Calcium and arrhythmogenesis

Abstract

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Fig. 1

A: Ca²⁺ release units in cardiac muscle (chick myocardium). Dyads are formed by junctional sarcoplasmic reticulum (SR) with feet on their cytosolic surface and containing calsequestrin (CSQ), associating with the surface membrane or the membrane of t tubules (T). Corbular SR contains the same components but does not associate with the cell membrane. B and C: peripheral couplings; docked, but not yet fully differentiated (embryo 2.5 days). D: freeze fracture of cell membrane arrows surrounding junctional domains containing dihydropyridine receptor (DHPR) particles. [From Franzini-Armstrong et al. (162).]

Fig. 2

Diagram of the excitation-contraction coupling system in the cardiac cell. During the action potential Ca²⁺ enters the cells as a rapid influx followed by a maintained component of the slow inward current. Ca²⁺ entry does not lead directly to force development as the Ca²⁺ that enter are rapidly bound to binding sites on the SR that envelops the myofibrils. The rapid influx of Ca²⁺ via the t tubules is thought to induce release of Ca²⁺ from a release compartment in the SR, by triggering opening of Ca²⁺ channels in the terminal cisternae, thus activating the contractile filaments to contract. Relaxation follows because the cytosolic Ca²⁺ is sequestered again in an uptake compartment of the SR and partly extruded through the cell membrane by the Na⁺/Ca²⁺ exchanger and by the low-capacity high-affinity Ca²⁺ pump. The force of contraction is thus determined by the circulation of Ca²⁺ from the SR to the myofilaments and back to the SR, and by the amount of Ca²⁺ that has entered during the preceding action potential. The relaxation rate of the twitch depends on the rate of Ca²⁺ dissociation from the myofilaments and on the rates of Ca²⁺ sequestration and extrusion. It is important to note that the process of Na⁺/Ca²⁺ exchange is electrogenic so that Ca²⁺ extrusion through the exchanger leads to a depolarizing current.

Fig. 3

A, a superimposed tracings are force (thick black line) and intracellular calcium (Ca_i) transient (thin black line) recordings of the electrically stimulated trabecula. Bottom tracing illustrates the slow change in Ca_i occurring in normal muscle during the diastolic period (between vertical dotted lines). A, b: force at increased gain and sarcomere length during the twitch and subsequent diastolic pause. Note that no sarcomere length fluctuations (>1.3 nm) occur (535). B: enlarged confocal image depicting the characteristics of line scans during propagation of one microscopic Ca²⁺ wave (top panel a) and during initiation and propagation of another (bottom panel b) in normal muscle. In Ba, the Ca²⁺ wave has an asymmetric appearance, as if it encounters a border or failed to propagate in one direction. In Bb, the wave begins as a “V,” indicating equal propagation in both directions; however, this wave stops propagating. The black arrows in both panels mark the same position in the two scans, indicating that the two waves started at the same place. The white arrows indicate the position of sparks at the leading edge of the wave in A. [From Wier et al. (590).]

Fig. 4

Dependence of ryanodine receptor (RyR) single-channel activities on cytosolic Ca²⁺ and SR-luminal Ca²⁺. A: original current traces from cardiac Ca²⁺ release channels at three differing Ca²⁺ levels. Upward deflections indicate openings from closed state (small bar at left). B: average single-channel open probability (P_o) values determined as in A at +35 mV (closed symbol) and −35 mV (open symbol). See Ref. 606 for more information. C: P_o-luminal [Ca²⁺] relationship of wild-type RyR2 expressed in HEK293 cells compared with the P_o-luminal [Ca²⁺] relationship of RYR2 channels with mutations linked to VT (L433P and R176Q/T2504M). These mutations displayed a leftward shift of the P_o-luminal [Ca²⁺] relationship without a change in the sensitivity to cytosol [Ca²⁺]. See Ref. 251 for details.

Fig. 5

A: ultrastructure of RyR1 at 9.5 Å resolution. The receptor is composed of a cytosolic assembly linked to a transmembrane assembly (TMA) through a neck region which conveys columns that form the vestibule of the TMA and the Ca²⁺ channel in the center of the TMA to the regulatory elements in the clamps and handle domain of the cytosolic assembly (see text for further details). [From Samso et al. (468).] B: schematic diagram of the reported mutation sites of RyR1 and RyR2. NH₂ terminus, central domain, and transmembrane (channel) regions are denoted. For more information, see text and http://pc4.fsm.it:81/cardmoc/. [From Yano et al. (617).]

Fig. 6

Large cell wide (CW) Ca²⁺ waves can lead to sufficient membrane depolarization to elicit an action potential (AP). A: selected image frames of Ca²⁺ from an IZPC (Purkinje cell aggregate from the infarcted heart) during the Ca²⁺-induced electrical activity. Time relative to t=0 of first frame is depicted by white numbers. Lower white light image is of aggregate during experiment. Large white arrowhead indicates probable cell border. B: transmembrane potential (black line) and Ca²⁺ (multicolored lines) changes of this aggregate during the CW wave induced electrical activity. μCaiT represents a small micro Ca²⁺ wavelet that occurred during the recording but that is not shown in these epifluorescent images (see Ref. 61 for more details).

Fig. 7

Multiple simultaneous spontaneous Ca²⁺ release events in the isolated canine wedge preparation under Ca²⁺-loaded conditions. A: representative transmural map (endocardium to the left, epicardium to the right) depicting activity of imaged Ca²⁺. Imaged area is 14 mm × 14 mm. SCR_amp means spontaneous calcium release amplitude as determined from recordings such as those shown in B. Relative fluorescent ratio units (ORU) are depicted by the various shades of gray in the bar. Note this map reveals that there are two “hot” spots of relatively high-amplitude SCR (denoted by a, c). Corresponding local Ca²⁺ transients during this time are seen in B. Note that while there is SCR near endocardium (site a) and midmyocardium (site c), there is no release at site d (epicardium). The SCRs are depicted as delayed after Ca²⁺ transients in B. [From Katra and Laurita (267).] C: one local Ca²⁺ transient in another preparation with selected images (0 to +660 ms) above showing the isochrones of Ca²⁺ levels during the inscription of the after Ca²⁺ transient. Note that SCR starts at one focus (∼1.2 mm²) and then during the course of the global transient propagates outward at ∼26 mm/s. At its maximum this SCR covers ∼35 mm². (From Laurita laboratory, unpublished data.)

Fig. 8

Initiation of Ca²⁺ waves in experimental multicellular preparation of nonuniform excitation-contraction coupling. A: Ca²⁺ waves induced by a local BDM exposure at various Ca²⁺ concentrations (1, 2, and 4 mM). BDM is delivered to the trabecula via a jet system that superfuses the region denoted by the dotted lines. Note that increasing Ca²⁺ led to the initiation of Ca²⁺ waves that propagate into the segment inside the jet and into the normal muscle. Both the amplitude of the initial and propagating Ca²⁺ transient as well as propagation velocity increased with increase in Ca²⁺. Arrows indicate initiation sites. B: collision of Ca²⁺ waves in the jet region (white arrow). C: Ca²⁺ traces, F records, and SL (sarcomere records) from average profiles indicated by the square bracket in B. Note that the onset of the initial Ca²⁺ rise of the wave in B (denoted by the black arrow in B) corresponds with the time at which the twitch had relaxed to 10% (F onset) and late during relaxation. [From Wakayama et al. (570).]