Store-operated Ca2+ influx and expression of TRPC genes in mouse sinoatrial node

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+ concentration ([Ca2+](i)) and stopped pacemaker activity. Reintroduction of Ca2+ in the presence of cyclopiazonic acid (CPA), a sarcoplasmic reticulum Ca2+ pump inhibitor, led to sustained elevation of [Ca2+](i), a characteristic of store-operated Ca2+ channel (SOCC) activity. Two SOCC antagonists, Gd3+ and SKF-96365, inhibited 72 +/- 8% and 65 +/- 8% of this Ca2+ influx, respectively. SKF-96365 also reduced the spontaneous pacemaker rate to 27 +/- 4% of control in the presence of CPA. Because members of the transient receptor potential canonical (TRPC) gene family may encode SOCCs, we used RT-PCR to examine mRNA expression of the 7 known mammalian TRPC isoforms. Transcripts for TRPC1, 2, 3, 4, 6, and 7, but not TRPC5, were detected. Immunohistochemistry using anti-TRPC1, 3, 4, and 6 antibodies revealed positive labeling in the SAN region and single pacemaker cells. These results indicate that mouse SAN exhibits store-operated Ca2+ activity which may be attributable to TRPC expression, and suggest that SOCCs may be involved in regulating pacemaker firing rate.