Normothermic transfer times up to 3 min will not precondition the isolated rat heart

Isolated, perfused heart preparations suffer an inevitable peri-operative delay (POD) before retrograde perfusion restores coronary flow. By varying this ischaemic period we investigated the threshold of POD-induced inadvertent preconditioning (PC) in the rat heart. Hearts subjected to POD at 37°C increasing from 1, 2, 3, 5, 10 up to 15 min prior to 20 min retrograde perfusion were further subjected to 30 min global, normothermic ischaemia and 30 min reperfusion (index I/R). The functional recovery was 32 ± 4.1% in hearts subject to 1 min POD. After 3 min POD functional recovery started to improve and peaked at 10 min POD (78 ± 7.1%, P < 0.001). At 4°C functional recovery started to improve after 5 min POD and peaked at 10 min POD. To demonstrate that the POD-mediated protection was true PC, two conventional models of PC were established. In both models, hearts were retrogradely perfused within 1 min POD prior to a standard PC protocol (one episode of 10 min ischaemia, or four episodes of 5 min ischaemia). In the conventional PC models protection against the index I/R was abolished using 100 μM 5-hydroxydecanoate (5-HD), the mitrochondrial K(ATP) channel inhibitor. Likewise, 10 min POD-mediated recovery at 37°C (70 ± 3.2%) was reversed by 100 μM 5-HD perfusion (36 ± 5.9%; NS vs 2 min POD). We conclude: (1) the threshold for PC is greater than 3 min at 37°C and greater than 5 min at 4°C; (2) blockade of the mitochondrial K(ATP) channel abolishes protection in three models of PC in the rat heart, including prolonged POD.