Calcium release from inositol 1,4,5-trisphosphate receptors influences cardiac pacemaker function in mouse sino-atrial node

Y.K. Ju,¹ B.H. Lee,¹ D. Lai,¹ E.A. Woodcock,² M.B. Cannell³ and D.G. Allen,¹ ¹School of Medical Sciences and Bosch Institute, University of Sydney, NSW 2006, Australia, ²Baker IDI Heart and Diabetes Institute, PO Box 6492, St Kilda Road Central, VIC 8008, Australia and ³The Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.

It has been found that Inositol 1,4,5-trisphosphate receptors (IP₃Rs), which function as inositol 1,4,5-trisphosphate (IP₃)-gated Ca²⁺ channels, are expressed in working cardiac myocytes (Nosek et al., 1986). In atrial cells, accumulated evidence shows that IP₃R signalling is involved in arrhythmic activity (Li et al., 2005; Woodcock, Kistler & Ju, 2009). However, there is little direct evidence whether IP₃Rs expression in adult mammalian sino-atrial node (SAN), the origin site of generating rhythm in the heart. In current studies, we quantified the level of the expression of IP₃Rs in the SANs by using a new cell direct qPCR technique. We find that both centre and peripheral SANs expression of IP₃Rs. We also studied the effect of IP₃R agonist, such as endothelin-1, IP₃-butyryloxymethyl ester (IP₃-BM), and antagonist 2-aminoethoxy diphenylborate (2-APB), on intracellular Ca²⁺ of spontaneously firing sinoatrial node preparations. In the presence of 10 nM endothelin-1, the resting [Ca²⁺]_i was increased by 36 ± 13 % (n = 5; P < 0.05) and the firing rate was increased by 20 ± 8 % (P < 0.05). The results were similar when IP₃-BM was used. IP₃R antagonist 2-APB reduced intracellular Ca²⁺ and slowed the firing rate. However, such effects were only seen in wild type but not in IP₃R2 knock out mice. The localisation of IP₃R2s and IP₃ induced Ca²⁺ sparks also further support that that IP₃Rs are involved in cardiac peacemaking through the release of Ca²⁺ from the intracellular Ca²⁺ stores that are near subsarcolemmal membrane.

Li X, Zima AV, Sheikh F, Blatter LA, Chen J. (2005) Endothelin-1-induced arrhythmogenic Ca²⁺ signaling is abolished in atrial myocytes of inositol-1,4,5-trisphosphate(IP3)-receptor type 2-deficient mice. Circ. Res. 96: 1274-81.

Nosek TM, Williams MF, Zeigler ST, Godt RE. (1986) Inositol trisphosphate enhances calcium release in skinned cardiac and skeletal muscle. Am. J. Physiol. 250: C807-C811.

Woodcock EA, Kistler PM, Ju YK. (2009) Phosphoinositide signalling and cardiac arrhythmias. Cardiovasc. Res. 82: 286-95.