Adenosine is a naturally occurring purine nucleoside that forms
from the breakdown of adenosine triphosphate (ATP). ATP is the
primary energy source in cells for transport systems and many
enzymes. Most ATP is hydrolyzed to ADP, which can be further
dephosphorylated to AMP. Most ADP and AMP that form in the cell
is rephosphorylated in the mitochondria by enzymatic reactions
requiring oxygen. If there are large amounts of ATP hydrolyzed,
and especially if there is insufficient oxygen available (i.e.,
hypoxia), then some of the AMP can be further dephosphorylated
to adenosine by the cell membrane associated enzyme,
5'-nucleotidase. Adenosine can bind to purinergic receptors in
different cell types where it can produce a number of different
physiological actions. One important action is vascular smooth
muscle relaxation, which leads to vasodilation. This is a
particularly important mechanism for matching coronary blood
flow to the metabolic needs of the heart. In coronary vascular
smooth muscle, adenosine binds