Stress synchronizes calcium release and promotes SR calcium leak

Activation of the sympathetic nervous system prepares the body for strenuous physical activity during an emergency or stressful situation. This ‘fight-or-flight’ response prepares the body to deal with a possible threat by rapidly increasing cardiac performance. It is well accepted that catecholamine-dependent stimulation of β-adrenergic receptors plays a central role in this process by boosting excitation–contraction (E-C) coupling.

β-Adrenergic stimulation has been shown to enhance intracellular Ca²⁺ release by activating L-type Ca²⁺ channels that initiate Ca²⁺-induced Ca²⁺ release from the sarcoplasmic reticulum (SR). Refilling of the SR is enhanced by phosphorylation of phospholamban, which stimulates the SR Ca²⁺-ATPase (SERCA2a) and increases the availability of SR Ca²⁺ for E-C coupling. Furthermore, it is believed that phosphorylation of the intracellular Ca²⁺ release channel, also known as ryanodine receptor type 2 (RyR2), enhances SR Ca²⁺ release (Wehrens et al. 2005).

Increased systolic Ca²⁺ release via RyR2 upon β-adrenergic stimulation may contribute to enhanced cardiac contractility by increasing the amplitude of the Ca²⁺ transient. Whereas it has been suggested that PKA-mediated phosphorylation of RyR2 might be responsible for this physiological enhancement of E-C coupling, this mechanism remains controversial (MacDonnell et al. 2008). On the other hand, RyR2 phosphorylation has also been shown to increase the propensity for spontaneous release of SR Ca²⁺ during diastole under conditions of chronic β-adrenergic stress (Marx et al. 2000). In a disease state such as chronic heart failure, SR Ca²⁺ leak via RyR2 may interfere with SR Ca²⁺ loading, thereby indirectly suppressing the amplitude of the Ca²⁺ transient and thus contractility (Wehrens et al. 2005).

The functional consequences of RyR2 phosphorylation have been a topic of debate in recent years, in part due to the discrepancies in findings obtained in isolated systems and in intact cardiomyocytes. It is challenging to determine the modulation of RyR2 open probability in a cellular environment, as β-adrenergic stimulation affects not only RyR2, but also the amplitude of the L-type Ca²⁺ channel and the extent of SR Ca²⁺ loading. In a recent issue of The Journal of Physiology, Ogrodnik & Niggli (2010) employed an elegant method to simulate the brief influx of Ca²⁺ which RyR2 channels normally encounter during E-C coupling. They demonstrated that UV flash uncaging of Ca²⁺ from DM-nitrophen produced a reproducible Ca²⁺ trigger that was not affected by β-adrenergic stimulation. In addition, the authors carefully ensured that all measurements of SR Ca²⁺ release were made at equal levels of SR Ca²⁺ loading before and after isoproterenol administration.

Under these well-defined experimental conditions, β-adrenergic stimulation increased SR Ca²⁺ release amplitude and coherence. The improved synchronization of triggered elementary SR Ca²⁺ release events may play a role in accelerating Ca²⁺ transients and enhancing myocyte contractility during the fight-or-flight response. The authors inferred that the enhanced Ca²⁺ release was caused by a higher RyR2 open probability due to an increased Ca²⁺ sensitivity. Under certain pathological conditions, however, Ca²⁺ sensitization of RyR2 may also promote the occurrence of spontaneous SR Ca²⁺ release events. These findings appear to support the theory that increased adrenergic stimulation in congestive heart failure promotes SR Ca²⁺ leak, associated with depressed cardiac contractility (Marx et al. 2000; Wehrens et al. 2005).

There has been great controversy in the literature over whether SR Ca²⁺ leak is caused by PKA or CaMKII phosphorylation of RyR2, and whether enhanced RyR2 sensitivity plays a role at all in this phenomenon (Eisner et al. 2009). In the study by Ogrodnik & Niggli (2010), membrane-permeant inhibitors of both PKA and CaMKII suppressed the increase in Ca²⁺ spark frequency following isoproterenol application. However, only the CaMKII inhibitor KN-93 prevented the depression of SR Ca²⁺ content, leading the authors to conclude that CaMKII, rather than PKA, plays a predominant role in causing SR Ca²⁺ leak. That observation is in line with those of other groups, who reported that inhibition of CaMKII reduces SR Ca²⁺ leak induced by β-adrenergic stimulation (Curran et al. 2007). Recent studies also suggest that β-adrenergic stimulation may activate CaMKII in a PKA-independent and, perhaps more surprisingly, a Ca²⁺-independent manner (Erickson et al. 2008). Thus, although the classic inotropic and lusitropic effects of β-adrenergic stimulation are dependent on downstream effects of PKA activation, other evidence including data presented by Ogrodnik & Niggli (2010) implicates an important role for CaMKII.

It is important to consider that these data were obtained in healthy myocytes, in which increased SR Ca²⁺ leak did not lead to a reduction in SR Ca²⁺ loading due to concomitant upregulation of SERCA2a activity following β-adrenergic stimulation. Under pathological conditions associated with a loss of SERCA2a responsiveness due to β-adrenergic stimulation, however, Ca²⁺ leak via sensitized RyR2 may cause contractile dysfunction because SERCA2a fails to maintain normal SR Ca²⁺ loading.

At present, it remains unclear whether defects in RyR2 sensitization and spatiotemporal incoherence play a role in arrhythmogenesis, for example in the context of catecholaminergic polymorphic ventricular tachycardia, which is caused by mutations in RyR2. Moreover, it would be interesting to further differentiate the contributions of RyR2 phosphorylation and SR Ca²⁺ loading, for example by repeating the aforementioned careful studies in ventricular myocytes isolated from mice in which the PKA phosphorylation site S2808 (Wehrens et al. 2006) or CaMKII phosphorylation site S2814 of RyR2 (Chelu et al. 2009) have been genetically inactivated. Finally, the study by Ogrodnik & Niggli (2010) highlights the importance of studying physiological mechanisms such as β-adrenergic signalling in the native cell type using carefully designed experiments.