Role of endogenous lipid droplets in lipolysis in rat adipocytes.

Sonication of rat fat cells caused an increase in lipolysis in the absence of lipolytic stimulants (basal lipolysis) and loss of epinephrine responsiveness. Sonication of endogenous lipid droplets from fat cells also induced an increase in lipolysis in the presence of hormone-sensitive lipase (HSL) and loss of epinephrine responsiveness in a cell-free system consisting of lipid droplets and HSL. This increase in lipolysis was found not to be due to an increase in the surface area resulting from sonication, but seemed to be due to a decrease in the phosphatidylcholine concentration on the surface of the lipid droplets. Addition of phosphatidylcholine to the sonicated lipid droplets reduced the hydrolysis of triglyceride by HSL in the cell-free system, consisting of HSL and intact lipid droplets or a lipid emulsion containing phosphatidylcholine increased lipolysis. These results suggest that phosphatidylcholine on the surface of the lipid droplets may be a regulatory factor for lipolysis in fat cells.