Endothelin-1–Induced Arrhythmogenic Ca ²⁺ Signaling Is Abolished in Atrial Myocytes of Inositol-1,4,5-Trisphosphate(IP ₃ )–Receptor Type 2–Deficient Mice

Abstract

Abstract-Recent studies have suggested that inositol-1,4,5-trisphosphate-receptor (IP 3 R)-mediated Ca 2ϩ release plays an important role in the modulation of excitation-contraction coupling (ECC) in atrial tissue and the generation of arrhythmias, specifically chronic atrial fibrillation (AF). IP 3 R type-2 (IP 3 R2) is the predominant IP 3 R isoform expressed in atrial myocytes. To determine the role of IP 3 R2 in atrial arrhythmogenesis and ECC, we generated IP 3 R2-deficient mice. Our results revealed that endothelin-1 (ET-1) stimulation of wild-type (WT) atrial myocytes caused an increase in basal [Ca 2ϩ ] i , an enhancement of action potential (AP)-induced [Ca 2ϩ ] i transients, an improvement of the efficacy of ECC (increased fractional SR Ca 2ϩ release), and the occurrence of spontaneous arrhythmogenic Ca 2ϩ release events as the result of activation of IP 3 R-dependent Ca 2ϩ release. In contrast, ET-1 did not alter diastolic [Ca 2ϩ ] i or cause spontaneous Ca 2ϩ release events in IP 3 R2-deficient atrial myocytes. Under basal conditions the spatio-temporal properties (amplitude, rise-time, decay kinetics, and spatial spread) of [Ca 2ϩ ] i transients and fractional SR Ca 2ϩ release were not different in WT and IP 3 R2-deficient atrial myocytes. WT and IP 3 R2-deficient atrial myocytes also showed a significant and very similar increase in the amplitude of AP-dependent [Ca 2ϩ ] i transients and Ca 2ϩ spark frequency in response to isoproterenol stimulation, suggesting that both cell types maintained a strong inotropic reserve. No compensatory changes in Ca 2ϩ regulatory protein expression (IP 3 R1, IP 3 R3, RyR2, NCX, SERCA2) or morphology of the atria could be detected between WT and IP 3 R2-deficient mice. These results show that lack of IP 3 R2 abolishes the positive inotropic effect of neurohumoral stimulation with ET-1 and protects from its arrhythmogenic effects. (Circ Res. 2005;96:1274-1281.)Key Words: IP 3 receptor Ⅲ intracellular calcium Ⅲ atrial arrhythmias Ⅲ excitation-contraction coupling Ⅲ endothelin C hronic atrial fibrillation (AF) is the most common sustained form of cardiac arrhythmia. AF is characterized by an atrial activation rate of typically Ͼ400 beats per minute, and is associated with 2 major complications including cardiac dysfunction and thrombus formation, resulting in an increased risk of morbidity because of heart failure and stroke. 1,2 In recent years, considerable attention has focused on the cellular and molecular mechanisms involved in AF (for review see Nattel 3,4 activates the RyR which leads to massive Ca 2ϩ release from the SR by a mechanism known as Ca 2ϩ -induced Ca 2ϩ release (CICR 12 ), which is required for inducing contraction.Cardiac myocytes also contain IP 3 R channels, however their functional importance in the heart has remained controversial. IP 3 Rs release Ca 2ϩ from intracellular Ca 2ϩ stores when activated by IP 3 , a product generated by phospholipase C (PLC) metabolism of phosphoinositol-4,5-bisphosphate (PIP 2 ) in response to G-protein-c...

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

“…It should be noted that mice deficient in cardiac expression of type 2 IP 3 Rs are viable and do not have any apparent cardiac defects [17]. Furthermore, complete single deletion of type 1, type 2 or type 3 IP 3 Rs in mice, or combined knockout of types 2 and 3 IP 3 Rs, does not prevent the murine heart (or whole animal) from developing [110,111].…”

“…It has been proposed that type 1 IP 3 Rs are dominant in human atrial and rat Purkinje myocytes [15,16], whereas atrial and ventricular myocytes from most other animal species express predominantly type 2 IP 3 R and, to a lesser extent, type 3 IP 3 R (e.g. [17,18]). Overall, the majority of studies have concluded that the most prevalent IP 3 R isoform within contractile cardiomyocytes is type 2 (see Table 1).…”

“…the IP 3 5' phosphatase), which does not necessarily have to be present at 1:1 stoichiometry to completely clamp IP 3 signaling. With respect to transgenic animals, it has been shown that the positive inotropic and pro-arrhythmic effects of endothelin-1 were completely abolished in mouse atrial cells lacking the IP 3 R type-2 isoform [17]. This loss of response to endothlelin-1 was specifically coupled to the absence of IP 3 Rs, since the myocytes retained a normal positive inotropic response to β-adrenergic stimulation.…”

“…This finding implies that, in addition to increasing SR Ca 2+ release during systole through activation of IP 3 Rs, IP 3 affects cellular Ca 2+ homeostasis in yet another way. In fact, G q protein-coupled agonists or IP 3 may also increase diastolic cytoplasmic Ca 2+ concentration [17,70] and alter the kinetics of the cytoplasmic Ca 2+ transient with a prolongation of the rising phase and an acceleration of the decay [72]. Clearly, future studies should aim at analyzing IP 3 effects on cellular Ca 2+ fluxes in a more quantitative way, which will help gain more insight into the actions of IP 3 on cellular Ca 2+ homeostasis.…”

“…Increased phosphoinositide breakdown and inositol phosphate generation in atrium is mediated by receptors for acetylcholine (muscarinic, M 1 and M 3 ), angiotensin II (AT 1 ), endothelin (ET A ), histamine (H 1 ), 5-hydroxytryptamine (5-HT 2 ), norepinephrine (α 1 ) and vasopressin in various species including cat, chick, guinea-pig, mouse, rat and human [10,[112][113][114][115][116][117][118][119][120][121][122][123][124][125]. Furthermore, expression of IP 3 Rs in atrial myocytes has been demonstrated at mRNA and protein levels in various organisms, including mouse, rabbit, rat and human [15,17,41,58]. Myocytes from animal atrium are particularly rich in type 2 IP 3 Rs and may also express type 3 IP 3 Rs [41].…”