Frogs (Rana catesbeiana) were obtained from Charles Sullivan, Inc., and kept in controlled lighting conditions (12 hours dark–light) for 2 weeks before use. Animals were treated in accordance with ARVO guidelines, and protocols were approved by the institutional animal care and use committee. Bovine retinas were purchased from W. L. Lawson, Inc. (Lincoln, NE); E4021 was a gift from Eisai Co., Ltd. (Tokyo, Japan); vardenafil was provided by Bayer Pharmaceuticals; and sildenafil and tadalafil were synthesized. All other reagents were from Sigma-Aldrich (St. Louis, MO). All PDE inhibitors were prepared as stock solutions in DMSO and diluted in buffer before use, so that the final concentration of DMSO was always <1%. Ringer’s solution consisted of (in mM): 105 NaCl, 2 KCl, 2 MgCl2, 1 CaCl2, 5 glucose, and 10 HEPES (pH 7.5). The ROS homogenization buffer contained (in mM): 100 Tris (pH 7.5), 10 MgCl2, 0.5 EDTA, 1 dithiothreitol, 0.5 mg/mL BSA, and mammalian protease inhibitor cocktail (Sigma-Aldrich). PDE assay buffer contained 20 mM Tris (pH 7.5), 10 mM MgCl2, and 0.5 mg/mL BSA.