MECHANISMS UNDERLYING DEPRESSED NA+ CA2+ EXCHANGER ACTIVITY IN THE DIABETIC HEART/

Objectives: Depression in Na+/Ca2+ exchanger activity is an important factor in the development of the diabetic cardiomyopathy. Since the me chanism underlying this depression remains unknown, the aim of this st udy was to determine the contribution of hyperglycemia and insulinopen ia towards the observed impairment in Na+/Ca2+ exchanger activity. Met hods: Non-insulin-dependent diabetes was induced in neonatal Wistar ra ts by injection of 90 mg/kg streptozotocin. Na+/Ca2+ exchange in sarco lemmal vesicles and isolated cardiomyocytes was determined by Na+-depe ndent Ca-45(2+) transport. To assess the role of insulin deficiency an d hyperglycemia on Na+/Ca2+ exchanger activity, neonatal cardiomyocyte s were incubated for 3 days in media containing either 5 mM glucose an d 56 U/l insulin (Control), 30 mM glucose and 56 U/l insulin (High glu cose) or 5 mM glucose and 0 insulin (Insulin deficiency). Since hyperg lycemia has been shown to affect protein kinase C activity, Ca2+-depen dent isoforms of protein kinase C were examined in non-diabetic and di abetic heart using hydroxylapatite chromatography. Also examined was N a+/Ca2+ exchanger mRNA levels in diabetic and non-diabetic hearts usin g Northern slot blot analysis. Results: Acute insulin produced a dose- dependent increase in Na+/Ca2+ exchanger activity, which was dramatica lly attenuated in diabetic membrane. Myocytes incubated in media conta ining 30 mM glucose exhibited a 33% reduction in Na+/Ca2+ exchanger ac tivity, while insulinopenia reduced activity by 63%. Exchanger mRNA le vels of the diabetic heart were normal; however, diabetes was associat ed with major changes in protein kinase C activity. Conclusions: Reduc ed Na+/Ca2+ exchanger activity resulting from diabetes, hyperglycemia or insulinopenia may be related to changes in protein kinase C activit y, but is not caused by altered expression of the transporter.