Increased glycolysis during ischaemia mediates the protective effect of glucose and insulin in the isolated rat heart despite the presence of cardiodepressant exogenous substrates
Background: Exogenous insulin is known to protect against ischaemia-reperfusion injury of the myocardium. We investigated whether these benefits of insulin could be explained by increased glycolysis during the ishaemic period, even in the presence of potentially cardiodepressant substrates such as fatty acids and lactate. Increased cardiac output during post-ischaemic reperfusion was used as a marker of protection by insulin. Methods: Isolated hearts from fasted rats were subjected to 5 minutes of Langendorff perfusion followed by a 15-minute working heart pre-ischaemic perfusion with glucose (11 mmol/l), followed by a 30-minute 0.2 ml/min low-flow Langendorff ischaemic period, using glucose (11 mmol/l) or glucose plus fatty acids (1 mmol/l) or lactate (10 mmol/l), or lactate plus glucose as substrates. During reperfusion, the hearts were again perfused with glucose. Insulin (1 mU/ml), when added, was present throughout the experimental protocol. Results: Post-ischaemic reperfusion cardiac output recovery was depressed in hearts perfused with glucose plus fatty acids or with lactate alone, when compared with glucose alone (p < 0.01). In each case, cardiac output improved (p < 0.01) with insulin pretreatment despite added fatty acids, or in hearts perfused with lactate alone during ischaemia. Improved cardiac output could be linked to improved glucose uptake (p < 0.05) during ischaemia and increased pre-ischaemic glycogen stores (p < 0.01). In hearts perfused with lactate alone, increased glycolysis occurred via glycogen breakdown as confirmed by increased cardiac tissue lactate levels at the end of the ischaemic period. Conclusion: Insulin treatment before ischaemia and throughout ischaemia and reperfusion, abolished the depressant effects of fatty acids added to glucose or of lactate alone, on reperfusion cardiac output. The consistent feature common to the protective effects of insulin in the various conditions tested was increased glycolysis, whether achieved by increased glucose uptake during the ischaemic period or by glycogen preloading. Increased glycolysis could also explain the protective effect of glucose added to lactate in the absence of insulin.