MODIFICATION OF THE TRANSIENT OUTWARD CURRENT OF RAT ATRIAL MYOCYTES BY METABOLIC INHIBITION AND OXIDANT STRESS

1. A putative function of the transient outward current (I(TO)) in car diac myocytes is to modulate the shape of the action potential and, co nsequently, 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-arrhyt hmic action of I(TO), we have examined its response to metabolic inhib ition 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 glycolysi s with or without the addition of 2 mm cyanide to block oxidative phos phorylation, led to inhibition of I(TO) at a holding potential of - 70 mV. Shifting the holding potential to - 80 mV restored -I(TO), sugges ting that metabolic inhibition had shifted the inactivation curve of I (TO) in a negative direction. 3. Quasi steady-state inactivation curve s revealed a shift in I(TO) inactivation induced by complete metabolic inhibition with 2-DG and cyanide. Myocytes typically contracted short ly after the shift was observed. In the presence of Ruthenium Red, con traction was delayed and myocytes could undergo several exposures to t he metabolic inhibitors, each time displaying a shift in I(TO) inactiv ation. The shifts ranged between - 7 and - 20 mV. 4. Recovery from ina ctivation 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 I(TO) from a holding potential of - 100 mV shifted in a negative direction during metabolic inhibition, from a half-activatio n voltage of 0.3 +/- 3-0 to - 14.7 +/- 2.5 mV (n = 5). Such a - 15 mV shift increases the amplitude of I(TO) by approximately 30 % at 0 mV. 6. A shift in I(TO) inactivation similar to that produced by metabolic inhibition could be shown when myocytes were subjected to oxidant str ess induced by either 1 mm t-butyl hydroperoxide (TBHP) or the photoac tivation of 100 nM Rose Bengal. Furthermore, an increase in pipette co ncentration of free Ca2+ from 20 to 200 nm also shifted I(TO) inactiva tion in a negative direction. 7. These results raise the possibility t hat the rise in intracellular [Ca2+] occurring during both metabolic i nhibition and oxidant stress modifies activation and inactivation of I (TO). Such a sensitivity to cytosolic Ca2+ may influence I(TO) during the action potential in the normal heart as well as under abnormal con ditions where [Ca2+]i rises. The depression of I(TO) resulting from th e shift in inactivation would lead to an elevation of the plateau pote ntial and widening of the action potential.