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

Yuichi Hattori, Naoyuki Matsuda*, Junko Kimura†, Toshiteru Ishitani*, Atsushi Tamada, Satoshi Gando*, Osamu Kemmotsu* and Morio Kanno

1. The present work was carried out in order to determine whether a decrease in cardiac Na⁺-Ca²⁺ exchanger (NCX) activity observed in diabetes is caused by a reduction in NCX protein and mRNA levels and to elucidate the significance of this decrease in alterations in [Ca²⁺]_i homeostasis in diabetic cardiomyocytes.

2. The NCX current was significantly reduced in ventricular myocytes freshly 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 of 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 NCX1 in diabetes were prevented by insulin therapy.

5. Although both diastolic and peak systolic [Ca²⁺]_i were not different between the two groups of myocytes, increasing external Ca²⁺ concentration to high levels greatly elevated diastolic [Ca²⁺]_i in diabetic myocytes. Inhibition of NCX by reduction in extracellular Na⁺ by 50 % could produce a marked rise in diastolic [Ca²⁺]_i in control myocytes in response to high Ca²⁺, as seen in diabetic myocytes. However, cyclopiazonic acid, an inhibitor of sarcoplasmic reticulum Ca²⁺ pump ATPase, did not modify the high Ca²⁺-induced changes in diastolic [Ca²⁺]_i in either control or diabetic myocytes.

6. Only in papillary muscles from diabetic rats did the addition of high Ca²⁺ 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 protein and mRNA. We suggest that this impaired NCX function may play an important role in alterations in Ca²⁺ handling when [Ca²⁺]_i rises to pathological levels.

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