Programme Contents

Store-operated Ca²⁺ entry and TRPC expression; possible roles in control of heart rate

Store-operated Ca²⁺ channels (SOCCs) were first identified in non-excitable cells by the observation that depletion of Ca²⁺ stores caused increased influx of extracellular Ca²⁺ (Putney, 1986). Recent studies have suggested that SOCCs might be related to the transient receptor potential (TRPC) gene family (Vazquez et al., 2004). In a previous study, we found that activation of the P2Y₁ purinergic receptor by ATP results in modulation of pacemaker firing due to receptor-coupled phospholipase C (PLC) activation and depletion of sarcoplasmic reticulum (SR) Ca²⁺ stores (Ju et al., 2003). Since activation of SOCCs also involves PLC, we speculated that SOCCs might also be present in pacemaker tissue (Ju & Allen, 2007).

To study SOCC in pacemaker tissue, we first developed a method for recording intracellular Ca²⁺ signals from mouse sinoatrial nodes (SAN) in which the structural integrity and activity of the node is preserved. Store-operated Ca²⁺ entry was investigated in isolated mouse sinoatrial nodes (SAN) dissected from right atria and loaded with Ca²⁺ indicators. Incubation of the SAN in Ca²⁺-free solution caused a substantial decrease in resting intracellular Ca²⁺ ([Ca²⁺]_i) and stopped pacemaker activity. Reintroduction of Ca²⁺ in the presence of cyclopiazonic acid, a selective sarcoplasmic reticulum Ca²⁺-ATPase inhibitor, led to sustained elevation of [Ca²⁺]_i; a characteristic of store-operated Ca²⁺ channel (SOCC) activity (Ju et al., 2007). Transcripts for all TRPC isoforms, except TRPC5 have been detected in SAN preparation. Immunohistochemistry studies also revealed the localizations of TRPC1, 3, 4, and 6 proteins in both the central and peripheral SAN (Ju et al., 2007).

The mechanism of cardiac pacemaking involves voltage-dependent pacemaker current; in addition there is growing evidence that intracellular sarcoplasmic reticulum (SR) Ca²⁺ release plays an important role. The SOCC antagonist, SKF-96365 (10 μmol/L) that inhibited Ca²⁺ influx reduced the spontaneous pacemaker rate and stopped pacemaker firing in the present of CPA (Ju et al., 2007). These newer findings suggest that Ca²⁺ entry and inward current triggered by store depletion might contribute to the pacemaker current and may play a role in control of heart rate.

Ju YK, Huang WB, Jiang L, Barden JA & Allen DG. (2003) Journal of Physiology, 552: 777-87.

Ju YK & Allen DG. (2007) Heart, Lung and Circulation, Sep 4; PMID: 17822952

Ju YK, Chu Y, Chaulet H, Lai D, Gervasio OL, Graham RM, Cannell MB & Allen DG (2007) Circulation Research, 100: 1605-14.

Putney JW. (1986) Cell Calcium, 7: 1-12.

Vazquez G, Wedel BJ, Aziz O, Trebak M, Putney JW Jr. (2004) Biochimica et Biophysica Acta, 1742: 21-36.