ILAR Journal Online, Volume 32(3) 1990: New Rat Models of Obesity and Type II Diabetes

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ILAR Journal V32(3) 1990
New Rat Models of Obesity and Type II Diabetes

The Spontaneous Hypertensive/NIH-corpulent Rat: A New Rodent Model for the Study of Non-insulin-dependent Diabetes Mellitus and Its Complications
Otho E. Michaelis IV and Carl T. Hansen

Otho E. Michaelis IV is a research biologist, Beltsville Human Nutrition Research Center, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland, and an adjunct associate professor, Division of Renal Diseases, Department of Medicine, George Washington University, Washington, D.C. Carl T. Hansen is a geneticist, Veterinary Resources Program, National Center for Research Resources, National Institutes of Health, Bethesda, Maryland.

Introduction

The spontaneous hypertensive/NIH-corpulent rat strain (SHR/N-cp) (Figure 1) is a recently developed genetic model for the study of obesity and diabetes (Hansen, 1983). Obese rats exhibit both metabolic and histopathologic characteristics associated with non-insulin-dependent diabetes mellitus (type II) in humans. This rodent model is unique in that glucose intolerance is expressed in both sexes.

The SHR/N-cp strain was one of two congenic rat strains developed for obesity studies at the National Institutes of Health Animal Genetic Resource (Hansen, 1988). The mutation corpulent cp arose spontaneously after several generations of inbreeding in a stock derived from an SHR/N female crossed with a male, normotensive SD rat (Koletsky, 1973). The gene was introduced into the SHRfN strain. A minimum of 12 backcrosses were conducted to eliminate the non-cp Koletsky genes. Back-crossing was Complicated by the fact that corpulent homozygotes (cp/cp) do not reproduce. This problem required test matings in each generation to identify the heterozygotes needed for the next cycle of backcrossing. The fully backcrossed SHR/N-cp strain differs genetically from its partner SHR/N strain only by the presence of the cp gene. Mating of heterozygotes yields obese to lean rats in a ratio of 1:3. Lean rats consist of two-thirds heterozygotes (cp/+) and one-third homozygotes (+/+).

Metabolic Characteristics

Obesity in the SHR/N-cp strain is characterized by an increase in both size and number of adipocytes in principal fat depots and by impaired thermogenesis (Tulp et al., 1986, 1988).

Obese male rats are mildly hypertensive and, when fed a high-carbohydrate diet, exhibit some of the metabolic alterations associated with human non-insulin-dependent diabetes mellitus, including hyperinsulinemia, hyperlipidemia, glucose intolerance, and glycosuria (Table 1) (Michaelis et al., 1984, 1988). In addition to serum insulin, other glucoregulatory hormones--corticosterone, glucagon, growth hormone, and pancreatic somatostatin--are also elevated in obese rats when compared to their lean littermates (Ellwood et al., 1988; Voyles et al., 1987).

When compared to obese male rats, obese female rats are smaller, less glucose-intolerant, and do not exhibit glycosuria (Table 1). Although levels of serum insulin are similar in both sexes, levels of serum triglyceride are substantially higher in obese female than in male rats and are associated with increased hepatic lipogenic enzyme activities (Yamini et al., 1988).

Using sucrose (or fructose) rather than starch as the source of dietary carbohydrate exaggerates adiposity and the expression of diabetes in obese rats (Michaelis et al., 1984, 1987; Tulp et al., 1988). In contrast to lean rats, obese rats demonstrate an improvement in glucose tolerance with aging (Michaelis et al., 1985).

Complications of Diabetes

Morphological changes associated with diabetes mellitus are observed in SHR/N-cp rats, including pancreatic islet hyperplasia, hepatocellular fatty change, nephropathy, and inner ear hair cell loss (Michaelis et al., 1986a; Triana et al., 1990). Obese female rats also exhibit hypertrophy of the adrenal cortex (Abraham and Michaelis, 1988).

Kidneys from obese rats show morphological changes consistent with both diabetes and inflammation (interstitial infiltration) (Abraham and Michaelis, 1988; Velasquez et al., 1989). Glomerular lesions associated with diabetes are characterized by segmental, diffuse, and nodular intercapillary mesangial expansion (Figure 2). Male and female obese rats show similar glomerular lesions, but females are less severely affected. Feeding a sucrose (or fructose) diet versus a starch diet accentuates the severity of the renal lesions.

