Self-protection by cardiac myocytes against hypoxia and hyperoxia

Cardiac muscle must maintain a continuous balance between its energy supply and work performed. An important mechanism involved in achievement of this balance is cross talk via chemical signals between cardiac myocytes and th e cardiac muscle vascular system. This has been demonstrated by incubating isolated cardiac myocytes in different concentrations of oxygen and then as saying the conditioned media for vasoactive substances on isolated aortic r ings and small-resistance arteries. With increasing oxygen concentrations a bove 6%, cardiac myocytes produce increasing amounts of angiotensin I, whic h is converted to angiotensin II by the blood vessel. The angiotensin II st imulates vascular endothelial cells to secrete endothelin and increase vasc ular tone. Below 6% oxygen, cardiac myocytes secrete adenosine, which acts directly on vascular smooth muscle to block the effect of alpha-adrenergic agonists and reduce vascular tone. In an intact heart, the net effect of th ese 2 regulatory systems would be the maintenance of oxygen concentration w ithin a narrow range at the cardiac myocytes. By acting as oxygen sensors, cardiac myocytes modulate vascular tone according to the needs of the myocy tes and reduce potential problems of hypoxia and extensive formation of rea ctive oxygen species.