Nicardipine-sensitive Na+-mediated single channel currents in guinea-pig sinoatrial node pacemaker cells

The Na+-dependent inward currents underlying slow diastolic depolarization of sinoatrial (SA) node cells were examined. Using a Na+-rich, Ca2+-free pipette solution a novel single channel current was recorded in addition to the conventional Na+ and L-type Ca2+ currents. The current (termed ist, as it reflects the whole-cell sustained inward current, Ist) does not show obvious inactivation during a 700 ms depolarization and is unique in having a smaller amplitude (1·1 ± 0·18 pA at −60 mV, n = 12) than the Na+ current through conventional Na+ (≈3·3 pA) and Ca2+ channels (9·6 ± 0·32 pA at −60 mV, n = 8). The mean unitary conductance of ist channels was 13·3 pS.The recording of ist was infrequent, was observed only in spontaneously beating SA node cells, and was facilitated by adding Bay-K 8644 to the pipette solution. Overlapping of ist events was observed and ist was abolished by bath application of nicardipine.In the ensemble average, the activation of ist was evident by depolarization beyond −70 mV, and the dynamic voltage range of activation (-70 to −30 mV) encompassed the extent of the slow diastolic depolarization. The current density of ist was 0·33 pA pF−1 at −60 mV, as estimated from the number of channels per membrane patch, the open probability and the unitary amplitude.Cumulative histograms for both open and closed times were fitted with a sum of two exponential components. The slow time constants decreased with depolarization, while the fast time constants and the fraction of the fast component were voltage independent. The number of bursts per sweep increased with depolarization. The time constant of the first latency histogram was about two orders of magnitude larger than those in cardiac L-type Ca2+ channels and decreased with depolarization.It is suggested that the ist channels might be responsible for the whole-cell Ist

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

Electrophysiological effects of endothelin-1 (ET-1) were studied in rabbit sinoatrial node (SAN) using conventional microelectrode and whole-cell voltage and current recordings.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.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.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.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.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