Unchanged Protein Level of Ryanodine Receptor but Reduced [<SUP>3</SUP>H] Ryanodine Binding of Cardiac Sarcoplasmic Reticulum from Diabetic Cardiomyopathy Rats

Unchanged Protein Level of Ryanodine Receptor but Reduced [<SUP>3</SUP>H] Ryanodine Binding of Cardiac Sarcoplasmic Reticulum from Diabetic Cardiomyopathy Rats

<P> The ryanodine receptor, a Ca<SUP>2&#8290;</SUP> release channel of the sarcoplasmic reticulum (SR), is responsible for the rapid release of Ca<SUP>2&#8290;</SUP> that activates cardiac muscle contraction. In the excitation-contraction coupling cascade, activation of SR Ca<SUP>2&#8290;</SUP> release channel is initiated by the activity of sarcolemmal Ca<SUP>2&#8290;</SUP> channels, the dihydropyridine receptors. Previous study showed that the relaxation defect of diabetic heart was due to the changes of the expressional levels of SR Ca<SUP>2&#8290;</SUP>ATPase and phospholamban. In the diabetic heart contractile abnormalities were also observed, and one of the mechanisms for these changes could include alterations in the expression and/or activity levels of various Ca<SUP>2&#8290;</SUP> regulatory proteins involving cardiac contraction. In the present study, underlying mechanisms for the functional derangement of the diabetic cardiomyopathy were investigated with respect to ryanodine receptor, and dihydropyridine receptor at the transcriptional and translational levels. Quantitative changes of ryanodine receptors and the dihydropyridine receptors, and the functional consequences of those changes in diabetic heart were investigated. The levels of protein and mRNA of the ryanodine receptor in diabetic rats were comparable to these of the control. However, the binding capacity of ryanodine was significantly decreased in diabetic rat hearts. Furthermore, the reduction in the binding capacity of ryanodine receptor was completely restored by insulin. This result suggests that there were no transcriptional and translational changes but functional changes, such as conformational changes of the Ca<SUP>2&#8290;</SUP> release channel, which might be regulated by insulin. The protein level of the dihydropyridine receptor and the binding capacity of nitrendipine in the sarcolemmal membranes of diabetic rats were not different as compared to these of the control. In conclusion, in diabetic hearts, Ca<SUP>2&#8290;</SUP> release processes are impaired, which are likely to lead to functional derangement of contraction of heart. This dysregulation of intracellular Ca<SUP>2&#8290;</SUP> concentration could explain for clinical findings of diabetic cardiomyopathy and provide the scientific basis for more effective treatments of diabetic patients. In view of these results, insulin may be involved in the control of intracellular Ca<SUP>2&#8290;</SUP> in the cardiomyocyte via unknown mechanism, which needs further study.