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

To determine whether the decreased contractile performance in diabetic hear ts is associated with a reduced energy reserve due to decreased creatine ki nase (CI() activity, we measured total CK activity (V-max) in vitro and CK reaction velocity in vivo using P-31 NMR spectroscopy in isolated perfused rat hearts after 4 and 6 weeks of diabetes. After 4 weeks of diabetes, V-ma x decreased by 22% with a larger decrease of CK MB than of CK MM and mitoch ondrial-CK isoenzymes. There was no further decrease in these parameters af ter 6 weeks of diabetes. Isovolumic contractile performance of 4 and 6 week diabetic hearts, estimated as rate-pressure product under identical perfus ion and loading conditions (EDP set at 6-8 mmHg), was only 50% of that of c ontrol. ATP, PCr and total creatine concentrations were not different in co ntrol and 4 or 6 weeks diabetic rat hearts. After 4 weeks of diabetes. CK r eaction 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. Ho wever, the further decline in the CIC reaction velocity after 6 weeks of di abetes (45%) was greater than that predicted from the CK rate equation (17% decrease), and cannot be explained by substrate control of the enzyme. Whe n hearts were inotropically stimulated by increasing perfusate calcium conc entration. CIC reaction velocity increased slightly (similar to 15%) in bot h control and diabetic hearts, thereby maintaining a constant ATP concentra tion. We conclude that in the diabetic myocardium, the CK reaction velocity decreases but does not limit the availability of high-energy phosphates fo r contraction over the range of workloads studied. Mie also conclude that a mechanism(s) in addition to substrate control regulates CK reaction veloci ty in the 6 week diabetic hearts. (C) 1999 Academic Press.