Calmodulin-dependent cyclic AMP phosphodiesterase in liver plasma membranes: stimulated by insulin

In vivo insulin consistently stimulates the plasma membrane, high-affinity (low Km) cyclic AMP phosphodiesterase (PDE) from diabetic rat liver or adipose tissue. In vitro stimulation of membrane PDE by insulin has been reported to be inconsistent. Also, the involvement of calmodulin (CaM) in insulin stimulation of PDE has been controversial. In this report, conditions for the isolation of rat liver plasma membranes containing PDE that is sensitive to in vitro insulin stimulation and the involvement of CaM in insulin stimulation of PDE were investigated. In vitro insulin raised the Vmax of the enzyme without altering its apparent Km and was dose dependent. Insulin stimulation was lost after freezing, sonication, solubilization with detergents, or storage of the membranes at 4 degrees C for 4 h after isolation. Insulin stimulation was completely blocked by the CaM antagonist compound 48/80, EGTA, or trifluoperazine. Two isoforms of membrane-bound PDE were separated by ion-exchange chromatography following solubilization of the plasma membranes. The activities of both isoforms were stimulated by exogenous CaM. Plasma membrane PDE eluted after the application of exogenous CaM plus Ca2+. The data support the concept of a critical involvement of CaM in insulin activation of liver membrane PDE.