Enzyme Assay: Materials: All chemicals are available from Sigma-Aldrich (St. Louis, Mo.) except for IMAP reagents (reaction buffer, binding buffer, FL-GMP and IMAP beads), which are available from Molecular Devices (Sunnyvale, Calif.).Assay: The following phosphodiesterase enzymes may be used: 3',5'-cyclic-nucleotide-specific bovine brain phosphodiesterase (Sigma, St. Louis, Mo.) and recombinant full length human PDE1A and PDE1B (r-hPDE1A and r-hPDE1B, respectively) which may be produced e.g., in HEK or SF9 cells by one skilled in the art. The PDE1 enzyme is reconstituted with 50% glycerol to 2.5 U/ml. One unit of enzyme will hydrolyze 1.0 umole of 3',5'-cAMP to 5'-AMP per min at pH 7.5 at 30 C. One part enzyme is added to 1999 parts reaction buffer (30 uM CaCl2, 10 U/ml of calmodulin (Sigma P2277), 10 mM Tris-HCl pH 7.2, 10 mM MgCl2, 0.1% BSA, 0.05% NaN3) to yield a final concentration of 1.25 mU/ml. 99 ul of diluted enzyme solution is added into each assay.