Qualifying patients who are scheduled for angiogram with endothelial function testing are approached to participate in this protocol (consent form reviewed with patient). Those who are willing to participate will have additional measurements taken during their angiogram as part of this protocol. Patients who are diagnosed with Endothelial Dysfunction as a result of a clinically indicated angiogram will be eligible to participate in a 6 month medication trial (Atrasentan vs. placebo - randomized, blinded trial). Patients are monitored closely during the 6 month trial via phone calls and blood testing for assessment. At the end of the 6 months they return to Mayo for a repeat angiogram to assess endothelial function for any changes or improvements.