Chronic exposure to endothelin-1 prolongs repolarization and hampers rapid delayed rectifier current of cardiac HL-1 cells.

Background: Cardiac hypertrophy and failure are consistently accompanied by remodeling of myocytic functional properties. A common cellular electrophysiological outcome is represented by the prolongation of action potential duration, which reduces membrane electrical stability, thus predisposing to arrhythmias. Experimental evidence indicates that a variety of locally released growing and hypertrophying factors mediate such cellular remodeling. Endothelin-1 is a potent promoter of myocardial hypertrophy and its synthesis and release are increased in the diseased heart. Recently, in cardiac HL-1 cells, ET-1 has been shown to induce the expression of GATA-4 transcription factor, which is known to be involved in mediating cellular response to hypertrophic stimuli. However, the direct effect of chronic exposure to ET-1 on cellular functional remodeling is largely unknown. Thus, we investigated the electrophysiological properties of cardiac HL-1 cells chronically exposed to ET-1 (100 nM). Methods and Results: Action potential (AP) was recorded in enzymatically dissociated HL-1 cells by patch-clamp technique and duration was measured at -10, -20, -40, -50, -60 mV. HL-1 cells chronically exposed to ET-1 (n=12) showed a significantly prolonged AP duration at -60 mV, as compared to cells grown in normal medium (CTR, n=18; 102.7±14.1 ms vs.174.9±25.7 ms, p<0.001). To investigate the ionic mechanism responsible for the prolonged AP duration in cells exposed to ET-1, we measured the rapid delayed rectifier K+ current (IKr), which mainly controls and stabilizes the terminal phase of repolarization. Tail maximal density of IKr was significantly reduced from 10.3±0.1 pA/pF in CTR cells (n=9) to 4.2±0.1 pA/pF in ET-1 exposed cells (n=14, p<0.001). Potential of half-maximal activation was significantly shifted from -6.3±0.3 mV in CTR cells (n=9) to 1.6±0.9 (n=14, p<0.001) in ET-1 exposed cells. Changes of current kinetics were estimated by mono-exponential fitting of envelope tail currents, which gave the time constant (tau) of IKr activation at +20 mV. Mean values of tau significantly increased from 21.4±1.2 ms in CTR (n=4) cells to 86.1±5.5 ms (n=7, p<0.001) in ET-1 exposed cells. Conclusions: Chronic exposure to ET-1 prolongs AP duration of HL-1 cells; the effect is associated and probably sustained by a decrease in IKr density as well as by a slowing of its activation rate. These results suggest that ET-1 acts directly at cardiac myocytic level, modulating cell functional properties thus leading to arrhythmogenic alterations.