Intracellular Levels of Na super(+) and TTX-sensitive Na super(+) Channel Current in Diabetic Rat Ventricular Cardiomyocytes

• Published in: Cardiovascular toxicology Vol. 13; no. 2; pp. 138 - 147
• Main Authors: Bilginoglu, Ayca, Kandilci, Hilmi Burak, Turan, Belma
• Format: Journal Article
• Language: English
• Published: 01-06-2013
• ISSN: 1530-7905; 1559-0259
• DOI: 10.1007/s12012-012-9192-9

Intracellular Na super(+) ([Na super(+)] sub( i )) is an important modulator of excitation-contraction coupling via regulating Ca super(2+) efflux/influx, and no investigation has been so far performed in diabetic rat heart. Here, we examined whether any change of [Na super(+)] sub( i ) in paced cardiomyocytes could contribute to functional alterations during diabetes. Slowing down in depolarization phase of the action potential, small but significant decrease in its amplitude with a slight depolarized resting membrane potential was traced in live cardiomyocytes from diabetic rat, being parallel with a decreased TTX-sensitive Na super(+) channel current (I sub(Na)) density. We recorded either [Na super(+)] sub( i ) or [Ca super(2+)] sub( i ) by using a fluorescent Na super(+) indicator (SBFI-AM or Na-Green) or a Ca super(2+) indicator (Fura 2-AM) in freshly isolated cardiomyocytes. We examined both [Na super(+)] sub( i ) and [Ca super(2+)] sub( i ) at rest, and also [Na super(+)] sub( i ) during pacing with electrical field stimulation in a range of 0.2-2.0 Hz stimulation frequency. In order to test the possible contribution of Na super(+)/H super(+) exchanger (NHE) to [Na super(+)] sub( i ), we examined the free cytoplasmic [H super(+)] sub( i ) changes from time course of [H super(+)] sub( i ) recovery in cardiomyocytes loaded with SNARF1-AM by using ammonium prepulse method. Our data showed that [Na super(+)] sub( i ) in resting cells from either diabetic or control group was not significantly different, whereas the increase in [Na super(+)] sub( i ) was significantly smaller in paced diabetic cardiomyocytes compared to that of the controls. However, resting [Ca super(2+)] sub( i ) in diabetic cardiomyocytes was significantly higher than that of the controls. Here, a lower basal pH sub( i ) in diabetics compared with the controls correlates also with a slightly higher but not significantly different NHE activity and consequently a similar Na super(+) loading rate at resting state with a leftward shift in pH sensitivity of NHE-dependent H super(+)-flux. NHE protein level remained unchanged, while protein levels of Na super(+)/K super(+) ATPase and Na super(+)/Ca super(2+) exchanger were decreased in the diabetic cardiomyocytes. Taken together, the present data indicate that depressed I sub(Na) plays an important role in altered electrical activity with less Na super(+) influx during contraction, and an increased [Ca super(2+)] sub( i ) load in these cells seems to be independent of [Na super(+)] sub( i ). The data with insulin treatment suggest further a recent balance between Na super(+) influx and efflux proteins associated with the [Na super(+)] sub( i ), particularly during diabetes.