Effects of gestation on glucose tolerance and plasma insulin in cynomolgus monkeys (Macaca fascicularis)

Characterization and heritability of obesity and associated risk factors in vervet monkeys

OBJECTIVE

The objective was to determine the prevalence and heritability of obesity and risk factors associated with metabolic syndrome (MS) in a pedigreed colony of vervet monkeys.

DESIGN

A cross-sectional study of plasma lipid and lipoprotein concentrations, glycemic indices, and morphometric measures with heritability calculated from pedigree analysis. A selected population of females was additionally assessed for insulin sensitivity and glucose tolerance.

SUBJECTS

All mature male (n=98), pregnant (n=40) and non-pregnant female (n=157) vervet monkeys were included in the study. Seven non-pregnant females were selected on the basis of high or average glycated hemoglobin (GHb) for further characterization of carbohydrate metabolism.

MEASUREMENTS

Morphometric measurements included body weight, length, waist circumference, and calculated BMI. Plasma lipids [total cholesterol (TC), triglycerides (TG), high-density lipoprotein cholesterol (HDL-C)] and glycemic measures (fasting blood glucose, insulin, and GHb) were measured. A homeostasis model assessment index was further reported. Glucose tolerance testing and hyperinsulinemic-euglycemic clamps were performed on 7 selected females.

CONCLUSION

Vervet monkeys demonstrate obesity, insulin resistance, and associated changes in plasma lipids even while consuming a low-fat (chow) diet. Furthermore, these parameters are heritable. Females are at particular risk for central obesity and an unfavorable lipid profile (higher TG, TC, and no estrogen-related increase in HDL-C). Selection of females by elevated GHb indicated impaired glucose tolerance and was associated with central obesity. This colony provides a unique opportunity to study the development of obesity-related disorders, including both genetic and environmental influences, across all life stages.

Old World Nonhuman Primate Models of Type 2 Diabetes Mellitus

Type 2 diabetes mellitus is a major health problem of increasing incidence. To better study the pathogenesis and potential therapeutic agents for this disease, appropriate animal models are needed. Old World nonhuman primates (NHPs) are a useful animal model of type 2 diabetes; like humans, the disease is most common in older, obese animals. Before developing overt diabetes, NHPs have a period of obesity-associated insulin resistance that is initially met with compensatory insulin secretion. When either a relative or absolute deficiency in pancreatic insulin production occurs, fasting glucose concentrations begin to increase and diabetic signs become apparent. Pathological changes in pancreatic islets are also similar to those seen in human diabetics. Initially there is hyperplasia of the islets with abundant insulin production typically followed by replacement of islets with islet-associated amyloid. Diabetic NHPs have detrimental changes in plasma lipid and lipoprotein concentrations, lipoprotein composition, and glycation, which may contribute to progression of atherosclerosis. As both the prediabetic condition (similar to metabolic syndrome in humans) and overt diabetes become better defined in monkeys, their use in pharmacological studies is increasing. Likely due to their genetic similarity to humans and the similar characteristics of the disease in NHPs, NHPs have been used to study recently developed agonists of the peroxisome proliferators-activated receptors. Importantly, agonists of the different receptor subclasses elicit similar responses in both humans and NHPs. Thus, Old World NHPs are a valuable animal model of type 2 diabetes to study disease progression, associated risk factors, and potential new treatments.

Medroxyprogesterone acetate antagonizes inhibitory effects of conjugated equine estrogens on coronary artery atherosclerosis

Although estrogen replacement therapy is associated with reduced risk of coronary heart disease and reduced extent of coronary artery atherosclerosis, the effects of combined (estrogen plus progestin) hormone-replacement therapy are uncertain. Some observational data indicate that users of combined hormone replacement consisting of continuously administered oral conjugated equine estrogens (CEE) and oral sequentially administered (7 to 14 days per month) medroxyprogesterone acetate (MPA) experience a reduction in risk similar to that of users of CEE alone. However, the effects of combined, continuously administered CEE plus MPA (a prescribing pattern that has gained favor) on the risk of coronary heart disease or atherosclerosis are not known. We studied the effects of CEE (monkey equivalent of 0.625 mg/d) and MPA (monkey equivalent of 2.5 mg/d), administered separately or in combination, on the extent of coronary artery atherosclerosis (average plaque size) in surgically postmenopausal cynomolgus monkeys fed atherogenic diets and treated with these hormones for 30 months. Treatment with CEE alone resulted in atherosclerosis extent that was reduced 72% relative to untreated (estrogen-deficient) controls (P < .004). Atherosclerosis extent in animals treated with CEE plus MPA or MPA alone did not differ from that of untreated controls. Although treatment had marked effects on plasma lipoprotein patterns, statistical adjustment for variation in plasma lipoproteins did not alter the between-group relationships in atherosclerotic plaque size, suggesting that these factors do not explain substantially the atheroprotective effect of estrogen or the MPA-associated antagonism. Although the mechanism(s) remains unclear, we conclude that oral CEE inhibits the initiation and progression of coronary artery atherosclerosis and that continuously administered oral MPA antagonizes this atheroprotective effect.

