In addition, patients will have blood flow studies. During the procedure, they will lie in an adjustable reclining chair for 5 to 6 hours. There will be 20- to 30-minute rests between specific activities and blood samples will be drawn intermittently for testing. Small tubes will be placed in the artery of the forearm at the inside of the elbow. Normal saline will be infused into one tube. A small pressure cuff will be applied to the wrist and a larger cuff to the upper arm. Both cuffs will be attached to an inflation device. A device like a rubber band, a strain gauge, will be placed around the widest part of the forearm. When the pressure cuffs are inflated, blood will flow into the arm, stretching the gauge proportion to blood flow, and information will be recorded. Then light reflected from the patients' hand and the blood flow in the forearm will be measured. Activity of the genes in the white blood cells will be measured as well. Small amounts of sodium nitroprusside, widely used to reduce blood pressure in people with dangerously high blood pressure, will be injected and blood flow will be measured. Later, small amounts of acetylcholine will be injected. It usually causes blood vessels to expand. After that, small amounts of L-NMMA will be injected. It usually decreases local blood flow by blocking the production of nitric oxide in the cells lining the arm's blood vessels. Then acetylcholine combined with L-NMMA will be injected. After that, oxypurinol, an agent taken by many patients to prevent gout, will be injected. The procedures will be repeated, with oxypurinol given along with each of the agents, and the measurement of blood flow in the forearm will be measured after each drug combination.