Physiological responses of adipose tissue to different models of physical activity and inactivity

Abstract

Childhood is a critical time of growth, and decisions made during this time lead to positive and negative health consequences, specifically pertaining to adiposity. There is convincing evidence that children who are overweight and obese are at higher risk for adult obesity and risk factors associated with obesity for their whole life course. Puberty is a critical time of adipose tissue expansion, and it is accepted by most that this is the age where the number of adipocytes is set for adulthood. Further it is believed that before puberty, adipose tissue expands by hyperplasia, while after puberty, adipose tissue expands by hypertrophy. The time leading up to puberty represents a critical time. Yet, little is known about the interaction between age, physical activity, and adipose tissue expansion in young, growing animals. Thus, I employed a unique model of childhood obesity to test whether: 1) seven days of decreased physical activity (wheel lock) in 49-56 day old rats and 70-77 day old rats would result in gains in visceral adipose mass seen in age-matched sedentary rats concurrent with an increase in adipocyte size and inflammatory mRNA expression (Chapter 2), and 2) glucocorticoid block would attenuate the visceral adipose depot gains seen with wheel lock in rats at sexual maturity (Chapter 3). Here, I present evidence that the age at which wheel lock occurs influences visceral adipose tissue growth and propose a mechanism for this growth at two ages of pre-pubertal growth (49-56 days of age and 70-77 days of age). Specifically, rats that undergo wheel lock for 7 days at 49-56 days of age show no differences in total body mass, mean adipocyte diameter, or mass of omental, epididymal, and perirenal adipose tissue depots compared to rats that remained physically active. However, rats that undergo wheel lock for 7 days at 70-77 days of age have an increased rate of body mass, fat mass, and % body fat gained and have increased depot mass and total number of adipocytes in epididymal and perirenal adipose tissue, r