Isolation and characterization of insulin-stimulated, high-affinity cAMP phosphodiesterases from rat liver

The ability of glucagon to elevate intracellular cAMP concentrations in hepatocytes is antagonized by insulin.l This effect is achieved through inhibition of adenylate cyclasC and activation of specific cAMP phosphodiesterases.
l,3,4 The activation of two specific cAMP phosphodiesterases can readily be assessed using a rapid Percoll gradient fractionation procedure to resolve the various membrane fractions. 1 Both of these enzymes express a high affinity and specificity for cAMP. One of them is a peripheral protein found associated exclusively with the plasma membrane 5 and is activated by its phosphorylation, which is triggered by insulin. 6 The other is associated with an as yet undefined intracellular dense-vesicle fraction.~ The mechanism of its activation has yet to be defined but may involve membrane processing/translocation reactions. 7 The dense-vesicle enzyme can also be activated by glucagon through a process which is distinct from that whereby insulin elicits its activation.