Regul Pept. 2014 Jan 10;188:52-9. doi: 10.1016/j.regpep.2013.12.005.

Unexpected effects of voluntary exercise training on natriuretic peptide and receptor mRNA expression in the ob/ob mouse heart.

 

Broderick TL1, Wang D2, Jankowski M2, Gutkowska J2.
  • 1Laboratory of Diabetes and Exercise Metabolism, Midwestern University, Glendale, AZ, USA. Electronic address: tbrode@midwestern.edu.
  • 2Laboratory of Cardiovascular Biochemistry, Centre Hospitalier de L’Université de Montréal-Hôtel-Dieu Research Centre, Montréal, Québec, Canada.

 

Abstract

Regular exercise is generally recommended for the treatment of obesity and type 2 diabetes. Exercise reduces body weight, improves glycemic control and cardiovascular (CV) function. This study was designed to determine the impact of voluntary wheel running on the cardiac oxytocin (OT)-natriuretic peptide (NP) system and plasma CV risk factors in the ob/ob mouse, a model of insulin resistance coupled with severe obesity. Five-week-old male ob/ob mice and non-obese heterozygote control littermates were assigned to either a sedentary or running group. Voluntary running was performed using a wheel system for a period of 8 weeks. Compared to non-obese mice, daily running activity expressed in kilometers, was significantly lower in ob/ob mice. In these mice, voluntary running improved body weight, but exacerbated CV markers, including plasma glucose and triglyceride levels. OT receptor gene expression was decreased in hearts of ob/ob mice compared to non-obese mice, and no improvement in the expression of this receptor was observed after voluntary running. Hearts from ob/ob mice also expressed lower BNP mRNA, whereas no differences in A- and C-type NP were observed between non-obese and ob/ob mice. After voluntary running, a downregulation in the expression of all three NPs coupled with increased apoptosis was observed in ob/ob hearts. Our results show that voluntary exercise running activity was decreased in the ob/ob mouse. Surprisingly, this was associated with a worsening of common CV plasma markers, reduced expression of peptides linked to the cardioprotective OT-NP system, and increased expression of cardiac apoptotic markers.

KEYWORDS: Glucose; Natriuretic peptides; Obesity; Oxytocin; Running; ob/ob

PMID: 24365091

 

Supplement:

Physical activity performed on a regular basis is an important strategy employed in healthcare avenues aimed at preventing the onset of cardiovascular disease in patients with insulin resistance, obesity, and type 2 diabetes. The benefits of exercise are attributed to reduced adiposity and increased muscle strength, improved blood glucose control and lipid control, and enhanced heart and vascular function.

Prevention of common risk factors and cardiovascular complications with exercise is well-documented in relevant rodent models, providing crucial results in the understanding the mechanisms of exercise and justification for the use of these models. Using the db/db mouse (hyperleptinemic from defective leptin receptor signaling) as a close counterpart of the human diabetes and obesity condition, we demonstrated that the benefits of exercise training on risk factors and metabolic control are dependent on the running environment (1).

While treadmill running has been used widely as a method to improve the obese diabetic state, studies showed both beneficial and detrimental effects. We have shown that moderate intensity treadmill running worsens the hyperglycemia and insulin resistance in the db/db mouse (1). Endogenous glucocorticoid production also enhanced and the liver acts as an addition source of the glucocorticoid corticosterone through enhanced expression 11β-hydroxysteroid dehydrogenase type I, leading to increased gluconeogenesis and hyperglycemia (2). Increased levels of plasma corticosterone resulting from treadmill running-related stress also inhibits gene transcription of important cardioprotective genes in this mouse model (3).

WBF- figure 1

Figure 1. The ob/ob mouse (left) is leptin deficient from a homozygous mutation of the ob gene. The onset of obesity is gradual, characterized with hyperphagia and a subsequent diabetes-like syndrome. The non-obese mouse littermate (lower right) with only a heterozygous mutation in the gene, served as control.

 

Wheel running, on the other hand, is regarded as a type of volitional exercise that is not associated with systemic stress responses. Indeed, diabetes-linked disturbances and the production of stress hormones are attenuated with nocturnal volitional running using a simple running wheel system (4). Based on our observations, it seems appropriate to use a wheel system as a means to investigate the role of exercise on glucose control in the ob/ob mouse. Further, the ob/ob mouse (hypoleptinemic) is profoundly obese from excessive eating and (figure 1), as expected, complicates the mouse’s ability to run at a constant speed on a treadmill.

For a period of 8 weeks, voluntary running activity was consistently higher in non-obese mice compared to obese mice (Figure 2). Non-obese mice ran on average 4.5 km per day while ob/ob mice averaged 0.5 km to 1.5 km per day. This very modest running activity reduced the extent of adiposity in ob/ob mice, but surprisingly, further increased plasma glucose, total glycated hemoglobin and triglycerides, suggesting that running deteriorated the metabolic state.

 
WBF -figure 2

Figure 2. The ob/ob mouse was housed in a running wheel cage equipped with a digital magnetic counter to monitor daily running activity over a period of 8 weeks.

