Effects of nifedipine and diltiazem on coronary reactive hyperaemia

The effects of nifedipine and diltiazem on the reactive hyperaemic response which followed 10 s occlusions of the lift circumflex coronary artery was studied in 18 chronically instrumented awake dogs. Nifedipine (0.010 mg.kg^-1, iv) and diltiazem (0.30 mg.kg^-1, iv) were administered to produce 2 1/2 to 3-fold peak increases in left circumflex coronary blood flow. Coronary flow returned to the control value within 30 min after drug administration, and reactive hyperaemia was examined from 30 min to 2 h after drug administration, when resting coronary blood flow, heart rate, arterial pressure and left ventricular end-diastolic pressure were not different from pretreatment values. Both nifedipine and diltiazem caused blunting of the reactive hyperaemic response following 10 s coronary occlusions with reduction of excess blood flow during reactive hyperaemia from 20.5 ± 2.65 ml during control conditions to 11.4 ± 3.2 ml 30 min after nifedipine (P < 0.001), and from 21.0 ± 3.74 ml to 13.8 ± 3.2 ml 30 min after diltiazem (P < 0.001). Coronary reactive hyperaemia remained depressed for 60 min after nifedipine and throughout the entire 2 h observation period after diltiazem. Measurements of myocardial blood flow with radioactive microspheres demonstrated that neither nifedipine nor diltiazem increased collateral flow during coronary occlusion. Thus, blunting of the reactive hyperaemic response could not be attributed to relief of ischaemia by enhanced collateral blood flow.