The major functional complication in obese SHR/N-cp rats is renal disfunction (Velasquez et al., 1988, 1989). After 3 months on a high-carbohydrate diet, obese male diabetic SHR/N-cp rats exhibit proteinuria with a reduction in glomerular filtration rate compared to obese male nondiabetic LA/N-cp rats (Table 2). Other functional complications associated with hyperglycemia in this model include abnormal secretion of insulin by the pancreas (Recant et al., 1989) and a decreased capacity for adaptive thermogenesis (Tulp et al., 1986).

Cause(s) of Diabetes

The cause, or causes, of hyperglycemia in obese SHR/N-cp rats is not fully understood. Abnormality of the insulin molecule does not appear to explain the hyperglycemia (Voyles et al., 1988). Glucose intolerance is associated with the expression of obesity (Michaelis et al., 1988) and is characterized by insulin and glucagon resistance with decreased hormone binding to liver plasma membranes (Bhathena et al., 1989), decreased insulin binding and insulin-stimulated glucose uptake by adipocytes (Baly et al., 1989), and increased gluconeogenic activity in the liver and kidney (Yamini et al., 1988). However, it is unlikely that these metabolic alterations can entirely explain the glucose intolerance, because other genetically obese rats (e.g., Zucker fatty and obese LA/N-cp) exhibit similar metabolic characteristics but are euglycemic or show only mildly impaired glucose tolerance (Michaelis et al., 1986b; Zucker and Antoniades, 1972).

The finding that liver insulin receptor tyrosine kinase activity is not different between obese and lean SHR/N-cp rats when expressed per receptor number (Adamo et al., 1989) suggests that a postreceptor defect may be more responsible for the insulin resistance observed in obese SHR/N-cp rats than a defect at the receptor level. However, insulin receptor kinase activity in peripheral tissues (i.e., adipose and skeletal muscle) needs to be determined.

Conclusion

These studies demonstrate that the SHR/N-cp rat strain is an appropriate model for investigating pathophysiologic alterations associated with obesity and non-insulin-dependent diabetes mellitus and show how diet, sex, and genetics interact to produce these alterations.

References

Abraham, A. A., and O. E. Michaelis IV. 1988. Histopathology of renal and adrenal gland lesions in SHR/NIH-corpulent (cp) rats. Pg. 85-92 in New Models of Genetically Obese Rats for Studies in Diabetes, Heart Disease and Complications of Obesity, Summaries of Workshop Papers and Current Bibliography, C. T. Hansen and O. E. Michaelis IV, eds. Bethesda, Md.: National Institutes of Health.

Adamo, M., J. Shemer, M. Aridor, J. Dixon, N. Carswell, S. J. Bhathena, O. E. Michaelis IV, and D. LeRoith. 1989. Liver insulin receptor tyrosine kinase activity in a rat model of type II diabetes mellitus and obesity. J. Nutr. 119:484-489.

Baly, D. L., M. J. Zarnowski, N. Carswell, and O. E. Michaelis IV. 1989. Insulin resistant glucose transport activity in adipose cells from the SHR/N-corpulent rat. J. Nutr. 119:682-632.

Bhathena, S. J., B. W. Kennedy, J. Jones, P. M. Smith, O. E. Michaelis IV, N. Carswell, C. T. Hansen, N. R. Voyles, and L. Recant. 1989. Effect of dietary carbohydrates on insulin and glucagon receptors in a new model of noninsulin-dependent diabetes-SHR/N-corpulent rat. Proc. Soc. Exp. Biol. Med. 192:66-71.

Ellwood, K. C., O. E. Michaelis IV, and S. J. Bhathena. 1988. Comparative hormonal profile of two newly developed obese congenic rat strains. Comp. Biochem. Physiol. 89A:371-375.

Hansen, C. T. 1983. Two new congenic rat strains for nutrition and obesity research. Fed. Proc. 42:573.

Hansen, C.T. 1988. The development of the SHR/N- and LA/N-cp (corpulent) congenic rat strains. Pg. 7-10 in New Models of Genetically Obese Rats for Studies in Diabetes, Heart Disease, and Complications of Obesity. Summaries of Workshop Papers and Current Bibliography, C. T. Hansen and O. E. Michaelis IV, eds. Bethesda, Md.: National Institutes of Health.

Koletsky, S. 1973. Obese spontaneously hypertensive rats--a model for study of atherosclerosis. Exp. Mol. Pathol. 19:53-60.

