<qa>  What is the receptor responsible for coronary vasodilation?Adenoscan adenosine is an approved pharmacological stress agent indicated as an adjunct to thallium-201 myocardial perfusion scintigraphy in patients unable to exercise adequately. The investigational drug, regadenoson CVT-3146 is a selective A2A adenosine receptor agonist, the receptor responsible for coronary vasodilation, and is being studied for potential use as a pharmacologic stress agent in myocardial perfusion imaging MPI studies. This study will compare the safety and efficacy of regadenoson to that of Adenoscan in detecting reversible myocardial perfusion defects. 