Yukihiro Ozaki

Electrophysiological analysis of the negative chronotropic effect of endothelin-1 in rabbit sinoatrial node cells

1 Electrophysiological effects of endothelin‐1 (ET‐1) were studied in rabbit sinoatrial node (SAN) using conventional microelectrode and whole‐cell voltage and current recordings. 2 In rabbit SAN, RT‐PCR detected ETA endothelin receptor mRNA. ET−1 (100 nm) increased the cycle length of action potentials (APs) from 305 ± 15 to 388 ± 25 ms; this effect was antagonised by the ETA receptor‐selective antagonist BQ−123 (1 μm). ET‐1 increased AP duration (APD50) by 22 %, depolarised the maximum diastolic potential (MDP) from −59 ± 1 to −53 ± 2 mV, shifted the take‐off potential by +5 mV and decreased the pacemaker potential (PMP) slope by 15 %. Under exactly the same experimental conditions, ET‐1 caused a positive chronotropic effect in guinea‐pig SAN with a decrease of 13 % in APD50, a shift of −4 mV in the take‐off potential and an increase of 8 % in the PMP slope. 3 Rabbit SAN exhibited two major cell types, distinguished both by their appearances and by their electrophysiological responses to ET‐1. Whereas the spontaneous pacing rate and the PMP slope were similarly decreased by ET‐1 (10 nm) in both cell types, ET‐1 depolarised MDP from −67 ± 1 to −62 ± 4 mV in spindle‐shaped cells but hyperpolarised it from −73 ± 1 to −81 ± 3 mV in rod‐shaped cells. ET‐1 decreased APD50 by 8 and 52 % and shifted the take‐off potential by +5 and −9 mV in spindle‐ and rod‐shaped cells, respectively. 4 ET‐1 decreased the high‐threshold calcium current (ICaL) by about 50 % in both cell types, without affecting its voltage dependence, and decreased the delayed rectifier K+ current (IK) with significant shifts (of +4.7 and +14.0 mV in spindle‐ and rod‐shaped cells, respectively) in its voltage dependence. It was exclusively in rod‐shaped cells that ET‐1 activated a sizeable amount of time‐independent inward‐rectifying current. 5 The hyperpolarisation‐activated current (If), observed exclusively in spindle‐shaped cells, was significantly increased by ET‐1 at membrane potentials between −74.7 and −84.7 mV whereas it was significantly decreased at more negative potentials. ET−1 significantly decreased the slope of the current‐voltage (I–V) relation of the If tail without changing its half‐maximum voltage. 6 The overall negative chronotropic influence of ET‐1 on the whole rabbit SAN is interpreted as resulting from the integration of its different actions on spindle‐ and rod‐shaped SAN cells through electrotonic interaction.