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).