The effects of ischaemic preconditioning, diazoxide and 5-hydroxydecanoate on rat heart mitochondrial volume and respiration

Studies with different ATP-sensitive potassium (K-ATP) channel openers and blockers have implicated opening of mitochondrial K-ATP (MitoK(ATP)) channels in ischaemic preconditioning (IPC). It would be predicted that this should increase mitochondrial matrix volume and hence respiratory chain activity. Here we confirm this directly using mitochondria rapidly isolated from Langendorff-perfused hearts. Pre-ischaemic matrix volumes for control and IPC hearts (expressed in mu1 per mg protein +/- S.E.M., n = 6), determined with (H2O)-H-3 and [C-14] sucrose, were 0.67 +/- 0.02 and 0.83 +/- 0.04 (P < 0.01), respectively, increasing to 1.01 +/- 0.05 and 1.18 +/- 0.02 following 30 min ischaemia (P < 0.0 1) and to 1.21 +/- 0.13 and 1.26 +/- 0.25 after 30 min reperfusion. Rates of ADP-stimulated (State 3) and uncoupled 2-oxoglutarate and succinate oxidation increased in parallel with matrix volume until maximum rates were reached at volumes of 1.1 mu1 ml(-1) or greater. The mitoK(ATP) channel opener, diazoxide (50 mum), caused a similar increase in matrix volume, but with inhibition rather than activation of succinate and 2-oxoglutarate oxidation. Direct addition of diazoxide (50 mum) to isolated mitochondria also inhibited State 3 succinate and 2-oxoglutarate oxidation by 30%, but not that of palmitoyl carnitine. Unexpectedly, treatment of hearts with the mitoK(ATP) channel blocker 5-hydroxydecanoate (5HD) at 100 or 300 muM, also increased mitochondrial volume and inhibited respiration. In isolated mitochondria, 5HD was rapidly converted to 5HD-CoA by mitochondrial fatty acyl CoA synthetase and acted as a weak substrate or inhibitor of respiration depending on the conditions employed. These data highlight the dangers of using 5HD and diazoxide as specific modulators of mitoK(ATP) channels in the heart.