Rabbit SAN exhibited two major cell types, distinguished both by their appearances and by their electrophysiological responses to ET-1. Under exactly the same experimental conditions, ET-1 caused a positive chronotropic effect in guinea-pig SAN with a decrease of 13 % in APD 50, a shift of −4 mV in the take-off potential and an increase of 8 % in the PMP slope. ET-1 increased AP duration (APD 50) 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 %. ET−1 (100 n m) increased the cycle length of action potentials (APs) from 305 ± 15 to 388 ± 25 ms this effect was antagonised by the ET A receptor-selective antagonist BQ−123 (1 μ m). In rabbit SAN, RT-PCR detected ET A endothelin receptor mRNA. Electrophysiological effects of endothelin-1 (ET-1) were studied in rabbit sinoatrial node (SAN) using conventional microelectrode and whole-cell voltage and current recordings.
