Hormonal regulation of adipocyte enzymes. The effects of epinephrine and insulin on the control of lipase, phosphorylase kinase, phosphorylase, and glycogen synthase.

The effects of epinephrine and insulin on three enzyme systems in adipocytes were investigated with regard to the role of adenosine 3’, 5’-monophosphate (cyclic AMP) in their control and the possibility that differential regulatory mechanisms might operate beyond cyclic AMP. When rat adipocytes were incubated with increasing concentrations of epinephrine (0.01 to 100 pM) elevation of cyclic AMP and glycerol production, activation of phosphorylase and deactivation of glycogen synthase appeared to be closely correlated. Activatability of hormone-sensitive lipase in cell extracts by cyclic AMP-dependent protein kinase decreased with increasing concentrations of epinephrine indicating conversion of the nonactivated to the activated form during incubation of the cells. Activation of lipase in cellfree extracts by cyclic AMP and MgATP was promptly arrested by addition of protein kinase inhibitor. This finding, together with results of previous studies, rules against the involvement of a “lipase kinase” analogous to that of phosphorylase kinase in phosphorylase activation. Phosphorylase kinase activities measured between pH 6.0 and 9.2 were unaffected at any concentration of epinephrine. Moreover, activation of phosphorylase in cell-free extracts was not inhibited by addition of protein kinase inhibitor. When adipocytes were incubated in Ca 2f-free Krebs-Ringer bicarbonate medium, activation of phosphorylase in response to epinephrine was not impaired. Phosphorylase kinase assayed in cell-free extracts did not require Ca*+ for the activation of endogenous phosphorylase. However, some Ca*+ dependence was shown when exogenous skeletal muscle phosphorylase b was used as substrate. Thus, Ca2f does not appear to regulate the catalytic activity of adipocyte phosphorylase kinase, nor could any change in the activation state of this enzyme in response to epinephrine be observed. These results distinguish adipocyte phosphorylase kinase from that in muscle and in heart which requires Ca2+ and is clearly convertible to an activated form by epinephrine. Incubation of adipocytes with glucose (10 II~M) significantly reduced the phosphorylase activity ratio (activity in the