Rate-limiting steps in isoproterenol and forskolin stimulated lipolysis

Using the flask-incubated fat cell system, effects of isoproterenol and forskolin on glycerol release, cyclic AMP levels and protein kinase were studied. Isoproterenol increased cyclic AMP levels, protein kinase activity and glycerol release over the same concentration range (10(-9) M to 10(-6) M). Forskolin also increased all three variables in a concentration-dependent manner (10(-7) M to 10(-4) M). The maximum response for each variable was significantly greater with forskolin than with isoproterenol. A combination of isoproterenol and forskolin resulted in an additional increase in cyclic AMP over forskolin alone, but no significant increase in protein kinase activity or glycerol release. These results support the concepts that the maximum lipolytic response to isoproterenol is limited by the accumulation of cyclic AMP and the maximum lipolytic response to forskolin is limited by some step distal to cyclic AMP production, possibly activation of protein kinase. At high concentrations of forskolin or with a combination of forskolin and isoproterenol, cyclic AMP levels were in excess of those needed to maximally activate protein kinase and lipolysis.