Mutants Y751A, D950A, and F1004A had reduced sensitivity to milrinone (K(i) changed from 0.66 microM for the recombinant PDE3A to 7.5 to 156 microM for the mutants), and diminished sensitivity to << cilostazol >> (K(i) of the mutants were 18- to 371-fold higher than that of the recombinant [[ PDE3A ]]).