Inositol 1,4,5-trisphosphate activates TRPC3 channels to cause extracellular Ca2+ influx in airway smooth muscle cells

Transient receptor potential-3 (TRPC3) channels play a predominant role in forming nonselective cation channels (NSCCs) in airway smooth muscle cells (ASMCs) and are significantly increased in their activity and expression in asthmatic ASMCs. To extend these novel findings, we have explored the regulatory mechanisms that control the activity of TRPC3 channels. Our data for the first time reveal that inositol 1,4,5-trisphosphate (IP3), an important endogenous signaling molecule, can significantly enhance the activity of single NSCCs in ASMCs. The analog of diacylglycerol (DAG; another endogenous signaling molecule), 1-ol-eyl-2-acetyl-sn-glycerol (OAG), 1-stearoyl-2-arachidonoyl-snglycerol (SAG), and 1-stearoyl-2-linoleoyl-sn-glycerol (SLG) all augment NSCC activity. The effects of IP3 and OAG are fully abolished by lentiviral short-hairpin (sh) RNA-mediated TRPC3 channel knockdown (KD). The stimulatory effect of IP3 is eliminated by heparin, an IP3 receptor (IP3R) antagonist that blocks the IP3-binding site, but not by xestospongin C, the IP3R antagonist that has no effect on the IP3-binding site. Lentiviral shRNA-mediated KD of IP(3)R1, IP(3)R2, or IP(3)R3 does not alter the excitatory effect of IP3. TRPC3 channel KD greatly inhibits IP3-induced increase in intracellular Ca2+ concentration. IP(3)R1 KD produces a similar inhibitory effect. TRPC3 channel and IP(3)R1 KD both diminish the muscarinic receptor agonist methacholine-evoked Ca2+ responses. Taking these findings together, we conclude that IP3, the important intracellular second messenger, may activate TRPC3 channels to cause extracellular Ca2+ influx, in addition to opening IP(3)Rs to induce intracellular Ca2+ release. This novel extracellular Ca2+ entry route may play a significant role in mediating IP3-mediated numerous cellular responses in ASMCs and other cells.