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).