Calcium-mobilizing receptors function to regulate ion channels located not only in the plasma membrane but also across the membranes of intracellular organelles, particularly the endoplasmic reticulum. A characteristic feature of such receptors is that they stimulate the hydrolysis of an inositol lipid to generate a pair of second messengers. Diacylglycerol remains within the plasma membrane where it activates protein kinase C leading to the phosphorylation of proteins some of which may regulate specific ionic channels, such as the calcium-dependent potassium channel or the Na+/H+ exchanger which regulates intracellular pH. The inositol trisphosphate (Ins 1,4,5P3) released to the cytosol functions as a second messenger to release calcium from the endoplasmic reticulum. The Ins 1,4,5P3 acts on a specific receptor to enhance the passive efflux of calcium while having no effect on the active calcium pump. There are indications that this Ins 1,4,5P3-induced release of calcium from an internal membrane store might provide an explanation of excitation-contraction coupling in skeletal muscle. Skinned skeletal muscle cells can be induced to contract by adding Ins 1,4,5P3. Mobilization of calcium from intracellular reservoirs by Ins 1,4,5P3 may thus prove to be a ubiquitous and fundamental mechanism for regulating cellular activity.

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