Approaches to the prevention of coronary vascular dysfunction caused by myocardial ischaemia and reperfusion.

The endothelium is an important regulator of coronary vascular tone due to its ability to release potent vasoactive substances such as the vasodilators nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF), prostacyclin (PGI2) and the potent vasoconstrictor endothelin. Endothelial dysfunction has been associated with a number of pathological states such as atherosclerosis, hypertension, diabetes and congestive heart failure. A disturbance of endothelial function may also contribute to the adverse effects that ischaemia and reperfusion exerts on the coronary vasculature. After ischaemia and reperfusion there is usually a selective impairment of endothelium-dependent relaxation in isolated coronary arteries. However, in the intact coronary circulation, there is a general loss of vasodilator reserve as responses to both endothelium-dependent and endothelium-independent agonists are attenuated. The release of vasoconstrictor(s) and plugging of capillaries with leukocytes may contribute to that impairment of the capacity of the coronary circulation to dilate together with the reduction in basal blood flow (no-reflow phenomenon). Ischaemic preconditioning is able to prevent ischaemic damage to the myocardium but the vasculature is less well protected as reperfusion is enhanced but the vasodilator reserve continues to be limited. Pharmacological preservation of vascular function has proved more successful with inhibitors of leukocyte adhesion, calcium channel blockers, endothelin receptor antagonists and inhibitors of oxygen radical generation all offering protection. Further refinement of protocols to preserve endothelial and vascular function after ischaemia will aid reperfusion, enhance vasodilator reserve and maximise recovery of myocardial function.