Inhibition of the acetylcholine-activated potassium current by endothelin-1 in atrial cardiomyocytes

Abstract

Endothelin-1 (ET-1), is a 21 amino-acid peptide with potent inotropic and chronotropic actions in the heart. Relatively little is known about the underlying electrophysiological effects of the peptide. In this study the effects of ET-1 on the acetylcholine-activated potassium current (IK(Ach)) were investigated in the absence and presence of the receptor-selective antagonists, PD155080 (ETA receptor-selective) and RES-701 (ETB receptor-selective) in atrial cardiomyocytes. Cells were obtained from New Zealand White rabbits (2·5–3 kg) by enzymatic dissociation using collagenase. Potassium currents were recorded, in the presence of nifedipine (5 μM), using the whole cell ruptured patch-clamp technique. Low resistance electrodes (1–3 MΩ) were filled with potassium gluconate solution (pH 7·2). Cells were superfused with a modified Tyrode's solution. Following stabilization, control recordings were made using standard pulse protocols, and drugs were applied using a gravity fed microperfusion system. Data were expressed as mean±standard error, and comparisons were made using Friedman's ANOVA and Wilcoxon's Signed Ranks tests; p<0·05 was considered significant ET-1 (10 nM) alone did not affect the potassium current. Acetylcholine (Ach, 1 μM) increased (p<0·05) the potassium current to −1321±290 pA, from a control value of −955±191 pA, at a step potential of −100 mV. This change in current is attributed to IK(Ach) as it was blocked by atropine (10 μM). ET-1 (10 nM) inhibited (p<0.05) the IK(Ach) obtained in the presence of Ach (−882±88 compared to a control value of −870±98 pA) Ach also increased the holding current from +80±9 to ±242±38 pA, and this effect was blocked (p<0·05) in the presence of ET-1 (44±13 pA). The ETA receptor-selective antagonist, PD155080 (1 μM), prevented (p<0·05) the ET-1 induced inhibition of IK(Ach) (control vs Ach±ET-1+PD155080: −804±224 vs −1030±210 pA; at −100 mV; n=6). The ETB receptor-selective antagonist, RES-701 (1 μM), also prevented (p<0·05) the inhibitory effect of ET-1 on IK(Ach) (control vs Ach+ET-1+RES-701: −733±116 vs −913±137 pA; at −100 mV; n=6). However, unlike PD155080, RES-701 failed to sustain this inhibitory effect after a further 90 s exposure to Ach and ET-1 (control vs Ach+ET-1+RES-701; −733±116 vs 768±96 pA; at −100mV; n=6). In summary, ET-1 clearly inhibits the effects of Ach on IK(Ach) in rabbit atrial cardiomyocytes. This effect is mediated by an ETA receptor-subtype, and an RES-701 -sensitive ETB receptor subtype.