The effects of sex steroids on energy balance in sheep
thesisposted on 17.02.2017, 04:03 authored by Clarke, Scott Daniel
Obesity develops during periods of prolonged positive energy balance, when energy intake exceeds that of energy expenditure. A greater understanding of the energy balance equation may provide new treatment strategies to combat obesity. The recent confirmation of function brown adipose tissue (BAT) in adult humans has renewed interest in adaptive thermogenesis – a component of energy expenditure whereby energy is dissipated via the production of heat. In BAT, adaptive thermogenesis can be induced by cold or dietary cues, and is mediated via the mitochondrial protein uncoupling protein 1 (UCP1). The thermogenic potential of skeletal muscle has also come into focus due to its similar embryonic origins to BAT, and its high expression of the UCP1 homologue UCP3 – which may also have uncoupling properties. As evidenced by the sexual dimorphism of body composition observed over the lifespan of males and females, the sex hormones (testosterone and estrogen) are thought to play an important role in energy balance. This thesis sought to characterise the role of testosterone and estrogen on energy balance, with a specific focus on adaptive thermogenesis. A model of postprandial temperature elevation in the sheep has previously been established by this laboratory. By restricting food intake to a daily “feeding window” temperature elevations during the postprandial period are entrained. They are measured by temperature recording devices (dataloggers) implanted in the peripheral tissue. Studies conducted in Chapter 3 of this thesis aimed to investigate the influence of regional blood flow on muscle temperature. Results confirm a disassociated of muscle temperature from regional blood flow in a number of paradigms including during entrained postprandial temperature elevation. In addition, intravenous infusion of high but not low doses of the β1/β2-adrenoreceptor agonist isoprenaline increased muscle temperature in addition to blood flow. This effect was attributed to “spillover” activation of β3-adrenoreceptors, the primary peripheral receptor involved in the initiation of adaptive thermogenesis. Collectively, these studies strengthen the case for a role of skeletal muscle thermogenesis in energy balance. Chapter 4 investigated the effects of chronic testosterone replacement in male and female castrate sheep. In males but not females testosterone decreased visceral and muscle temperature at baseline and during the postprandial period. This occurred without affecting the duration or amplitude of the postprandial temperature response. In males, testosterone decreased postprandial plasma glucose levels without changes to activation of skeletal muscle Akt or AMP- activated protein kinase (AMPK), key regulators of glucose uptake and fatty acid oxidation. In males, Expression of UCP1 and UCP3 mRNA was shown to be elevated in muscle taken during the postprandial period compared to baseline. This occurred independent of testosterone treatment. Elevations in UCP1 and UCP3 expression may be responsible for the temperature increases observed. Chapter 5 investigated the effects of acute and chronic estrogen replacement on energy balance in castrated ewes. Acute intramuscular injection of estradiol-17β benzoate (50μg) increased skeletal muscle and visceral fat temperature within one hour, compared to vehicle control. Skeletal muscle blood flow was unaltered during this period. Gluteal fat had no such reaction to acute estradiol-17β benzoate administration. Chronic (7 days) estrogen replacement (3x3cm subcutaneous estradiol-17β benzoate implants) had no effect on tissue temperature compared to control (blank implants). Acute estradiol-17β benzoate replacement was shown to increase activation of Akt and AMPK in skeletal muscle during the postprandial period. This indicates increased fuel utilisation. Repeated intravenous bolus infusion of estrogen (25μg, 3 infusions 3 hours apart) induced temperature elevations in muscle with a greater duration and amplitude than a single intramuscular dose. Expression of UCP mRNA could not account for the estrogen-induced temperature elevations. Collectively, the body of work presented in this thesis strengthens the notion that skeletal muscle is capable of thermogenesis. This work also presents evidence that estrogens and testosterone differentially regulate energy balance. Estrogens regulate energy expenditure acutely but not chronically in a tissue specific manor, increasing tissue temperature and fuel utilisation. Testosterone regulates energy expenditure in males but not females, decreasing tissue temperature. Understanding the actions of sex steroids on energy balance may lead to novel targets in combatting obesity.