We also plan to characterize the effect of NO delivery on microvascular perfusion in study subjects with and without sickle cell anemia. Measurements in study subjects will be made prior to and while receiving either NO, nitroglycerin, or placebo. These perfusion measurements will occur at rest and during concentric dorsiflexion exercise. Magnetic resonance imaging (MRI) of lower extremity skeletal muscle enhancement during first passage of intravenously injected gadolinium contrast will be used to evaluate regional skeletal muscle perfusion. Perfusion measurements will be paired with a [31]Phosphorus Magnetic Resonance Spectroscopy ([31]P-MRS) study of the concentration of muscle high energy phosphate compounds. Changes in their levels reflect the energy state of muscle and are dependent on the adequacy of blood flow.