Damage to the mitochondrial electron transport chain (ETC) occurs during ischaemia. Blockade of electron flow in the ETC just before ischaemia with the reversible complex I inhibitor amobarbital protects isolated mitochondria against ischaemic damage and preserves oxidative phosphorylation and cytochrome c content. We hypothesized that brief amobarbital perfusion just before ischaemia would improve cardiac recovery and decrease infarct size after ischaemia and reperfusion (IR) by preserving the mitochondrial redox state and reducing mitochondrial superoxide (O₂^−•) generation, in turn would decrease mitochondrial Ca²⁺ accumulation (mt[Ca²⁺]).

Guinea pig Langendorff-perfused hearts were treated with Krebs Ringer solution (KR; untreated) or amobarbital (2.5 mM) in KR for 1 min immediately before 30 min of no flow, global ischaemia, followed by reperfusion without additional treatment. Cardiac function, mitochondrial NADH, FAD, mt[Ca²⁺], and O₂^−• levels were assessed during the 1 min perfusion period and throughout IR.

Amobarbital perfusion alone before ischaemia significantly increased O₂^−• levels and NADH, without altering FAD, and decreased mt[Ca²⁺]. During ischaemia, mitochondrial NADH was higher, O₂^−• levels were lower, and mt[Ca²⁺] was less elevated in the amobarbital group. On reperfusion O₂^−• levels and mt[Ca²⁺] were significantly reduced, NADH-FAD redox state was preserved and cardiac function was markedly improved in the amobarbital group; infarct size was smaller in the amobarbital group compared to the untreated group.

Temporary blockade of mitochondrial complex I activity by amobarbital protects hearts by reducing production of O₂^−• and mtCa²⁺ loading during IR injury.