Intracellular calcium levels are unchanged in the diabetic heart

The quality control indices of myocyte isolation (viability, yield, survival time, cell response, etc.) suggest that the adult rat myocyte model is stable and useful in Ca2+i measurements and functional studies at the cellular level. Moreover, diabetic cardiomyocytes are a valuable model for studying cellular functions of the diabetic heart as they retain most of the features of cardiac dysfunction of intact rat. Data from our studies indicate that the basal Ca2+i in both quiescent and electrically-stimulated cells is not changed. Thus, resting levels of Ca2+i and basal Ca2+i transients may not reflect the abnormalities observed in diabetes until the system is challenged by certain stimuli. Ca2+i responses to isoproterenol are depressed in both resting and stimulated diabetic cells. This suggests an alteration in the beta-adrenergic pathway, possibly related to the beta-adrenoceptor deficiency reported in the diabetic heart. SR Ca-ATPase is also involved in the isoproterenol-induced Ca2+i changes. Moreover, the decreased maximum response to 8-bromo-cAMP provides evidence of a post-receptor alteration in the pathway. Diabetic myocytes are more sensitive to ouabain, whereas the maximum response to ouabain was depressed. This may be the result of depressed Na-K ATPase and increased Na+i. In diabetic myocytes, rapid cooling contractures and caffeine contractures are depressed, whereas caffeine-induced Ca2+ transients are decreased. Ryanodine binding suggests a decreased number of high-affinity binding sites in the SR of diabetic myocytes. Additionally, there are indications that SR releasable calcium is reduced and that the major functions of SR, notably uptake, release and storage, may be depressed in diabetic myocytes. Finally, L-type Ca(2+)-channels are quantitatively and qualitatively altered in diabetes. Insulin treatment normalizes most of the diabetes-induced changes in cardiomyocytes, suggesting that metabolic alterations due to insulin deficiency play an important role in diabetic cardiomyopathy. Results from several studies show that in diabetes the function of major organelles which handle Ca2+i in myocytes is depressed, which in turn causes the alteration of Ca2+i mobilization in myocytes. Different second messenger systems involved in E-C coupling may also be altered due to the metabolic impairments. The rapid increase in our understanding of the pathophysiology of calcium homeostasis in cardiomyocytes will be forthcoming as the powerful new tools of molecular and structural biology are used to investigate the regulation of the Ca2+ transport system.