Measuring respiratory activity of adipocytes and adipose tissues in real time.

The realization that obesity and its associated diseases have become one of modern society's major challenges to the health of the world's population has fueled much effort to understand white adipocyte biology and elucidate pathways to increase energy expenditure. One strategy has been to increase the oxidative capacity and activity of the adipocytes themselves. This has the advantage that free fatty acids (FAs) would not be released into the circulation in copious amounts, which can have detrimental effects. This is particularly true for obese individuals, who often already display severe dyslipidemia, putting them at increased risk for cardiovascular diseases. It was recently discovered that adult humans, in addition to infants, possess active brown adipocytes, characterized by expression of the mitochondrial electron gradient dissipater uncoupling protein 1 (UCP1). This has generated renewed interest in finding ways to "convert" or "adapt" white adipocytes into a more brown adipocyte-like state by increasing mitochondrial content and expression of UCP1 and activating UCP1 via lipolysis-mediated free FAs. Another approach to consider is elevating the activity of the not insignificant amount of mitochondria found in white adipocytes. The invention of the XF Flux Analyzer by Seahorse Bioscience has revolutionized this line of research as it allows for real-time measurements of respiration in multiple samples simultaneously. In this chapter, we describe our approaches and experience with employing this technology to study the metabolism of mouse and human primary and immortalized cells and mouse white adipose tissue.