Effects of androgens on coronary artery atherosclerosis and atherosclerosis-related impairment of vascular responsiveness

The factors responsible for the marked gender differences in risk of coronary heart disease and atherosclerosis severity remain largely undetermined. While some clinical and experimental evidence supports a protective effect of endogenous estrogen on the initiation and progression of atherosclerosis and incidence of coronary heart disease, much of the epidemiological data do not support this conclusion. The possibility that endogenous androgens may have adverse effects on atherosclerosis progression and coronary risk has received little attention. We investigated the effects of experimentally induced hyperandrogenism in female cynomolgus monkeys with diet-induced atherosclerosis. Animals were assigned randomly to one of four treatment groups: (1) untreated controls, (2) ovariectomized (sex hormone-deficient) controls, (3) treated with androstenedione and estrone (mild hyperandrogenism), or (4) treated with testosterone (male plasma androgen pattern). At necropsy, coronary atherosclerosis was approximately twice as extensive (P < .05) in testosterone-treated animals relative to untreated controls, while treatment with androstenedione and estrone had no effect on atherosclerosis extent. Coronary plaque size was positively correlated with lumen size in intact and ovariectomized controls; however, there was no evidence of a similar relation between animals in either androgen treatment group. The atherogenic effects of testosterone were independent of variations in plasma lipoprotein and nonlipoprotein risk variables. Although chronic hyperandrogenism had adverse effects on atherosclerosis progression, it reversed (P < .03) atherosclerosis-related impairment of endothelium-dependent vasodilator responses. We conclude that an experimentally induced male plasma androgen pattern results in exacerbation of diet-induced atherosclerosis-related arterial remodeling in female monkeys.(ABSTRACT TRUNCATED AT 250 WORDS)

Social status and coronary artery atherosclerosis in female monkeys

While coronary heart disease is the leading cause of death in women in the United States, research in the area is lacking, especially concerning psychosocial risk factors. The purpose of this experiment was to study the effect of a known psychosocial risk factor in female monkeys, social status, and the effect of alteration of social status on coronary artery atherosclerosis. In previous experiments it has been demonstrated that social status is an enduring characteristic of the individual and that socially subordinate female monkeys have poor ovarian function and exacerbated coronary artery atherosclerosis. In the present experiment, adult female monkeys were fed an atherogenic diet and housed in small social groups, and social status was altered in half of the animals (subordinates became dominant and dominants became subordinate). The manipulation of social status had minimal effects on risk factors but significantly affected coronary artery atherosclerosis, supporting the hypothesis that social status affects atherogenesis in these females. However, all animals that changed social positions had worsened coronary artery atherosclerosis whether they became dominant or became subordinate, and this effect was independent of ovarian function. Subordinates that became dominant had 44% more and dominants that became subordinate had 500% more atherosclerosis than their counterparts that did not change social status. Thus, modification of this psychosocial risk factor was not effective in reducing coronary artery atherosclerosis. The manipulation of social status may have deleteriously altered a complex interaction between individuals and their psychosocial environment.

Inhibition of coronary artery atherosclerosis by 17-beta estradiol in ovariectomized monkeys. Lack of an effect of added progesterone

Although controversy continues, the preponderance of evidence indicates that estrogen replacement therapy favorably influences the risk of coronary heart disease in postmenopausal women. It remains uncertain how this effect is mediated and whether the cyclic addition of a progestin may influence adversely an estrogen-related cardioprotective effect. We investigated the influence of sex hormone replacement therapy on diet-induced coronary artery atherosclerosis in estrogen-deficient (ovariectomized) adult female cynomolgus monkeys. Monkeys were assigned randomly to one of three treatment groups: 1) no hormone replacement (n = 17), 2) continuously administered 17-beta estradiol plus cyclically administered progesterone (n = 20), and 3) continuously administered 17-beta estradiol (n = 18). The physiologic patterns of plasma estradiol and progesterone concentrations were maintained by administering the hormones in sustained-release subcutaneous Silastic implants. The experiment lasted 30 months. At necropsy, coronary artery atherosclerosis was inhibited similarly (reduced by approximately one-half) in animals in both hormone replacement groups (p less than or equal to 0.05). Antiatherogenic effects of hormone replacement were independent of variation in total plasma cholesterol, lipoprotein cholesterol, apoprotein A-1 and B concentrations, high density lipoprotein subfraction heterogeneity, and low density lipoprotein molecular weight. We conclude that physiologic estrogen replacement therapy with or without added progesterone inhibits atherosclerosis progression in ovariectomized monkeys. This may explain why estrogen replacement therapy results in reduced risk of coronary heart disease in postmenopausal women.

