Sudden decrease in physical activity evokes adipocyte hyperplasia in 70- to 77-day-old rats but not 49- to 56-day-old rats.

The cessation of physical activity in rodents and humans initiates obesogenic mechanisms. The overall purpose of the current study was to determine how the cessation of daily physical activity in rats at 49-56 days of age and at 70-77 days of age via wheel lock (WL) affects adipose tissue characteristics. Male Wistar rats began voluntary running at 28 days old and were either killed at 49-56 days old or at 70-77 days old. Two cohorts of rats always had wheel access (RUN), a second two cohorts of rats had wheel access restricted during the last 7 days (7d-WL), and a third two cohorts of rats did not have access to a voluntary running wheel after the first 6 days of (SED). We observed more robust changes with WL in the 70- to 77-day-old rats. Compared with RUN rats, 7d-WL rats exhibited greater rates of gain in fat mass and percent body fat, increased adipocyte number, higher percentage of small adipocytes, and greater cyclin A1 mRNA in epididymal and perirenal adipose tissue. In contrast, 49- to 56-day-old rats had no change in most of the same characteristics. There was no increase in inflammatory mRNA expression in either cohort with WL. These findings suggest that adipose tissue in 70- to 77-day-old rats is more protected from WL than 49- to 56-day-old rats and responds by expansion via hyperplasia.