Role of albumin in hormone-stimulated lipolysis

Abstract

The addition of epinephrine (10 −6 M) to isolated fat cells resulted in increased lipolytic activity. In the presence of 1, 2 and 4% bovine serum albumin (BSA), lipolytic rates were linear for a 60 min period of time. Rates of lipolysis were increased by increasing the BSA concentration. In other experiments, the effect of glucose (10 mg/ml) was tested on basal, epinephrine-stimulated, and theophylline-stimulated lipolytic activity. In the absence of BSA, glucose resulted in a nearly 3-fold increase in lipolytic rates under all three conditions. In the presence of BSA (4%), the addition of glucose resulted in a further increase in lipolytic activity. The time course of cyclic AMP accumulation following the addition of epinephrine (3 × 10 −6M) was determined in the absence and presence of BSA (4%). In the presence of BSA, accumulation of the cyclic nucleotide continued for a longer period of time and reached a value nearly twice that in the absence of BSA. In another experiment, cyclic AMP accumulation at 5 min following the addition of epinephrine was increased by the addition of 1% BSA. An additional increase was observed by the addition of 2% BSA. When the BSA concentration was increased to 4%, no additional increase was observed. BSA (4%) was also shown to increase the lipolytic response to dibutytyl cyclic AMP. It was concluded that the presence of albumin promoted lipolysis by preventing a negative feedback effect of free fatty acids on cyclic AMP accumulations at some point distal to the production of cyclic AMP. Additionally, it was suggested that albumin facilitates the lipolytic process by some mechanism in addition to reducing intracellular free fatty acid levels.