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

Abstract

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 ET(A) 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 ET(A) receptor-selective antagonist BQ-123 (1 microM). 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 (I(CaL)) by about 50% in both cell types, without affecting its voltage dependence, and decreased the delayed rectifier K+ current (I(K)) 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 (I(f)), 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 I(f) 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.

Pathophysiological Significance of T-type Ca2+ Channels: T-type Ca2+ Channels and Drug Development

“…The change in action potential waveform produced by efonidipine was very similar to that by 40 µM nickel, which specifically inhibits T-type Ca 2+ channels in myocardial cells (4,29). Similar change in action potential waveform was observed after application of mibefradil (40) and after application of efonidipine to isolated sino-atrial node cells (41) (42). Voltage clamp exeriments with isolated guinea-pig ventricular cardiomyocytes showed that efonidipine inhibits both L-type and T-type Ca 2+ channels concentration-dependently (30).…”

Modulation by Endothelin-1 of Spontaneous Activity and Membrane Currents of Atrioventricular Node Myocytes from the Rabbit Heart

“…Data from patients with angina pectoris have shown left bundle-branch block to be associated with raised ET-1 levels, suggesting that the hormone may be involved in conduction abnormalities [20]. Consistent with an ability of ET-1 to exert a direct effect on the pacemaker-conduction system, experiments on cells isolated from the rabbit SAN have demonstrated that ET-1 produces a negative chronotropic effect that is associated with direct ion channel modulation [21]–[23]. By contrast, to our knowledge, there is no current information available concerning direct effects of this peptide hormone on the AVN.…”

Brain Death Does Not Change Epicardial Action Potentials and Their Response to Ischemia–Reperfusion in Open-chest Pigs

“…After stabilization of the amplitude of the signal (30 seconds to 1 minute), APs were recorded during 1 minute at a stable heart rate. APs were detected and analyzed, as previously described, 24 using MATLAB (The MathWorks, Inc., Natick, MA) for AP plateau amplitude and action potential at 50% re-polarization from the plateau level (termed action potential duration, APD). Time zero for computing APD was taken at the time of maximum up-stroke velocity of the AP.…”