Intracellular Ca²⁺ release contributes to automaticity in cat atrial pacemaker cells

Abstract

The cellular mechanisms governing cardiac atrial pacemaker activity are not clear. In the present study we used perforated patch voltage clamp and confocal fluorescence microscopy to study the contribution of intracellular Ca2+ release to automaticity of pacemaker cells isolated from cat right atrium. In spontaneously beating pacemaker cells, an increase in subsarcolemmal intracellular Ca2+ concentration occurred concomitantly with the last third of diastolic depolarization due to local release of Ca2+ from the sarcoplasmic reticulum (SR), i.e. Ca2+ sparks. Nickel (Ni2+; 25–50 μM), a blocker of low voltage‐activated T‐type Ca2+ current ((ICa,T), decreased diastolic depolarization, prolonged pacemaker cycle length and suppressed diastolic Ca2+ release. Voltage clamp analysis indicated that the diastolic Ca2+ release was voltage dependent and triggered at about ‐60 mV. Ni2+ suppressed low voltage‐activated Ca2+ release. Moreover, low voltage‐activated Ca2+ release was paralleled by a slow inward current presumably due to stimulation of Na+‐Ca2+ exchange (INa‐Ca). Low voltage‐activated Ca2+ release was found in both sino‐atrial node and latent atrial pacemaker cells but not in working atrial myocytes. These findings suggest that low voltage‐activated ICa,T triggers subsarcolemmal Ca2+ sparks, which in turn stimulate INa‐Ca to depolarize the pacemaker potential to threshold. This novel mechanism indicates a pivotal role for ICa,T and subsarcolemmal intracellular Ca2+ release in normal atrial pacemaker activity and may contribute to the development of ectopic atrial arrhythmias.

Normal Heart Rhythm is Initiated and Regulated by an Intracellular Calcium Clock Within Pacemaker Cells

“…3A) [5,10]. Of note, however, in cat, latent atrial pacemaker cells, T-type Ca 2+ current, activated during DD, is thought to trigger LCRs, as these are blocked by 50 μM Ni 2+ in concentrations that inhibit T-type Ca 2+ channels [8]; in contrast, in primary rabbit SANC, Ni 2+ at these concentrations does not inhibit LCRs (Fig. 4) [11].…”

“…In rabbit SANC, fluorescence imaging over the last decade has documented the occurrence of an AP-induced Ca 2+ transient, and, importantly, in the absence of Ca 2+ overload, confocal imaging has permitted detection of the occurrence of localized, Ca 2+ releases (LCRs) during normal cell function [7,8]. During spontaneous beating, LCRs occur during late DD in the form of multiple locally propagating wavelets beneath the cell surface membrane (Fig.…”

Potassium channels in the sinoatrial node and their role in heart rate control

“…Calcium is released from the sarcoplasmic reticulum even during diastole when the cell is quiescent and in ventricular cells this is manifest as calcium sparks [18]. In SAN myocytes however these local calcium releases are larger involving a number of release sites [19–21]. In permeabilised cells the effects of membrane ion channels are removed and in these conditions these events are periodic in contrast to the stochastic nature of sparks in ventricular myocytes [22].…”

“…The SA node does not have t-tubules and calcium release is sub-sarcolemmal. The timing of the spontaneous calcium release is in the latter part of the diastolic depolarization and the activation of the inward sodium-calcium exchanger current is exponential ultimately leading to the threshold potential [15,21]. The SAN seems to have higher levels of the calcium pump, SERCA2, present in the membranes of the sarcoplasmic reticulum and in inducible knockout mice the heart rate is substantially slowed [23,24].…”

Cardiac pacemaker cell failure with preserved If, ICaL, and IKr: A lesson about pacemaker function learned from ischemia-induced bradycardia

“…The net effects of this phosphorylation create conditions that are required for LCR spontaneous activation during the DD. The LCR activation in rabbit SANC is indeed truly spontaneous as it does not depend on membrane depolarization (but see [29] for cat latent atrial pacemaker cells), and can be observed under voltage clamp or in permeabilized SANC [27,30]. Importantly, it is the spontaneous emergence of LCRs during the late DD that activates I NCX and thus changes the DD dynamics from a linear to a nonlinear, exponentially rising function, culminating in I CaL activation and membrane excitation (Fig.1A) [17,18].…”