Diminished function and expression of the cardiac Na+-Ca2+ exchanger in diabetic rats: implication in Ca2+ overload

1. The present work was carried out in order to deter mine whether a decrea se in cardiac Na+-Ca2+ exchanger (NCX) activity observed in diabetes is cau sed by a reduction in NCX protein and mRNA levels and to elucidate the sign ificance of this decrease in alterations in [Ca2+](i) homeostasis in diabet ic cardiomyocytes. 2. The NCX current was significantly reduced in ventricular myocytes freshl y isolated from streptozotocin-induced diabetic rat hearts, and its current density was about 55% of age matched controls. 3. Diabetes resulted in a 30% decrease in cardiac protein and mRNA levels o f NCX1, a NCX isoform which is expressed at high levels in the heart. 4. The reduced NCX current, and the decreased protein and mRNA levels of NC X1 in diabetes were prevented by insulin therapy. 5. although both diastolic and peak systolic [Ca2+](i) were not different b etween the two groups of myocytes, increasing external Ca2+ concentration t o high levels greatly elevated diastolic [Ca2+](i) in diabetic myocytes. In hibition of NCX by reduction in extracellular Na+ by 50% could produce a ma rked rise in diastolic [Ca2+](i) in control myocytes in response to high Ca 2+ as seen in diabetic myocytes. However, cyclopiazonic acid, an inhibitor of sarcoplasmic reticulum Ca2+ pump ATPase, did not modify the high Ca2+-in duced changes in diastolic [Ca2+](i) in either control or diabetic myocytes . 6. Only in papillary muscles from diabetic rats did the addition of high Ca 2+ cause a marked rise in resting tension signifying a partial contracture that was possibly due to an increase in diastolic [Ca2+](i). 7. In conclusion, the diminished NCX function in diabetic myocytes shown in this: study results in part from the decreased levels. of cardiac NCX prot ein and mRNA. We suggest that this impaired NCX function may play an import ant role in alterations in Ca2+ handling when [Ca2+](i) rises to pathologic al levels.