Calcium signaling in non-excitable cells:: Ca2+ release and influx are independent events linked to two plasma membrane Ca2+ entry channels

The regulatory mechanism of Ca2+ influx into the cytosol from the extracellular space in non-excitable cells is not clear. The "capacitative calcium entry" (CCE) hypothesis suggested that Ca2+ influx is triggered by the IP3-mediated emptying of the intracellular Ca2+ Stores. However, there is no clear evidence for CCE and its mechanism remains elusive. In the present work, we have provided the reported evidences to show that inhibition of IP3-dependent Ca2+ release does not affect Ca2+ influx, and the experimental protocols used to demonstrate CCE can stimulate Ca2+ influx by means other than emptying of the Ca2+ Stores. In addition, we have presented the reports showing that IP3-mediated Ca2+ release is linked to a Ca2+ entry from the extracellular space, which does not increase cytosolic [Ca2+] priorto Ca2+ release. Based on these and other reports, we have provided a model of Ca2+ signaling in non-excitable cells, in which IP3-mediated emptying of the intracellular Ca2+ store triggers entry of Ca2+ directly into the store, through a plasma membrane TRPC channel. Thus, emptying and direct refilling of the Ca2+ stores are repeated in the presence of IP3, giving rise to the transient phase of oscillatory Ca2+ release. Direct Ca2+ entry into the store is regulated by its filling status in a negative and positive manner through a Ca2+-binding protein and Stim1/Orai complex, respectively. The sustained phase of Ca2+ influx is triggered by diacylglycerol (DAG) through the activation of another TRPC channel, independent of Ca2+ release. The plasma membrane IP3 receptor (IP3R) plays an essential role in Ca2+ influx, by interacting with the DAG-activated TRPC, without the requirement of binding to IP3.