Programme Contents

Ca²⁺ influx through store-operated Ca²⁺ channel in mouse sinoatrial node

In both excitable and non-excitable cells, the depletion of intracellular Ca²⁺ stores causes a flux of Ca²⁺ into the cells and refills the Ca²⁺ store to its original level. The inward Ca²⁺ flux resulting from depletion of Ca²⁺ store is through the store-operated cation channels (SOCCs). There is growing evidence that SOCCs play an important role in muscle cell signalling (for review see Gailly, 2002).

In previous studies, we found that intracellular Ca²⁺ stores are involved in cardiac pacemaking (for review see Ju & Allen, 2001). To examine if store–operated Ca²⁺ entry is present in cardiac pacemaker tissue and its possible role in regulating heart rate, sinoatrial node (SAN) tissue was dissected from mouse right atria of the heart and loaded with the Ca²⁺ indicator indo-1 AM. In the presence of extracellular Ca²⁺ ([Ca²⁺]_o ), cyclopiazonic acid (CPA), a selective sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) inhibitor, significantly increased resting [Ca²⁺]_i and gradually reduced the amplitude of [Ca²⁺]_i transients. Incubating SAN in Ca²⁺ free solution caused a substantial decline in resting [Ca²⁺]_i and stopped pacemaker activity. Reintroduction of Ca²⁺ (1.8 mM) to the perfusate in the presence of CPA evoked a striking increase in resting [Ca²⁺]_i, a characteristic of SOCC activity. The Ca²⁺ influx in response to reintroduction of [Ca²⁺]_o was 7.1 ± 3.2 fold greater in the presence of CPA than in its absence (p < 0.03, n = 11), which suggested that the Ca²⁺ influx was dependent on the SR store depletion. It is known that the Na⁺-Ca²⁺ exchanger exists in cardiac pacemaker tissue. After a period of incubation in zero Ca²⁺ solution, the reintroduction of Ca²⁺ could also activate the reverse mode of Na⁺-Ca²⁺ exchanger and increase Ca²⁺ influx. To test this possibility, we applied Na⁺-Ca²⁺ exchanger inhibitor KBR -7943. We found that in the presence of KBR -7943, there was still a significant rise of [Ca²⁺]_i in response to the depletion of SR the Ca²⁺ store. Moreover, gadolinium (100 μM), a known SOCC inhibitor, significantly inhibited 72 ± 8% of Ca²⁺ influx in the present of CPA (P< 0.01, n = 4).

Recent studies have suggested that SOCCs might be related to the transient receptor potential canonical (TRPC) gene family. We examined SAN mRNA expression of the seven known mammalian TRPC isoforms by RT-PCR. mRNA for TRPC1, 2, 3, 4, 6 and 7 was detected in SAN, whereas that for the TRPC5 was not. These results suggest that cardiac pacemaker tissue exhibits store-operated Ca²⁺ activity which may be due to expression of TRPCs in these cells.

Gailly P. (2002) Biochimica et Biophysica Acta 1600, 38-44.

Ju, Y.K. & Allen, D.G. (2001) Clinical and Experimental Pharmacology and Physiology 28, 703-8.