Altered creatine kinase enzyme kinetics in diabetic cardiomyopathy. A(31)P NMR magnetization transfer study of the intact beating rat heart

Abstract

To determine whether the decreased contractile performance in diabetic hearts is associated with a reduced energy reserve due to decreased creatine kinase (CK) activity, we measured total CK activity (V(max)) in vitro and CK reaction velocity in vivo using(31)P NMR spectroscopy in isolated perfused rat hearts after 4 and 6 weeks of diabetes. After 4 weeks of diabetes, V(max)decreased by 22% with a larger decrease of CK MB than of CK MM and mitochondrial-CK isoenzymes. There was no further decrease in these parameters after 6 weeks of diabetes. Isovolumic contractile performance of 4 and 6 week diabetic hearts, estimated as rate-pressure product under identical perfusion and loading conditions (EDP set at 6-8 mmHg), was only 50% of that of control. ATP, PCr and total creatine concentrations were not different in control and 4 or 6 weeks diabetic rat hearts. After 4 weeks of diabetes, CK reaction velocity decreased by 22%. This was in proportion to the decline of V(max)and therefore predicted by the rate equation for the CK reaction. However, the further decline in the CK reaction velocity after 6 weeks of diabetes (45%) was greater than that predicted from the CK rate equation (17% decrease), and cannot be explained by substrate control of the enzyme. When hearts were inotropically stimulated by increasing perfusate calcium concentration, CK reaction velocity increased slightly (approximately 15%) in both control and diabetic hearts, thereby maintaining a constant ATP concentration. We conclude that in the diabetic myocardium, the CK reaction velocity decreases but does not limit the availability of high-energy phosphates for contraction over the range of workloads studied. We also conclude that a mechanism(s) in addition to substrate control regulates CK reaction velocity in the 6 week diabetic hearts.