A rapidly activating delayed rectifier K⁺ current regulates pacemaker activity in adult mouse sinoatrial node cells

Abstract

We have investigated the physiological role of the "rapidly activating" delayed rectifier K+ current (IKr) in pacemaker activity in isolated sinoatrial node (SAN) myocytes and the expression of mouse ether-a-go-go (mERG) genes in the adult mouse SAN. In isolated, voltage-clamped SAN cells, outward currents evoked by depolarizing steps (greater than -40 mV) were strongly inhibited by the class III methanesulfonanilide compound E-4031 (1-2.5 microM), and the deactivation "tail" currents that occurred during repolarization to a membrane potential of -45 mV were completely blocked. E-4031-sensitive currents (IKr) reached a maximum at a membrane potential of -10 mV and showed pronounced inward rectification at more-positive membrane potentials. Activation of IKr occurred at -40 to 0 mV, with half-activation at about -24 mV. The contribution of IKr to action potential repolarization and diastolic depolarization was estimated by determining the E-4031-sensitive current evoked during voltage clamp with a simulated mouse SAN action potential. IKr reached its peak value (approximately 0.6 pA/pF) near -25 mV, close to the midpoint of the repolarization phase of the simulated action potential, and deactivated almost completely during the diastolic interval. E-4031 (1 microM) slowed the spontaneous pacing rate of Langendorff-perfused, isolated adult mouse hearts by an average of 36.5% (n = 5). Expression of mRNA corresponding to three isoforms coded by the mouse ERG1 gene (mERG1), mERG1a, mERG1a', and mERG1b, was consistently found in the SAN. Our data provide the first detailed characterization of IKr in adult mouse SAN cells, demonstrate that this current plays an important role in pacemaker activity, and indicate that multiple isoforms of mERG1 can contribute to native SAN IKr.

Cardiac performance in the zebrafishbreakdancemutant

“…This suggested that not only the QT interval of the conductance system was affected, but that the pacemaker cells in the sinoatrial node must R. Kopp, T. Schwerte and B. Pelster be affected as well. Electrophysiological studies revealed that the ERG channels (I Kr ) are expressed in mouse pacemaker cells and pharmacological block of these channels results in a bradycardia (Clark et al, 2004). The bradycardia observed in bre animals thus provides additional support for the conclusion that this mutation is due to a defect in the delayed rectifier potassium channel.…”

Culture and adenoviral infection of sinoatrial node myocytes from adult mice

“…Data were acquired at 5 kHz and low-pass filtered at 1 kHz using an Axopatch 1D or 200B amplifier, Digidata 1322a or 1440a A/D converter, and ClampEx software (Molecular Devices). Spontaneous AP firing rate was measured in the presence of 1 nM isoproterenol (ISO; EMD Millipore/Calbiochem, Billerica, MA) in the bathing Tyrode's solution, which was used to promote stable AP firing in both acutely isolated and cultured SAMs as previously described (6,14,16,18,38).…”

Creation of a Biopacemaker: Lessons from the Sinoatrial Node

“…ERG1B knock out mice did not display a reduction in overall heart rate, but 6 out 21 mice did show abrupt and spontaneous bradycardias that were never seen in control littermates (Lees-Miller et al, 2003). In rabbit and mouse SA node cells, blockade of I Kr with E-4031 prolonged cycle length and action potential duration, and depolarized the MDP (Clark et al, 2004;Verheijck et al, 1995). Since I Kr is absent from pig SA node, blocking of I Kr did not have any affect on the action potential or cycle length in this species (Ono et al, 2000).…”