Relationships between lipolysis induced by various lipolytic agents and hormone-sensitive lipase in rat fat cells

Norepinephrine induced lipolysis in rat fat cells, in vitro, in a time- and concentration-dependent manner, without concomitantly increasing hormone-s ensitive lipase (HSL) activity. It also induced, time and concentration dep endently, HSL translocation from the cytosol to the lipid droplets in fat c ells. Isoproterenol, forskolin, dibutyryl cyclic AMP, and theophylline also induced lipolysis in fat cells, but did not stimulate HSL activity. These agents also induced HSL translocation from the cytosol to the lipid droplet s in fat cells: about 80% to 90% of all HSL was located in lipid droplets a fter incubation for 1 h. These results suggest that the critical event in l ipolytic activation of fat cells induced by lipolytic agents is not an incr ease in the catalytic activity of HSL but translocation of HSL to its subst rate on the surfaces of lipid droplets in fat cells.