Exogenous Adenosine, Supplied Transiently During Reperfusion, Ameliorates Depressed Endogenous Adenosine Production in the Post-ischemic Rat Heart

Abstract

Adenosine (ADO) is an important endogenous protective metabolite of the heart which also exerts beneficial effects when exogenously supplied before or after ischemia. Previous studies established that after initial massive release of ADO, its endogenous production could be significantly reduced following myocardial ischemia. However, the mechanism and consequences of this phenomenon are not clear. We investigated whether this suppressed endogenous ADO production could be reversed by a transient supply of exogenous ADO during reperfusion. Furthermore, we studied the recovery of mechanical function, coronary flow and myocardial nucleotide levels after this intervention. Three concentrations of ADO were applied: 1μm, which exerts maximal vasodilatation; 30μm, optimal for adenylate resynthesis; and 1 mmwhich exerts a cardioplegic effect. Rat hearts perfused in the Langendorff mode were divided into five groups (n=6–9 per group): all hearts had transient (30-s) ischemia at 20 min (TI-1) and 70 min (TI-3) of perfusion. Group 1 (control) had an additional transient (30-s) ischemia at 45 min (TI-2). Group 2 (ischemic control) had 10-min ischemia at 30 min; groups 3, 4 and 5 also had 10-min ischemia at 30 min but were reperfused for the initial 15 min with 1μm, 30μmor 1 mmADO. Developed tension, coronary flow and coronary effluent purines and pyrimidines were measured throughout the 75-min experimental period. Nucleotide content was evaluated in freeze-clamped hearts at the end of the experiment. Endogenous ADO release to the coronary effluent increased immediately after TI-1 in all groups. This increase was similar after TI-1 and after TI-3 in control, while it was reduced to 30% in ischemic control group. In the 30μmADO group the increase in endogenous ADO release after TI-3 was restored and was similar to that after TI-1. A similar trend was observed with 1 mmADO, while in 1μmgroup recovery of endogenous ADO release after TI-3 was not observed. The highest recovery of developed tension (+s.e.) occurred with 1μmand 30μmADO (72±3% and 72±5% of pre-ischemic value, respectively) compared to 53±5% and 63±5% in ischemic control and 1 mmADO groups, respectively (P<0.05). Coronary flow was restored 30 s after 10 min ischemia in hearts treated with 1μmand 30μmADO, whereas more than 2 min were necessary in ischemic control or 1 mmADO groups. Furthermore, hyperemic response after TI-3 was significantly enhanced in the 1μmor 30μmADO groups. ATP content at the end of reperfusion was highest in the 30μmADO group (18.9±0.5μmol/g dry wt.) as compared to ischemic control, 1μmor 1 mmADO groups (15.2±0.6, 16.4±0.4, and 17.2±0.4μmol/g dry wt. respectively). Concentrations of other nucleotide triphosphates (GTP, UTP and CTP) were similar in all hearts subjected to 10-min ischemia. In summary, depressed endogenous ADO production in the post-ischemic heart could be ameliorated by transient supply of exogenous ADO during reperfusion at 30μmconcentration. This effect was found to be related to the elevation of the adenine nucleotide pool. However, restoration of endogenous ADO production was not necessary for improvement in the recovery of mechanical function by exogenous ADO.