ENDOTHELIN-1 MEDIATED INHIBITION OF THE ACETYLCHOLINE-ACTIVATED POTASSIUM CURRENT FROM RABBIT ISOLATED ATRIAL CARDIOMYOCYTES

1 Endothelin-1 is a 21 amino acid peptide with potent inotropic and ch ronotropic actions in the heart. Relatively little is known about the underlying electrophysiological effects of the peptide. In this study, the effects of endothelin-1 (ET-1) on the acetylcholine-activated pot assium current (I-K(ACh) were investigated in the absence and presence of the receptor-selective antagonists, PD155080 (ET(A) receptor-selec tive adn RES-701 (ET(B) receptor-selective) in rabbit atrial cardiomyo cytes. 2 Cells were obtained from New Zealand White rabbits (2.5-3 kg) by enzymatic dissociation with collagenase. Potassium currents were r ecorded, in the presence of nifedipine (5 mu M), by use of the whole c ell ruptured patch-clamp technique. Following stabilization, control r ecordings were made with standard pulse protocols, and drugs were appl ied by a gravity fed microperfusion system. 3 Endothelin-1 (10 mM) alo ne did not affect the 'steady state' potassium current. Acetylcholine (1 mu M) increased (P<0.05) the potassium current to -1321+/-290 pA, f rom a control value of -955+/-191 pA, at a step potential of -100 mV. Acetylcholine also increased the holding current at -40 mV from +80+/- 9 pA to +242+/-38 pA, and this effect was abolished (P<0.05) in the pr esence of endothelin-1 (+44+/-13 pA). The responses to acetylcholine w ere attributed to activation of the atrial muscarinic-activated potass ium current (I-K(ACh)) as they were blocked by atropine (10 mu M). End othelin-1 (10 mM) in the presence of acetylcholine did not affect the 'steady state' potassium current (-882+/-88 pA compared to a control v alue of -870+/-98 pA, at -100 mV). 4 The ET(A) receptor-selective anta gonist, PD155080 (1 mu M), prevented (P<0.05) the ET-1 induced inhibit ion of I-K(ACh) at all potentials. PD155080, in the presence of endoth elin-1 and acetylcholine, increased the inward component of the 'stead y state' potassium current to -1030+/-210 pA from a control value of - 804+/-224 pA at a step potential of -100 mV. Also the outward componen t was increased at a potential of -20 mV from +90+/-17 pA to +241+/-47 pA. 5 Unlike PD155080, the ET(B) receptor-selective antagonist, RES-7 01 (1 mu m), only prevented (P<0.05) the inhibitory effect of endothel in-1 on the inward component of the I-K(ACh); at -100 mV, RES-701, in the presence of endothelin-1 and acetylcholine, increased the 'steady state' potassium current to -913+/-137 pA from -733+/-116 pA. Furtherm ore, RES-701, in contrast to PD155080, failed to sustain this inhibito ry effect as, in the presence of endothelin-1 and acetylcholine, the ' steady state' potassium current returned to a value of -768+/-96 pA, a t a step potential of -100 mV. 6 In conclusion, endothelin-1 clearly i nhibits the effects of acetylcholine on I(K(ACh)) in rabbit atrial car diomyocytes. This effect is primarily mediated by an ET(A) receptor-su btype, but is transiently and partially mediated by a RES-701-sensitiv e ET(B) receptor subtype. Inhibition of the I(K(ACh)) may account for the positive chronotropic properties of endothelin-1.