Type 1 and 4 PDE activity differs between a murine B lymphoma cell line (Bal-17) and CLL cells. Lineweaver-Burk analysis of PDE enzymatic activity in lysates of CLL cells and Bal-17 cells. (Left) PDE activity in CLL cells was inhibited by 10 μmol/L rolipram but was not augmented by the addition of 0.2 mmol/L calcium and 20 nmol/L calmodulin. (Right) Bal-17 PDE activity is augmented by the addition of calcium and calmodulin. The CLL data are representative of enzymatic assays on three patients.