Manipulations in glycogen metabolism and the failure to influence infarct size in the ischaemic rabbit heart

Abstract

Background and objective: Myocardial ischaemic preconditioning is characterized by a reduction in the rate of glycolysis. Brief myocardial ischaemia also reduces the glycogen content of the heart. The first objective was to determine whether augmenting glucose oxidation by activation of the pyruvate dehydrogenase complex would prevent the infarct limitation of ischaemic preconditioning. The second part of the study evaluates whether glycogen depletion before ischaemia mimics the infarct-limiting effect of ischaemic preconditioning.

Methods: Dichloroacetate (300 + 150 mg kg⁻¹), an activator of the pyruvate dehydrogenase complex, was administered intravenously in the anaesthetized open-chest rabbit. All animals underwent 45 min of regional ischaemia and 3 h of reperfusion. Ischaemic preconditiong was elicited by 5 min of coronary occlusion. Control rabbits, those with ischaemic preconditioning with no dichloroacetate, received a saline vehicle. An isolated perfused rabbit heart model was employed to test the second hypothesis. Hearts were depleted of glycogen by perfusing them with a substrate-free buffer. Infarction was assessed by triphenyl tetrazolium chloride and area at risk determined with fluorescent particles.

Results: (a) Pyruvate dehydrogenase complex activation experiments. Treatment with dichloroacetate alone did not alter infarct size (58 ± 7% control vs. 60 ± 5% dichloroacetate). Addition of dichloroacetate did not attenuate the infarct-limiting effect of ischaemic preconditioning as evidenced by a similar reduction in infarct size in the ischaemic preconditioning group (22 ± 5%) and in the ischaemic preconditioning + dichloroacetate group (27 ± 7%). (b) Glycogen depletion experiments. Compared with control hearts with a normal glycogen content (4.84 ± 0.15 mg g⁻¹ wet weight), glycogen depleted and ischaemic preconditioning hearts had reduced glycogen content before ischaemia (2.15 ± 0.26, 1.62 ± 0.17 mg g⁻¹ wet weight, respectively; P < 0.01). Glycogen depletion did not reduce infarct size: 25.0 ± 4.5% cf. 27.9 ± 3.4% in the control group. However, ischaemic preconditioning resulted in a significant reduction of infarct size (11.5 ± 2.3% vs. 27.9 ± 3.4% control; P < 0.01).

Conclusions: Augmentation of oxidative glycolysis by dichloroacetate in in situ rabbit hearts does not alter the effect of ischaemic preconditioning, and glycogen depletion in the isolated rabbit heart does not influence infarct size after subsequent coronary occlusion.