Inositol phosphates have an important role in Ca
2+ mobilization and especially inositol 1, 4, 5-trisphosphate (IP
3) is now believed to release Ca
2+ from the endoplasmic reticulum (ER) . On the other hand, the mechanism of activation of Ca
2+ entry is unknown. Non-excitable cells have only receptoroperated Ca
2+ channels, lacking voltage-operated Ca
2+ channels, and are a useful system for studying signal transduction. In this review, some mechanisms for the regulation of Ca
2+ entry in non-excitable cells are discussed and a new hypothesis originally proposed by Putney (1986), the capacitative Ca
2+ entry model, is focussed. In this model, Ca
2+ influx across the plasma membrane is increased when the IP3-sensitive Ca
2+ pools is emptied. Capacitative Ca
2+ entry is now confirmed in rat parotid acinar cells by studies on the refilling process for intracellular Ca
2+ pools and by using the microsomal Ca
2+ATPase inhibitor thapsigargin, which does not increase cellular IP
3. Finally, capacitative Ca
2+ entry is expected to exist in a variety of cell types including excitable cells.
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