Role of adenosine kinase and AMP deaminase in the regulation of cardiac purine release

To analyze the relation between cardiac energy status, adenosine formation, and purine release, reliable measurements of the cytosolic concentrations of ATP, ADP, AMP, and adenosine are required. Based on the creatine kinase and myokinase equilibrium, ADP and AMP are determined by P-31 nuclear magne tic resonance spectroscopy, whereas free cytosolic adenosine is measured by the S-adenosyl-homocysteine (SAH) technique. Combining these methods with efflux measurements, selective enzyme blockade and a comprehensive model an alysis enables a description of both concentrations and flux rates in purin e metabolism. In the well-oxygenated heart, adenosine is predominantly form ed intracellularly from AMP, but also from S-adenosyl-homocysteine. Net ade nosine formation (2.3 nmol/min per g) exceeds coronary venous release (0.07 nmol/min per g) more than 30-fold, because most of the adenosine formed is rephosphorylated by adenosine kinase. This enzyme maintains a low intracel lular adenosine and limits both adenosine release and deamination to inosin e. In fact, inosine is mainly formed from IMP (1.8 nmol/ min per g) the pro duct of AMP deaminase. Inosine, hypoxanthine, xanthine, and uric acid (1.1, 0.4, 0.2, 1.4 nmol/min per g) are the main purine catabolites released. In the oxygen-limited heart, energy status is impaired and AMP increased. U nder these conditions, a linear relation between AMP (200-3,000 nmol/liter) , net adenosine formation, as well as net inosine formation is observed. It is, thus, the AMP substrate concentration that directly controls adenosine formation by cytosolic 5'-nucleotidase and most likely flux through AMP de aminase. Hypoxia-induced inhibition of adenosine kinase shunts adenosine fr om the salvage pathway to venous release and causes the amplification of sm all changes in AMP into a major rise in adenosine. This mechanism plays an important role in the high sensitivity of the cardiac adenosine system to i mpaired oxygenation. Drug Dev. Res. 45:295-303, 1998. (C) 1998 Wiley-Liss, Inc.