Voltage-dependent calcium channels and cardiac pacemaker activity: From ionic currents to genes

Abstract

The spontaneous activity of pacemaker cells in the sino-atrial node controls the heart rhythm and rate under physiological conditions. Compared to working myocardial cells, pacemaker cells express a specific array of ionic channels. The functional importance of different ionic channels in the generation and regulation of cardiac automaticity is currently subject of an extensive research effort and has long been controversial. Among families of ionic channels, Ca²⁺ channels have been proposed to substantially contribute to pacemaking. Indeed, Ca²⁺ channels are robustly expressed in pacemaker cells, and influence the cell beating rate. Furthermore, they are regulated by the activity of the autonomic nervous system in both a positive and negative way. In this manuscript, we will first discuss how the concept of the involvement of Ca²⁺ channels in cardiac pacemaking has been proposed and then subsequently developed by the recent advent in the domain of cardiac physiology of gene-targeting techniques. Secondly, we will indicate how the specific profile of Ca²⁺ channels expression in pacemaker tissue can help design drugs which selectively regulate the heart rhythm in the absence of concomitant negative inotropism. Finally, we will indicate how the new possibility to assign a specific gene activity to a given ionic channel involved in cardiac pacemaking could implement the current postgenomic research effort in the construction of the cardiac Physiome.

Introduction

The cardiac automaticity is a complex physiological function requiring the coordinated activity of the primary rhythmogenic centre of the heart, the sino-atrial node (SAN) and that of the conduction system (CCS), formed by the atrioventricular node (AVN) and the Purkinje fibres network (Bouman and Jongsma, 1986; Boyett et al., 2000; Efimov et al., 2004; Moorman and Christoffels, 2003; Moorman et al., 1998; Oosthoek et al., 1993a, Oosthoek et al., 1993b). Automaticity is initiated in the SAN by primary pacemaker cells which generate spontaneous periodic oscillations of their membrane potential. The pacemaker impulse then spreads from the SAN to the CCS, driving the contraction of the whole working myocardium. The spontaneous activity of the pacemaker cell is due to the diastolic depolarisation, a slow phase of the pacemaker cycle which drives the membrane voltage from the end of the repolarisation phase of an action potential toward the threshold of the following action potential (Weidmann, 1980). At the cellular level, pacemaker activity requires the intervention of several families of ionic channels, as well as second messenger molecules and proteins regulating Ca²⁺ handling and ionic homeostasis (for review see Boyett et al., 2000; Irisawa et al., 1993). There is now substantial evidence indicating that Ca²⁺ ions play an important role in the generation and regulation of the pacemaker cycle, by carrying inward current through voltage-dependent Ca²⁺ channels (VDCCs) (Mangoni et al., 2003; Verheijck et al., 1999), or by stimulating Na⁺–Ca²⁺ exchange activated by subsarcolemmal Ca²⁺ release during the diastolic depolarisation (Bogdanov et al., 2001; Huser et al., 2000; for review see Lakatta, 2004). VDCCs also participate to the upstroke phase of the action potential (Doerr et al., 1989; Hagiwara et al., 1988; Kodama et al., 1997).

The best characterised route for Ca²⁺ entry in pacemaker cells is through VDCCs (Catterall et al., 2003). In this review, we will primarily focus on two distinct families of VDCCs, the L- and T-type Ca²⁺ channels. L-type Ca²⁺ channels are expressed in heart tissues, are sensitive to dihydropyridines (DHPs) such as nifedipine and BayK-8644 and are strongly stimulated by protein kinase A (PKA)-dependent phosphorylation (for review see Striessnig, 1999). As compared to T-type channels, L-type channels activate upon membrane depolarisation for more positive voltages and have slow mixed Ca²⁺- and voltage-dependent inactivation. T-type channels activate at negative voltages, display typical slow criss-crossing activation and fast voltage-dependent inactivation. In addition, T-type channels have smaller single channel conductance than L-type channels (Perez-Reyes, 2003). During the past 20 years, the electrophysiological study of heart tissues has allowed the description of the biophysical properties of VDCCs in cells coming from the SAN, the CCS and the working myocardium (for review see Roden et al., 2002; Schram et al., 2002), and the key role of L-type VDCCs in the initiation of myocardial contraction is well established (Bers, 2002b; Tanabe et al., 1990). Here we will review the expression profile and the properties of the different classes of VDCCs present in pacemaker cells and focus on the profound differences in their activation and inactivation properties, sensitivity to intracellular second messengers and pharmacological agents susceptible to act on heart rhythm and rate. The heterogeneity in VDCCs biophysical properties is of particular interest for cardiac automaticity, since different classes of VDCCs can play distinct roles in the generation and regulation of pacemaker activity. To illustrate this point, we will first try to give to the reader an historical account of how the connection between Ca²⁺ channels and cardiac pacemaking has been progressively developed from the early 1970s (Beeler and Reuter, 1970; Reuter and Beeler, 1969), with the first evidence for the existence of Ca²⁺-dependent currents in voltage-clamped heart tissue, to the identifications of the different VDCCs in the heart and SAN and the description of their pharmacological properties during the 1980s (Bean, 1985; Hagiwara et al., 1988; Lee and Tsien, 1983) and 1990s (Bois and Lenfant, 1991; Fermini and Nathan, 1991). Importantly, this latter research phase has also been rich of seminal works by many independent groups dealing with the regulation of VDCCs by autonomic agonists and the complex regulatory network constituted by intracellular second messengers (see for example: Han et al., 1994; Petit-Jacques et al., 1993; Tanaka et al., 1996). The role of VDCCs in cardiac pacemaking has been investigated by a combination of electrophysiological, pharmacological and numerical modelling approaches (see below). However, the partially overlapping pharmacology between channels belonging to different classes of VDCCs has limited the possibility to clearly separate the specific contribution of VDCCs in the different phases of the pacemaker cycle.

At the dawn of the 1990s, we have witnessed the advent of the mouse gene-targeting approach in the study of cardiac physiology. Indeed, in the domain of ionic channels, a wide set of mouse models have been developed in the aim of investigating the basis of arrhythmias and sudden death (Charpentier et al., 2004; Nerbonne et al., 2001). Also, genetically modified mouse strains showing dysfunctions of SAN pacemaking have been created (Ludwig et al., 2003; Platzer et al., 2000; Stieber et al., 2004; Zhang et al., 2002). We have recently described the isolation and electrophysiological recording of pacemaker cells coming from the mouse SAN (Mangoni and Nargeot, 2001; Mangoni et al., 2001). These studies have opened the way to investigate the physiology of cardiac automaticity in genetically modified mice. Thanks to this new possibility, a new picture of the contribution of VDCCs in pacemaking is emerging.