Low insulin and high glucose induce abnormal relaxation in cultured adult rat ventricular myocytes

One of the most prominent myocardial defects associated with diabetes is abnormal diastole. We have recently reported that this dysfunction involves prolonged relaxation (relengthening) in isolated ventricular myocytes that occurs within days after the induction of diabetes. The present study was designed to evaluate the role of insulin and glucose in the etiology of this dysfunction with a serum-free myocyte culture system. Adult rat ventricular myocytes were cultured for 1-4 days in a "diabetic-like" medium containing five times less insulin and approximately five times more glucose than in our normal medium. Mechanical properties and Ca2+ transients (fura 2) were evaluated with a high-resolution (120-Hz) video-based edge-detection/spectro-fluormetric system. The cells were held stimulated to contract at slow and physiologically relevant rates, and indexes of contraction and relaxation were evaluated. Relengthening was markedly longer in myocytes cultured in low-insulin-high-glucose (LIHG) medium compared with those in normal medium, whereas contraction was unaffected. Intracellular Ca2+ transients showed slower rates of decay in myocytes cultured in LIHG medium. These data demonstrate that maintaining normal ventricular myocytes in an LIHG environment prolongs relaxation in a manner similar to the effects of in vivo diabetes. Furthermore, the abnormal relaxation is inducible in 1 day, suggesting rapid alterations in processes regulating relaxation, which likely include impaired Ca2+ sequestration and/or extrusion.