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Department of Physiology, University of Adelaide, North Terrace, Australia.
1. A putative function of the transient outward current (ITO) in cardiac myocytes is to modulate the shape of the action potential and, consequently, cardiac contractility. In addition, it has been suggested that this current may help protect against arrhythmias during periods of cardiac ischaemia. In our investigation of the possible anti-arrhythmic action of ITO, we have examined its response to metabolic inhibition and oxidant stress. 2. Whole-cell recordings were obtained from rat atrial myocytes using standard patch-clamp techniques. Inhibition of metabolism, using 10 mM 2-deoxy-D-glucose (2-DG) to block glycolysis with or without the addition of 2 mM cyanide to block oxidative phosphorylation, led to inhibition of ITO at a holding potential of -70 mV. Shifting the holding potential to -80 mV restored ITO, suggesting that metabolic inhibition had shifted the inactivation curve of ITO in a negative direction. 3. Quasi steady-state inactivation curves revealed a shift in ITO inactivation induced by complete metabolic inhibition with 2-DG and cyanide. Myocytes typically contracted shortly after the shift was observed. In the presence of Ruthenium Red, contraction was delayed and myocytes could undergo several exposures to the metabolic inhibitors, each time displaying a shift in ITO inactivation. The shifts ranged between -7 and -20 mV. 4. Recovery from inactivation was determined using a two-pulse protocol. The time constant of recovery at a holding potential of -80 mV reversibly shifted from 48 +/- 8 to 129 +/- 21 ms during metabolic inhibition (n = 4). 5. The activation of ITO from a holding potential of -100 mV shifted in a negative direction during metabolic inhibition, from a half-activation voltage of 0.3 +/- 3.0 to -14.7 +/- 2.5 mV (n = 5). Such a -15 mV shift increases the amplitude of ITO by approximately 30% at 0 mV. 6. A shift in ITO inactivation similar to that produced by metabolic inhibition could be shown when myocytes were subjected to oxidant stress induced by either 1 mM t-butyl hydroperoxide (TBHP) or the photoactivation of 100 nM Rose Bengal. Furthermore, an increase in pipette concentration of free Ca2+ from 20 to 200 nM also shifted ITO inactivation in a negative direction. 7. These results raise the possibility that the rise in intracellular [Ca2+] occurring during both metabolic inhibition and oxidant stress modifies activation and inactivation of ITO.(ABSTRACT TRUNCATED AT 400 WORDS)
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