Q. A 52-year-old man comes to the clinic because of intermittent chest pain. The pain started two weeks ago, and is relieved with sublingual nitroglycerin. The patient is a previous veteran who lost his left leg while on active duty in Iraq. His medical history is significant for diabetes and smoking. His vital signs are normal. Cardiac and chest examination reveals no abnormalities. Pharmacological stress testing using dipyridamole is ordered. Which of the following describes how cardiac stress is induced in this procedure?