OXYGEN-DERIVED FREE-RADICAL STRESS ACTIVATES NONSELECTIVE CATION CURRENT IN GUINEA-PIG VENTRICULAR MYOCYTES - ROLE OF SULFHYDRYL-GROUPS

Oxygen-derived free radicals (O-Rs) cause alterations in cardiac elect rical activity, including sustained depolarization, which may contribu te to arrhythmic activity in reperfusion after ischemia. The ionic cur rent(s) and cellular mechanism(s) underlying the sustained depolarizat ion are not well defined. We used the whole-cell variant of the patch- clamp technique to study sustained depolarization in guinea pig ventri cular myocytes during the extracellular application of O-Rs (generatin g system: dihydroxyfumaric acid, 3 to 6 mmol/L; FeCl3/ADP, 0.05:0.5 mm ol/L). Myocytes superfused with O-Rs (pipette EGTA, 0.1 mmol/L) showed (1) sustained depolarization to between -40 and -10 mV, (2) oscillati ons in membrane potential, and (3) triggered activity. The depolarizat ion resulted from an increase in quasi-steady state difference current reversing at approximate to-18 mV, and the oscillations were due to t ransient inward current. The latter were inhibited with ryanodine (10 mu mol/L) or high pipette EGTA (5 mmol/L), but the steady state curren t was unaffected. Nonselective cation current (I-NSC) (recorded with C s+, Li+, and Mg2+ replacing K+, Na+, and Ca2+, respectively; 20 mmol/L tetraethylammonium chloride [TEA] and 5 mmol/L; BAPTA in the pipette solution and 10 mmol/L TEA, 10 mu mol/L tetrodotoxin, and 10 mu mol/L nicardipine in the bath solution) was activated by O-Rs; the increase in current was unaffected by preventing changes in [Ca2+](i) but was i nhibited with dithiothreitol. Oxidizing agents (diamide and thimerosal ) or caffeine (pipette EGTA, 0.1 mmol/L) produced a similar increase i n membrane conductance. I-NSC activated with O-Rs, oxidizing agents, o r caffeine was sensitive to SK&F 96365. O-R treatment was without effe ct when INSC was already activated with caffeine. The data suggest tha t (1) extracellular O-Rs activate a Ca2+-sensitive I-NSC in the absenc e of changes in [Ca2+](i), (2) oxidative modification of extracellular sulfhydryl groups may be involved, and (3) this mechanism is differen t from the Ca2+-dependent activation of I-NSC by intracellular O-Rs, i ndicating that O-Rs may alter ion channel activity by differential mec hanisms, depending on the compartment, extracellular or intracellular, in which they are present.