CELL SWELLING HAS DIFFERENTIAL-EFFECTS ON THE RAPID AND SLOW COMPONENTS OF DELAYED RECTIFIER POTASSIUM CURRENT IN GUINEA-PIG CARDIAC MYOCYTES

Cell swelling has been shown to cause activation of a variety of cardi ac sarcolemmal ionic conductances including potassium channels. The ai m of this study was to investigate the effect of swelling on the two s ubtypes of delayed rectifier potassium current (J(Kr) and I-Ks) in sin gle guinea pig myocytes using the whole-cell configuration of the patc h clamp technique. When the holding potential was set at -40 mV and st epped to +40 mV for 1 s under isoosmotic conditions (300 mOsm) a delay ed rectifier current (I-K) was activated (0.86 +/- 0.05 nA; n = 43). S witching to a hypoosmotic solution (200 mOsm) caused a rapid increase in I-K to a mean value of 1.43 +/- 0.10 nA (p < 0.05; n = 43), The eff ect of swelling on the two subtypes of I-K was studied by analysis of deactivating tail currents using an. envelope of tails protocol (stepp ing from -40 to +40 mV for 18 different pulse durations between 50 ms and 2.9 s; n = 16). Swelling caused a decrease in current amplitude me asured at the end of the pulse (and I-Ktail) at short durations (less than or equal to 150 ms) however, when the pulse duration was >1 s swe lling caused a significant increase in current. Using a pulse protocol to measure I-Kr with minimal contamination by I-ks (voltage step from -40 to -10 mV for 250 ms) a 50-100 pA current was elicited which coul d be completely blocked by dofetilide (0.2 mu M; n = 3). Introduction of hypoosmotic solution caused a significant decrease in IK, and when dofetilide (0.2 or 1.0 mu M) was introduced the current remaining was decreased further (p < 0.05; n = 5), but was not completely blocked, t hus suggesting that swelling had decreased the ability of dofetilide t o block I-Kr. Similar results were obtained over a range of dofetilide concentrations and with a second I-Kr blocker, La3+. In Ca2+-free ext ernal solutions, pulsing to -10 mV for 500 ms to measure I-Kr in the a bsence of I-Ks, and to + 60 mV for 5 s (with 0.2 mu M dofetilide) to e voke only I-Ks, it was clear that swelling significantly increased I-K s (pulse and tail currents) and decreased I-Kr. In addition, when meas ured using the perforated patch method, swelling modulated I-Kt and I- Ks in a similar fashion. We conclude that swelling has differential ef fects on the subtypes of the classical cardiac I-K, which may have imp ortant implications in our understanding of the mechanisms underlying ischaemia- and reperfusion-induced arrhythmogenesis.