High extracellular glucose impairs cardiac E-C coupling in a glycosylation-dependent manner

Hyperglycemia is a major manifestation of all forms of diabetes mellitus and is associated with increased risk of cardiovascular disease. It is well established that cardiac excitation-contraction (E-C) coupling is adversely affected in diabetic animals such that ventricular myocyte action potential duration is prolonged and intracellular Ca2+ clearing and mechanical relaxation are slowed. We now report that ventricular myocytes incubated in a culture medium containing high extracellular glucose (25.5 mM) also exhibit these same changes in E-C coupling. These effects are not manifested for similar to 24 h after exposure. Furthermore, in the presence of normal glucose (5.5 mM), relaxation is also prolonged by fructose (20 mM, yet is unaffected by equimolar concentrations of nonmetabolizable sugars such as L-glucose and mannitol, implying that the high glucose effects require glucose entry into the cell and metabolic processing. The prolonged relaxation can also be produced by 5 mM glucosamine (an intermediate of glycosylation) and is blocked by 0.5 mu g/ml tunicamycin (an inhibitor of N-linked glycoprotein synthesis). Culturing myocytes with an inhibitor of glycation (10 mM aminoguanidine) does not prevent the high extracellular glucose concentration effects. Thus our data indicate that high extracellular glucose impairs cellular mechanisms contributing to myocardial relaxation and that this impairment may involve glycosylation of nascent proteins.