Several receptors for neurotransmitters, hormones and growth factors cause accelerated phosphodiesteratic breakdown of poly-phosphoinositides when activated1,2. One of the soluble products of this reaction, inositol-1,4,5-trisphosphate (Ins(l,4,5)P3) is thought to act as a second messenger signalling the release of Ca2+ from intracellular stores3. In support of this hypothesis, several studies have shown that Ins(l,4,5)P3 releases sequestered Ca2+ from permeable cells4-6 and microsomes7+9. On the basis of certain structural requirements for Ca2+-releasing activity by inositol phosphates, it has been postulated that Ins(l,4,5)P3 acts by binding to a specific intracellular receptor, probably on a component of the endoplasmic reticulum10. Here we report that 32P-Ins(l,4,5)P 3 binds to a specific saturable site in permeabilized guinea pig hepatocytes and rabbit neutrophils, and that the properties of this binding site suggest that it is the physiological receptor for Ins(l,4,5)P3.
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