Coronary reactions to cardiac hyperactivity and to hypoxia in isolated perfused heart of rat.

1. Continuous recording of cardiac force of contraction, heart rate and coronary flow from isolated perfused hearts of rats was used to study coronary reactions: (a) to cardiostimulation with noradrenaline, CaCl2, or electrically induced tachycardia; (b) to short duration stoppage of coronary inflow (hypoxia). 2. The heart rate was controlled by electrical pacing. Coronary vasodilatation resulted from cardiostimulation or hypoxia. This coronary response was greater at higher heart rates. 3. In parallel experiments administration of noradrenaline to hearts paced at different frequencies resulted in a rate-dependent elevation of adenosine-3',5'-cyclic monophosphate (cyclic AMP). 4. Duration of hypoxia leading to different degrees of reactive hyperaemia did not change the cardiac cyclic AMP levels. 5. Coronary vasodilatation due to increased cardiac metabolism produced by noradrenaline, Ca2+ or tachycardia was enhanced by the phosphodiesterase inhibitors diazoxide and papaverine while it was inhibited during the administration of prostaglandin E2.6. Reactive hyperaemia was unaffected by diazoxide, papaverine or prostaglandin E2. 7. Catecholamine depletion by reserpine did not influence metabolic coronary dilatation nor the reactive hyperaemic responses. 8. We postulate that there are at least two types of coronary reactions: one in response to hypoxia, 'reactive hyperaemia', and another resulting from cardiac hyperactivity, 'metabolic coronary dilatation'. The latter, blocked by prostaglandin E2 and enhanced by diazoxide or papaverine, would be triggered by cyclic AMP while reactive hyperaemia would result from other mechanisms.