Relationship among isoproterenol, cyclic AMP, cyclic AMP-dependent protein kinase and lipolysis in perfused fat cells

By using perfused fat cells the effect of isoproterenol on adenosine 3':5'-monophosphate (cAMP) levels, cAMP-dependent protein kinase activity and lipolysis was studied. An infusion of isoproterenol (10(-7) M) resulted in a time-dependent increase in cAMP levels and protein kinase activity in the fat cells. Both parameters reached maximum values after 5 min of drug infusion, then declined to steady-state values by 10 min. At 60 min, cAMP levels were still significantly (P less than .05) elevated over basal. Dose-response curves were determined for isoproterenol on cAMP levels, protein kinase activity and glycerol release. All three parameters were increased by isoproterenol over the same concentration range (10(-9)--10(-7) M). A plot of cAMP levels or protein kinase activity ratios vs. glycerol release resulted in linear relationship with high degrees of correlation (r = 0.98). The rates at which cAMP levels and glycerol release decline after termination of isoproterenol infusion were studied. Half-life values of 5.8 and 6.9 min were obtained for cAMP levels and glycerol release, respectively. These results support the hypotheses that cAMP, acting through protein kinase, is an intracellular mediator of the lipolytic response to isoproterenol. It is concluded that cAMP is not formed in great excess of that necessary to maximally increase lipolysis.