Pregnancy-associated inhibition of coronary artery atherosclerosis in monkeys. Evidence of a relationship with endogenous estrogen

We investigated the influence of repeated pregnancy on diet-induced atherosclerosis in cynomolgus monkeys and sought to determine if circulating endogenous reproductive steroid levels were associated with the extent of coronary artery atherosclerosis. At necropsy, females which were pregnant one or more times were found to have coronary artery atherosclerosis which was one-fourth as extensive as that of intact females which had not been pregnant. Extent of coronary artery atherosclerosis correlated positively with mean total plasma cholesterol (Rho = 0.52, p less than 0.01) and inversely with high density lipoprotein (HDL) cholesterol (Rho = -0.48, p less than 0.01) concentrations; both decreased during pregnancy. Additionally, the extent of coronary artery atherosclerosis was found to have a strong inverse association (Rho = -0.66, p less than 0.001) with an index (area-under-the-curve) of magnitude and duration of the pregnancy-induced elevation in plasma 17-beta estradiol concentration. This association could not be explained by an interrelationship between estradiol area-under-the-curve and either plasma total or HDL cholesterol concentrations. There was no relationship between atherosclerosis extent and a similar index of plasma progesterone concentrations. These findings provide evidence for an inhibitory effect of endogenous estrogen on the progression of coronary artery atherosclerosis.

Nonhuman primate models of atherosclerosis: potential for the study of diabetes mellitus and hyperinsulinemia

Nonhuman primates have been used for many years to investigate the pathogenesis and progression of atherosclerosis. The use of these animal models has resulted in a better understanding of the risk factors associated with atherosclerosis. Nonhuman primates that have consumed an atherogenic diet for several years develop lesions that are comparable to those found in human beings. Diabetes, both spontaneous and chemically induced, has been described in a number of nonhuman primate species. These diabetic models may be used to understand the accelerated progression and vascular complications of atherosclerosis in diabetic human beings.

Lipids, lipoproteins, and endocrine profiles during pregnancy in the African green monkey (Cercopithecus aethiops)

In an attempt to establish relationships between the endocrine and lipid metabolism during pregnancy, the changes in total plasma cholesterol (TPC) and lipoprotein cholesterol that occur during pregnancy in the African green monkey were investigated longitudinally in ten females in relation to the changes in progesterone, estradiol, and fasting insulin concentrations. Respective means for TPC, high-density lipoprotein (HDL) cholesterol, and low-density lipoprotein (LDL) plus very low-density lipoprotein (VLDL) cholesterol were 343 +/- 35, 108 +/- 9, and 235 +/- 36 mg/dL prior to the estimated date of conception in ten females fed a high-fat, high-cholesterol diet. The concentration of these lipids fell to 225 +/- 31, 54 +/- 4, and 168 +/- 29 mg/dL for TPC (P less than 0.001), HDL cholesterol (P less than 0.001), and LDL + VLDL cholesterol (P less than 0.001), respectively, by midpregnancy (84 days). Progesterone concentrations increased during the first 60 days of pregnancy and were negatively correlated with HDL cholesterol concentrations (r = -0.57, P less than 0.02). After reaching their highest mean value, progesterone concentrations then plateaued at lower concentrations until parturition. The decrease in progesterone concentrations was associated with an initial rise in estradiol concentrations, which reached their highest concentrations in late pregnancy and were inversely correlated with HDL-cholesterol concentrations (r = -.32, P less than 0.01). Although glucose concentrations remained steady during gestation, insulin concentrations were elevated compared to postpartum concentrations (P less than 0.05) suggesting that insulin resistance occurred during the pregnancy in this nonhuman primate.(ABSTRACT TRUNCATED AT 250 WORDS)

Pregnancy alters diurnal variation of plasma glucose concentration

Human pregnancy introduces a diurnal variation of fasting plasma glucose concentrations, with significantly lower values found after a daytime fast of eight hours compared with a night-time fast of the same duration. The diurnal variation of plasma glucose clearance, found in nonpregnant subjects, is not demonstrable during pregnancy. Insulin tolerance does not exhibit a circadian valuation but is substantially decreased during pregnancy.