Adenine nucleotide release from isolated perfused guinea pig hearts and extracellular formation of adenosine

The quantification of adenine nucleotides released from the heart is hampered by their rapid dephosphorylation to adenosine in the extracellular space catalyzed by highly active ectonucleotidases. To determine the total release of adenine nucleotides from isolated Langendorff-perfused guinea pig hearts, ecto 5'-nucleotidase was effectively blocked by infusion of alpha, beta-methylene-ADP (AOPCP, 50 microM). Adenine nucleotides were measured in the coronary venous effluent by the luciferin-luciferase method after enzymatic rephosphorylation to ATP. In hearts perfused at a constant flow rate (10 ml/min) with normoxic buffer (95% O2, 5% CO2) the release +/- SEM of adenine nucleotides and adenosine was 0.06 +/- 0.01 (n = 11) and 0.04 +/- 0.01 (n = 13) nmol/min. In the presence of AOPCP, the release of adenine nucleotides increased to 0.43 +/- 0.04 nmol/min (n = 9; p less than 0.05), whereas adenosine remained unchanged. Hypoxic perfusion (10% O2, 85% N2, 5% CO2) caused a threefold increase in adenine nucleotide release but a 40-fold increase in adenosine. In contrast, global ischemia (30 seconds) caused adenine nucleotide and adenosine release to rise to similar values of 1.06 +/- 0.10 and 0.80 +/- 0.14 nmol/min (n = 9). Stimulation of hearts with isoproterenol (4 nM) likewise increased the release of adenine nucleotides (0.50 +/- 0.04 nmol/min) and adenosine (0.87 +/- 0.21 nmol/min) (n = 6). To determine the cellular source of adenine nucleotides released from the heart, the coronary endothelial adenine nucleotide pool was selectively prelabeled by [3H]adenosine. Global ischemia increased the specific radioactivity of released adenine nucleotides by 57%. The findings indicate that 1) adenine nucleotides and adenosine are released at the same order of magnitude from the well-oxygenated heart; 2) beta-adrenergic stimulation and ischemia stimulate the release of adenine nucleotides and adenosine, both purines reaching vasoactive concentrations in the effluent perfusate; 3) during hypoxic perfusion only the release of adenosine is greatly enhanced; and 4) the coronary endothelium preferentially contributes to the ischemia-induced adenine nucleotide release.