Does interstitial adenosine mediate acute hibernation of guinea pig myocardium?

The aim was to test the role of interstitial adenosine in protective downregulation of myocardial energy demand during myocardial hibernation.

Isolated working guinea pig hearts, perfused with glucose fortified Krebs-Henseleit, were subjected to 60 min global low flow ischaemia followed by 30 min reperfusion. Left ventricular performance was assessed from heart rate-developed pressure product and pressure-volume work. Cytosolic energy level was indexed by creatine phosphate and ATP phosphorylation potentials measured in snap frozen myocardium. Lactate and purine nucleosides (adenosine, inosine) were measured in venous effluent.

When coronary flow was lowered by 80% for 60 min, heart rate-pressure product and pressure-volume work fell 87% and 75%, respectively, and stabilised at these low levels, but fully recovered when flow was restored. Myocardial ATP phosphorylation potential fell by 67% during the first 10 min of ischaemia, but subsequently recovered to preischaemic levels despite continuing ischaemia, indicating down-regulation of myocardial energy demand. Lactate release increased about 10-fold during ischaemia and remained increased until reperfusion. Purine nucleoside release varied reciprocally with phosphorylation potential, peaking at 10 min of ischaemia, then gradually returning to the preischaemic level during the subsequent 50 min of ischaemia. The ecto 5'-nucleotidase inhibitor alpha,beta-methylene adenosine 5'-diphosphonate (50 microM) decreased ischaemic purine nucleoside release by 41%, but did not attenuate postischaemic contractile recovery. The unspecific adenosine receptor antagonist 8-p-sulphophenyl theophylline (8-SPT, 20 microM) doubled ischaemic lactate release and lowered coronary venous purine nucleoside release by 21%. 8-SPT increased phosphorylation potential at 10 min ischaemia relative to untreated hearts, but blunted the subsequent rebound of phosphorylation potential. 8-SPT treatment during ischaemia resulted in a significantly higher cytosolic phosphorylation potential at 30 min of reperfusion, but did not affect postischaemic contractile function.

We conclude that activation of adenosine receptors results in recovery of cytosolic energy level of moderately ischaemic working myocardium, but this energetic recovery is not solely responsible for postischaemic contractile recovery.