| Abstract
| - Stimulation of animal cells with a number of agonists causes a rise in intracellular Ca2+ which can occur at least temporarily even in the absence of any extracellular Ca2+. The explanation for this phenomenon lies in the mobilisation of Ca2+ from intracellular stores, and a recent consensus of opinion now focuses on the endoplasmic reticulum as the principal organella involved. The mitochandrion probably does not contribute any mobilized Ca2+, but instead increases its Ca2+ content in response to the cytosolic rise. The endoplasmic reticular (e.r.) Ca2+ pool which is responsive to agonists, is only a part of the total e.r. Ca2+, and this particular pool may well be closely associated with the plasma membrane.The compound which carries the message to mobilise Ca2+ from the receptor in the plasma membrane to the e r is inositol (1,4,5)trisphosphatens(1,4,5)P3 After it has interacted with a specific receptor in the e.r.to release Ca2+,Ins(1,4,5)P3 is deactivated by a specific 5-phosphatase which converts it to inositol(1,4)bisphosphate.
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