Michaelis IV, O. E., K. C. Ellwood, J. M. Judge, N. W. Schoene, and C. T. Hansen. 1984. Effect of dietary sucrose on the SHR/N-corpulent rat: A new model for insulin-independent diabetes. Am. J. Clin. Nutr. 39:612-618.

Michaelis IV, O. E., K. C. Ellwood, J. J. Emberland, C. T. Hansen, and J. J. Canary. 1985. Effect of dietary carbohydrate on expression of diabetes in the SHR/N-corpulent rat: A new model for obesity-induced diabetes. J. Obesity Wt. Reg. 4:168-178.

Michaelis IV, O. E., D. H. Patrick, C. T. Hansen, J. J. Carnary, R. M. Werner, and N. Carswell. 1986a. Spontaneous hypertensive/NIH-corpulent rat. Animal model of human disease insulin-independent

diabetes mellitus (type II). Am. J. Pathol. 123:398-400.

Michaelis IV, O. E., K. C. Ellwood, O. L. Tulp, and M. R. C. Greenwood. 1986b. Effect of feeding sucrose or starch diets on parameters of glucose tolerance in the LA/N-corpulent rat. Nutr. Res. 6:95-99.

Michaelis IV, O. E., N. Carswell, and O. L. Tulp. 1987. Effect of long-term sucrose feeding on metabolic parameters of female corpulent and lean SHR/N-cp rats. Fed. Proc. 46:577.

Michaelis IV, O.E., H. Carswell, C. T. Hansen, J. J. Canary, and P. Kimmel. 1988. A new genetic model of non-insulin dependent diabetes mellitus (type II) and hypertension: The spontaneous hypertensive/NIH-corpulent rat. Pg. 257-264 in Frontiers in Diabetes Research. Lessons from Animal Diabetes II, E. Shafrir and A. E. Renold, eds. London: John Libbey.

Recant, L., N. R. Voyles, K. I. Timmers, S. J. Bhathena, D. Solomon, S. Wilkins, and O. E. Michaelis IV. 1989. Comparison of insulin secretory patterns in obese nondiabetic LA/N-cp and obese diabetic SHR/N-cp rats. Diabetes 38: 691-697.

Triana, R., G. W. Suits, S. Garrison, J. Prazma, and H. C. Pillsbury. 1990. Hearing loss in diabetes mellitus. Proceedings of the midwinter meeting of the Association for Research in Otolaryngology, Clearwater, Fla.

Tulp, O. L., C. T. Hansen, and O. E. Michaelis IV. 1986. Nonshivering thermogenesis in the diabetic SHR/N-cp (corpulent) rat. Physiol.' Behav. 36:127-131.

Tulp, O. L., S. P. DeBolt, N. Carswell, and O. E. Michaelis IV. 1988. Effects of fructose consumption on adiposity and glycemic status in the diabetic SHR/N-cp rat. Pg. 75-70 in New Models of Genetically Obese Rats for Studies in Diabetes, Heart Disease, and Complications of Obesity. Summaries of Workshop Papers and Current Bibliography, C. T. Hansen and O. E. Michaelis IV, eds. Bethesda, Md.: National Institutes of Health.

Velasquez, M. T., P. L. Kimmel, O. E. Michaelis IV, N. Carswell, and J. P. Bosch. 1988. Effect of metabolic glucose control on renal function and structure in a new rat model of type II diabetes mellitus. Diabetes 37:215A.

Velasquez, M. T., P. L. Kimmel, O. E. Michaelis IV, N. Carswell, A. Abraham, and J.P. Bosch. 1989. Effect of carbohydrate intake on kidney function and structure in SHR/N-cp rats. Diabetes 38:679-685.

Voyles, N. R., S. J. Bhathena, B. W. Kennedy, S. D. Wilkins, O. E. Michaelis IV, C. M. Zalenski, K. I. Timmers, and L. Recant. 1987. Tissue somatostatin levels in three models of genetic obesity in rats. Proc. Soc. Exp. Biol. Med. 185:49-54.

Voyles, N. R., A. M. Powell, K. I. Timmers, S. D. Wilkins, S. J. Bhathena, C. T. Hansen, O. E. Michaelis IV, and L. Recant. 1988. Reversible impairment of glucose-induced insulin secretion in SHR/N-cp rats. Genetic model of type II diabetes. Diabetes 37:398-404. Yamini, S., O. E. Michaelis IV, N. Carswell, and B. Szepesi. 1988.

