What determines the initiation of the heartbeat?

The origin of the heartbeat in the sino-atrial (SA) node is usually thought to arise from the sequential activation of a variety of ionic currents in pacemaker cells (Irisawa et al. 1993). Recently, the possibility has been considered that heart rate might be influenced by transient changes in cytosolic Ca²⁺. Rubenstein & Lipsius (1989) demonstrated that in cat subsidiary pacemaker cells the late phase of diastolic depolarization was slowed in the presence of ryanodine to selectively inhibit Ca²⁺ release from the sarcoplasmic reticulum (SR). Primary pacemaker cells also have a SR-dependence of cardiac pacemaking since the rate of beating of guinea-pig SA node/atrial preparations was slowed in the presence of either ryanodine or cyclopiazonic acid (an inhibitor of the SR Ca²⁺-ATPase). The reduction in rate was associated with changes in action potential characteristics recorded intracellularly (Rigg & Terrar, 1996). The role of Ca²⁺ release from the SR in influencing pacemaker rate appears to be a common mechanism in many types of pacemaking tissue since the rate reducing effects of ryanodine have been observed in other mammalian cells (rabbit SA, e.g. Hata et al. 1996; and atrioventricular node, Hancox et al. 1994) and in amphibian pacemaker cells (Ju & Allen, 1998).

In this issue of TheThe Journal of Physiology, Hüser and colleagues (Hüser et al. 2000) provide exciting new information on the association between Ca²⁺ release from the SR and spontaneous electrical activity of pacemaker cells from the cat. A rise in cytosolic Ca²⁺, detected by fluo-3 fluorescence, occurred in association with the last third of the pacemaker depolarization (before the rapid upstroke of the action potential). When fluorescence was recorded in small regions beneath the sarcolemma, localized rapid rises in Ca²⁺ were detected which resemble Ca²⁺ sparks seen in ventricular cells (Cheng et al. 1993). The ‘sparks’ were suppressed by Ni²⁺ at concentrations of 25–50 μM, conditions which also decreased the rate of pacemaker depolarization and slowed spontaneous beating. It was suggested that Ca²⁺ entry through Ni²⁺-sensitive low voltage-activated T-type Ca²⁺ channels may trigger subsarcolemmal Ca²⁺ sparks (involving SR Ca²⁺ release), and that the rise in subsarcolemmal Ca²⁺ may increase the rate of pacemaker depolarization through the action of electrogenic Na⁺-Ca²⁺ exchange.

It will be interesting to see whether the T-type-dependent spark-like activity seen in cat SA node cells by Hüser et al. (2000) is also prominent in pacemaker cells from other species. It seems from preliminary evidence that the pattern of a slow rise in Ca²⁺ fluorescence preceding the rapid upstroke of the Ca²⁺ transient similar to that reported by Hüser et al. (2000) can also be recorded from guinea-pig SA node cells, as can spark-like activity (Rakovic et al. 2000).

Hüser et al. (2000) also show that, under voltage clamp conditions, slow ramps of depolarization from -70 to -45 mV (on a time scale similar to that of the pacemaker depolarization) caused a Ni²⁺-sensitive rise in cytosolic Ca²⁺, which did not occur spontaneously when the membrane potential was maintained at -70 mV. The rise in cytosolic Ca²⁺ in response to a ramp depolarization that was seen in pacemaker cells was not detected in non-pacemaker atrial cells under the same experimental conditions. However, latent pacemaker cells isolated from the inferior right atrium at its junction with the inferior vena cava (like the primary pacemaker cells isolated from the SA node) did show Ni²⁺-sensitive rises in cytosolic Ca²⁺ during ramp depolarizations. In spontaneously beating latent pacemaker cells, Ni²⁺ decreased the slope of the late phase of pacemaker depolarization and slowed the rate of beating. It was therefore proposed that the mechanism suggested above in relation to normal activity of pacemaker cells (Ca²⁺ entry though T-type Ca²⁺ channels leading to subsarcolemmal Ca²⁺ release from the SR followed by depolarization mediated by Na⁺-Ca²⁺ exchange) might contribute, in latent pacemaker cells, to the development of ectopic atrial arrhythmias.

The experiments of Hüser et al. (2000) therefore provide important insights concerning mechanisms regulating pacemaker activity in the SA node, and perhaps in the generation of certain types of atrial arrhythmia. It remains to be seen whether the subsarcolemmal rises in cytosolic Ca²⁺ may regulate other ionic currents in addition to Na⁺-Ca²⁺ exchange current. For example, it has been suggested that I_f activated by hyperpolarization and also the delayed rectifier K⁺ current may be regulated by cytosolic Ca²⁺ (Zaza et al. 1997). It is also possible that increases in cytosolic Ca²⁺, including that released from the SR, might play a role in autonomic influences on pacemaking. For example, it might be questioned how increases in L-type Ca²⁺ current as a consequence of sympathetic stimulation lead to a positive chronotropic effect: these currents contribute to a small fraction of the cardiac pacemaking cycle (mainly the upstroke of the action potential and a brief period preceding this) and it is therefore difficult to see how a major reduction in the beat interval could be a direct consequence of enhancement of L-type Ca²⁺ current. However, the additional Ca²⁺ entering the cell by this pathway would be expected to lead to a greater loading of the SR with Ca²⁺, and larger Ca²⁺ transients could exert important indirect effects on other currents determining pacemaker activity. In the context of the paper by Hüser et al. (2000), although direct effects of autonomic influences on T-type Ca²⁺ currents have not been reported, a triggering effect of Ca²⁺ entering via T-type Ca²⁺ channels to release SR Ca²⁺ might be amplified by increased loading of the SR with Ca²⁺. The possible importance of SR-released Ca²⁺ for the effects of β-adrenoceptor stimulation on pacemaker activity is supported by the observation that the log(concentration)- response curve for the action of isoprenaline on the rate of beating of guinea-pig atria showed a lower maximum and less steep slope following pretreatment of the preparations with 2 μM ryanodine (Rigg & Terrar, 1998). Evidence from amphibian pacemaker cells also supports the suggestion that the effects of β-adrenoceptor stimulation on firing rate are mediated largely by an increase in the amplitude of the cytosolic Ca²⁺ transient (Ju & Allen, 1999).

Since the observations of Hüser et al. (2000) support the proposal that Ca²⁺ released from the SR in response to Ca²⁺ entry via T-type Ca²⁺ channels can exert an important amplifying influence on pacemaker mechanisms in the SA node, the possibility arises that this mechanism might under some circumstances be the primary mechanism leading to initiation of action potentials in the SA node. This will be an exciting area for future investigation.