Effects of insulin on glucose uptake by rat hearts during and after coronary flow reduction

We tested the hypothesis that low-flow ischemia increases glucose uptake and reduces insulin responsiveness. Working hearts from fasted rats were perfused with buffer containing glucose alone or glucose plus a second substrate (lactate, octanoate, or beta-hydroxybutyrate). Rates of glucose uptake were measured by 3H2O production from [2-3H]glucose. After 15 min of perfusion at a physiological workload, hearts were subjected to low-flow ischemia for 45 min, after which they were returned to control conditions for another 30 min. Insulin (1 mU/ml) was added before, during, or after the ischemic period. Cardiac power decreased by 70% with ischemia and returned to preischemic values on reperfusion in all groups. Low-flow ischemia increased lactate production, but the rate of glucose uptake during ischemia increased only when a second substrate was present. Hearts remained insulin responsive under all conditions. Insulin doubled glucose uptake when added under control conditions, during low-flow ischemia, and at the onset of the postischemic period. Insulin also increased net glycogen synthesis in postischemic hearts perfused with glucose and a second substrate. Thus insulin stimulates glucose uptake in normal and ischemic hearts of fasted rats, whereas ischemia stimulates glucose uptake only in the presence of a cosubstrate. The results are consistent with two separate intracellular signaling pathways for hexose transport, one that is sensitive to the metabolic requirements of the heart and another that is sensitive to insulin.