among the hypertensive, control, and noncholesterol-fed monkeys. During the 3-year period the serum cholesterol concentrations of the insulin-deficient and hypothyroid monkeys averaged 575 ± 79 (mean ± SEM) and 414 ± 18 mg 100 ml , respectively, both being greater than that of the controls which averaged 228 ± 18 (P