RAPID TURNOVER OF THE AMP-ADENOSINE METABOLIC CYCLE IN THE GUINEA-PIGHEART

The intracellular flux rate through adenosine kinase (adenosine --> AM P) in the well-oxygenated heart was investigated, and the relation of the AMP-adenosine metabolic cycle (AMP reversible adenosine) to transm ethylation (S-adenosylhomocysteine [SAH] --> adenosine) and coronary f low was determined. Adenosine kinase was blocked in isolated guinea pi g hearts by infusion of iodotubercidin in the presence of the adenosin e deaminase blocker erythro-9-(2-hydroxy-3-nonyl)adenine (5 mumol/L). Iodotubercidin (1 nmol/L to 4 mumol/L) caused graded increases in veno us effluent concentrations of adenosine, from 8 +/- 3 to 145 +/- 32 nm ol/L (mean+/-SEM, n = 3), and in coronary flow, which increased to max imal levels. Flow increases were completely abolished by adenosine dea minase (5 to 10 U/mL). Interstitial adenosine concentrations, estimate d using a mathematical model, increased from 22 nmol/L during control conditions to 420 nmol/L during maximal vasodilation. The possibility that iodotubercidin caused increased venous adenosine by interfering w ith myocardial energy metabolism was ruled out in separate P-31 nuclea r magnetic resonance experiments. To estimate total normoxic myocardia l production of adenosine (AMP --> adenosine <-- SAH), the time course of coronary venous adenosine release was measured during maximal inhi bition of adenosine kinase with 30 mumol/L iodotubercidin. Adenosine r elease increased more than 15-fold over baseline, reaching a new stead y-state value of 3.4 +/- 0.3 nmol . min-1 . g-1 (n = 5) after 4 minute s. In parallel experiments, the relative roles of AMP hydrolysis and t ransmethylation (SAH hydrolysis) were determined, using adenosine dial dehyde (10 mumol/L) to block SAH hydrolase. In these experiments, aden osine release increased to similar levels of 3.4 +/- 0.5 nmol . min-1 . g-1 (n = 6) during inhibition of adenosine deaminase and adenosine k inase. It is concluded that (1) maximal increases in coronary flow are elicited by increases in interstitial adenosine concentration to appr oximately 400 nmol/L, (2) more than 90% of the adenosine produced in t he heart is normally rephosphorylated to AMP without escaping into the venous effluent, (3) AMP hydrolysis is the dominant pathway for cardi ac adenosine production under normoxic conditions, and (4) the high ra te of adenosine salvage is due to rapid turnover of a metabolic cycle between AMP and adenosine. Rapid cycling may serve to amplify the rela tive importance of AMP hydrolysis over transmethylation in controlling cytosolic adenosine concentrations.