Albumin is an important factor in the control of serum free fatty acid flux in both male and female mice.

Albumin knockout (Alb-/-) mice exhibit a low plasma free fatty acid (FFA) concentration, but it was not known if the suppressed concentration reflects a lower rate of appearance (Ra) of FFA in the circulation (i.e., lower FFA flux) or if the absence of albumin alters the relationship between FFA flux and concentration. For understanding the role of albumin in FFA transport through the bloodstream, it is not sufficient to rely on FFA concentration data alone. Therefore, we developed a method to study FFA kinetics in Alb-/- mice. Using an albumin-free formulation of [U-13C]palmitate tracer, serum FFA kinetics were tested in Alb-/- and wild-type (WT) mice. Results indicate that the flux of FFA in serum of Alb-/- mice was significantly lower than in WT mice (P < 0.05), while albumin deficiency did not alter the relationship between FFA flux and concentration. Next, to test if suppressed lipolysis might have also been involved in the suppressed FFA kinetics, gene expression of a lipolytic enzyme (adipose triglyceride lipase, Atgl) and a marker of lipolysis (phosphorylation of hormone-sensitive lipase, p-HSL) were measured in adipose tissue. In contrast to the low FFA flux in Alb-/-, both Atgl gene expression and p-HSL protein were significantly higher in adipose tissue of Alb-/- than in WT mice (P < 0.05). Thus, the low FFA flux in Alb-/- appeared to be driven by the absence of albumin's FFA binding functions rather than through regulation of lipolysis, indicating that albumin is an important factor in determining the flux of FFA in circulation.NEW & NOTEWORTHY To improve understanding of the albumin protein's function in vivo, we tested plasma free fatty acid kinetics in albumin knockout mice compared with wild-type mice. Using a new tracer formulation strategy, it was discovered that the appearance rate of free fatty acids in serum is lower in albumin knockout mice than in wild-type mice. The results indicate that albumin is a major controller of free fatty acid kinetics.

Transient elevation of triacylglycerol content in the liver: a fundamental component of the acute response to exercise.

Exercise is well appreciated as a therapeutic approach to improve health. Although chronic exercise training can change metabolism, even a single exercise session can have significant effects upon metabolism. Responses of adipose tissue lipolysis and skeletal muscle triacylglycerol (TAG) utilization have been well appreciated as components of the acute exercise response. However, there are other central components of the physiological response to be considered, as well. A robust and growing body of literature depicts a rapid responsiveness of hepatic TAG content to single bouts of exercise, and there is a remaining need to incorporate this information into our overall understanding of how exercise affects the liver. TAG content in the liver increases during an exercise session and can continue to rise for a few hours afterwards, followed by a fairly rapid return to baseline. Here, we summarize evidence that rapid responsiveness of hepatic TAG content to metabolic stress is a fundamental component of the exercise response. Adipose tissue lipolysis and plasma free fatty acid concentration are likely the major metabolic controllers of enhanced lipid storage in the liver after each exercise bout, and we discuss nutritional impacts as well as health implications. Although traditionally clinicians would be merely concerned with hepatic lipids in overnight-fasted, rested individuals, it is now apparent that the content of hepatic TAG fluctuates in response to metabolic challenges such as exercise, and these responses likely exert significant impacts on health and cellular homeostasis.