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.