Effect of carbohydrate feeding on enzymatic activity in the liver of SHR/N-corpulent rats. FASEB J. 2:A1222.

Zucker, L. M., and H. N. Antoniades. 1972. Insulin and obesity in the Zucker genetically obese rat"fatty." Endocrinology 90:1320-1330.

Address correspondence to Otho E. Michaelis IV, Carbohydrate Nutrition Laboratory, BHNRC, ARS, USDA, Building 307, Room 317, BARC-East, Beltsville, Maryland 20705 (301/344-2396).

TABLE 1 Metabolic Characteristics of the SHR/N-cp Rat Strain

	Male		Female
	Obese	Lean	Obese	Lean
Body weight (g)	600 + 8.7	426 + 8.0	493 + 9.5	238 + 5.0
Systolic blood pressure (mm Hg)	135 + 2.7	181 + 4.0	141 ± 2.6	162 ± 2.1
Serum insulin (mU/ml)
Fasting	809 ± 104.3	70 ± 4.3	868 + 63.1	66 + 3.7
1-Hour response	1213 + 85.8	101 ± 5.7	1378 + 97.3	94 ± 6.5
Serum triglyceride (mg/dl)
Fasting	323 + 39.0	77 + 4.4	1130 ± 122.2	111 ± 8.7
Serum total cholesterol (mg/dl)
Fasting	204 ± 17.1	94 + 6.1	169 + 19.6	85 ± 3.6
Serum glucose (mg/dl)
Fasting	134 + 5.5	103 ± 1.9	144 + 4.6	116 + 2.7
1-Hour response	345 ± 17.7	174 + 5.0	276 + 15.5	181 ± 5.1
Urine glucose (mg/17 hr)	104.6 ± 11.70	2.0 + 0.22	4.6 ± 0.50	2.0 ± 0.28

SOURCE: Adapted in part from Michaelis et al. (1988).
NOTE: Young rats (5 to 6 weeks of age) were fed a nutritionally adequate semipurified dietcontaining 54 percent carbohydrate for 3 months. Systolic blood pressure was determined by indirect tail-cuff method. Blood was obtained by tail bleeding. Fasting serum parameters were determined in the early portion of the dark cycle after rats were fasted for 12 to 14 hours. Glucose tolerances and insulin responses were determined in fasted rats following an oral glucose load of 250 mg per 100 g body weight. Urine was collected from nonfasted rats. Six hours into the dark cycle, rats were put into metabolic cages without food for 17 hours. Results are shown as mean values ± SEM for at least 12 animals per group. Statistical analysis between sexes was not conducted because experiments were not run concurrently. For all parameters within sex, obese different than lean, p < 0.05.

TABLE 2 Parameters of Glucose Tolerance and Indices of Renal Function in Obese MaJe Diabetic SHR/N-cp and Nondiabetic LA/N-cp Rats

	LA/N-cp	SHR/N-cp
Body weight (g)	595 + 11	600 + 14
Serum insulin (mU/mD
Fasting	263 + 20	809 + 104*
Serum glucose (mg/dl)
1-Hour response	190 + 8	345 + 18*
Urinary glucose (mg/17 hr)	2.0 + 0.5	92 + 22*
Urinary protein (mg/17 hr)	11 + 2	90 + 16*
Creatinine clearance (ml/min/kg)	4.3 + 1.0	2.1 + 0.2*

*Different from LA/N-cp, p < 0.01.
SOURCE: Adapted from Velasquez et al. (1988). NOTE: Young rats (5 to 6 weeks of age) were fed a nutritionally adequate semipurified diet containing 54 percent carbohydrate for three months. Blood was obtained by tail bleeding. Fasting serum parameters were determined in the early portion of the dark cycle after rats were fasted for 12 to 14 hours. Urine was collected from nonfasted rats. Six hours into the dark cycle, rats were put into metabolic cages without food for 17 hours. Blood for measurement of serum creatinine was obtained at the end of the metabolic collections. Results are shown as mean values + SEM for at least nine rats per group.

Figure 1 Obese and lean male SHR/N-cp rat (14 weeks of age).

Figure 2 Renal glomerulus from an obese SHR/N-cp rat showing diffuse and nodular intercapillary mesangial expansion (methylene blue and basic fuchsin stain, 400x). Photograph courtesy of A. Abraham, Department of Pathology, George Washington University Medical Center, Washington, D.C.