 

 

With ob/ob mice exhibiting hyperglycemia and hypertriglyceridemia following running, we then determined if this would downregulate the expression of the oxytocin (OT) and natriuretic peptide (NPs; atrial NP, brain NP, C-type NP) system (5). This system is cardioprotective by virtue of its effects on autonomic regulation of heart rate, glucose uptake and oxidation, diuresis and natriuresis, peripheral vasodilation, and anti-fibrotic and anti-hypertrophic effects. Expression of the OT receptor was lower in hearts of ob/ob mice and voluntary running had no effect on this receptor. However, voluntary running inhibited synthesis of ANP, BNP, and CNP in hearts of ob/ob mice (table 1). With this reduction in NP gene expression, we were expecting to observe lower GATA4 levels, since this transcription factor regulates the expression of NPs (6). To our surprise, running increased GATA4 expression in hearts of obese mice, suggesting that NPs are not solely derived from GATA4 and may involve other transcription factors.

The effects of voluntary running of the expression of GLUT4, a key protein regulating glucose uptake in tissues, and on markers of apoptosis, are also interesting. Running decreased the expression of GLUT4 expression in hearts of obese mice compared with sedentary obese mice. Although this could explain the hyperglycemia after voluntary running, comprehensive experiments on glucose uptake in major tissues in the obese mouse are warranted. For apoptosis, we used the ratio of Bax to BCL2 as a simple indicator of cell death and found that hearts from obese running mice exhibiting the highest ratio, again suggesting a form of maladaptation to exercise.

WBF - figure3

Figure 3. The effects of exercise on the cardiac OT-NP system in mouse models of obesity and diabetes. NPR; natriuretic peptide receptor, VEGF; vascular endothelial growth factor, eNOS; endothelial nitric oxide synthase, MHCα; myosin heavy chain-alpha.

 

Key points of this study. Based on our studies using mice with aberrant leptin function, the synthesis of the cardiac OT-NP system is reduced and voluntary running further decreases the synthesis of beneficial NPs and exacerbates common plasma markers of cardiovascular health. With the dramatic transition from physical activity to sedentary living, we have witnessed an unprecedented increase in the prevalence of obesity, diabetes, and cardiometabolic disorders. While the importance of exercise is warranted, it is possible that a certain population of patients; those with severe obesity from leptin deficiency or exhibit leptin resistance from excess adiposity may not respond well to exercise.

 

Table 1. Changes in gene expression in hearts from ob/ob mice after voluntary running.

TB tab1

To highlight the changes in gene expression, values are presented change relative to control. For these data, hearts from sedentary ob/ob mice served control. OTR; oxytocin receptor, ANP; atrial natriuretic peptide, BNP; brain natriuretic peptide, CNP; C-type natriuretic peptide, GATA; binding protein 4, GLUT; glucose transporter protein 4. For all genes, significant differences from control were observed.

 

Acknowledgements

The study was funded a grant from the Diabetes Action Research and Education Foundation (TLB), the Midwestern University Office of Research and Sponsored Programs (TLB), and the Canadian Institute for Health Research (JG, MJ).

 

References

  1. Sennott J, Morrissey J, Standley PR, Broderick TL. 2008. Treadmill exercise fails to reverse defects in glucose, insulin and muscle GLUT4 content in the db/db mouse model of diabetes. Pathophysiology 15:173-79
  2. Brust K, Corbell K, Al-Nakkash L, Babu JR, Broderick TL. 2014 Expression of gluconeogenic enzymes and 11β-hydroxysteroid dehydrogenase type 1in liver of diabetic mice after acute exercise. Diabetes Metab Syndr Obes 7:495-504
  3. Broderick TL, Parrott CR, Wang D, Jankowski M, Gutkowska J. 2012 Expression of GATA4 and downstream genes after exercise training in the db/db mouse. Pathophysiology 19:193-203
  4. Parrott CR, Ghosh P, Tedeschi J, Gunasekara G, Broderick TL. 2011 Urinary corticosterone and normetanephrine levels after voluntary and forced treadmill running in the db/db mouse. J Diab Mell 1:40: 71-8 (doi:10.4236/jdm.2011.14011)
  5. Gutkowska J, Broderick TL, Bogdan D, Wang D, Lavoie JM, Jankowski M. 2009 Downregulation of oxytocin and natriuretic peptides in diabetes: possible implications in cardiomyopathy. J Physiol 587:4725-4736
  6. Broderick TL, Jankowski M, Wang D, Danalache BA, Parrott CR, Gutkowska J. 2012 Downregulation in GATA4 and downstream structural and contractile genes in the db/db mouse heart. ISRN Endocrinology doi:10.5402/2012/736860.

 

Contact:

Tom L. Broderick, Ph.D.

Midwestern University

Department of Physiology

Laboratory of Diabetes and Exercise Metabolism

19555 North 59th Avenue

Glendale, AZ, USA 85308

tel: 623.572.3664

fax: 623.572.3673

e-mail: tbrode@midwestern.edu

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