Coronary Vessels/pathology; Erectile Dysfunction/pathology; Erectile Dysfunction/physiopathology; Penis/blood supply; Prediabetic State/pathology; Prediabetic State/physiopathology; Acetylcholine/pharmacology; Animals; Arteries/pathology; Arteries/physiology; Cholesterol/blood; Coronary Vessels/physiopathology; Endothelium, Vascular/pathology; Endothelium, Vascular/physiopathology; Enzyme Inhibitors/pharmacology; Male; Nitric Oxide/metabolism; Nitric Oxide Synthase Type III/antagonists & inhibitors; Nitric Oxide Synthase Type III/metabolism; Nitroarginine/pharmacology; Obesity/pathology; Obesity/physiopathology; Piperazines/pharmacology; Purines/pharmacology; Rats; Rats, Zucker; Sildenafil Citrate; Sulfones/pharmacology; Superoxides/metabolism; Triglycerides/blood; Vasoconstriction/drug effects; Vasoconstriction/physiology; Vasodilation/drug effects; Vasodilation/physiology; Vasodilator Agents/pharmacology