To elucidate the physiological role of the
AMP-
adenosine metabolic cycle and to investigate the relation between
AMP and
adenosine formation, the O2 supply of isolated guinea pig hearts was varied (95% to 10% O2). The net
adenosine formation rate (
AMP-->
adenosine) and coronary venous effluent
adenosine release rate were measured; free cytosolic
AMP was determined by 31P-nuclear magnetic resonance. Switching from 95% to 40% O2 increased free
AMP and
adenosine formation 4-fold, whereas free cytosolic
adenosine and venous
adenosine release rose 15- to 20-fold. In the
AMP range from 200 to 3000 nmol/L, there was a linear correlation between free
AMP and
adenosine formation (R2 = .71); however,
adenosine release increased several-fold more than formation. At 95% O2, only 6% of the
adenosine formed was released; however, this fraction increased to 22% at 40% O2, demonstrating reduced
adenosine salvage. Selective blockade of
adenosine deaminase and
adenosine kinase indicated that flux through
adenosine kinase decreased from 85% to 35% of
adenosine formation in
hypoxia. Mathematical model analysis indicated that this apparent decrease in
enzyme activity was not due to saturation but to the inhibition of
adenosine kinase activity to 6% of the basal levels. The data show (1) that
adenosine formation is proportional to the
AMP substrate concentration and (2) that
hypoxia decreases
adenosine kinase activity, thereby shunting myocardial
adenosine from the salvage pathway to venous release. In conclusion, because of the normal high turnover of the
AMP-
adenosine metabolic cycle,
hypoxia-induced inhibition of
adenosine kinase causes the amplification of small changes in free
AMP into a major rise in
adenosine. This mechanism plays an important role in the high sensitivity of the cardiac
adenosine system to impaired oxygenation.