Alpha-2 adrenergic activation inhibits forskolin-stimulated adenylate cyclase activity and lipolysis in human adipocytes

Forskolin at 10 μM caused a 100-fold increase in the intracellular concentration of cyclic AMP and a 6-fold increase in glycerol release in the human adipocyte. These responses are comparable to those prompted by 10 μM isoproterenol. The effects of forskolin on cyclic AMP and lipolysis were dose-dependent. Alpha-2 adrenergic activation, achieved with 10 μM epinephrine and 30 μM propranolol, significantly inhibited forskolin-stimulated cyclic AMP accumulation and glycerol release, shifting the dose-response curves to the right. Forskolin at 10 μM caused a 4.5-fold increase in the adenylate cyclase activity of human adipocyte membranes. When either isoproterenol or epinephrine (0.1 mM) was combined with forskolin, the magnitude of response was substantially greater than the sum of responses achieved by each agent incubated alone.