Adenosine and AMP are substances normally present in the body. Adenosine is also given for the treatment of some heart rhythm problems and may be used to reduce heart damage during heart attacks. The problem in using adenosine is that it is taken up by cells and, therefore, very little of the adenosine we give by vein or in the artery actually reaches the tissue. We propose to use AMP as a way to improve delivery of adenosine. AMP is inactive by itself, but is converted to adenosine in tissue. We hope that by giving AMP we will increase levels of adenosine in tissue. To see if this is true, we will give either adenosine or AMP into the forearm artery while we measure how much adenosine reaches the forearm tissue.