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.