IP3-induced release

Other receptor systems are coupled via GTP-binding proteins (G ), which activate phospholipase C. Activation of this enzyme releases the second messengers inositol triphosphate (IP3) and diacylglycerol (DAG) from the membrane phospholipid phosphatidylinositol bisphosphate (P1P2). The IP3 induces release of Ca2+ from the sarcoplasmic reticulum (SR), which, together with DAG, activates protein kinase C. The protein kinase C serves then to phosphorylate a set of tissue-specific substrate enzymes, usually not phosphorylated by protein kinase A, and thereby affects their activity. 

New IP3 5-phosphatase inhibitors have been prepared L- nyo-inositol 1,4,5-trisphosphorothioate and myo-inositol 1,3,5-trisphosphorothioate, which inhibited IP3 metabolism with concomitant elevation of the heparin-sensitive IP3-induced release of 45Ca2+ in T cells (Ward et al, 1994) and L-c/zira-inositol 1,4,6-triphosphate and the corresponding trisphosphoro-thioate compound L-c/n>o-I(l,4,6)PS3, which potentially and selectively inhibited inositol polyphosphate metabolism without affecting Ca + stores in electrically permeabilized neuroblastoma cells (Hansbro et al., 1994). These novel 5-phosphatase inhibitors may provide a starting point for development of cell-permeable analogues for further studies of the functions of IP3 and IP4 in the regulation of [Ca " ] in smooth muscle. 

As discussed above, biphasic regulation of IP3-induced Ca2+ release by Ca2+ is reminiscent of the behavior of the CICR channel from skeletal muscle. In addition, the inhibitory action of Ca2+ on IP3-induced Ca2+ release... 

Further support for the hypothesis that Ca2+ plays a central role in regulating phytoalexin accumulation is provided by experiments in which the turnover of phosphatidylinositol was measured in the plasma membrane of elicitor-treated carrot cells [17]. The carrot cells were first labelled with [3H]myo-inositol and, after the addition of elicitors, acid extracts of the cells were analyzed chromatographically for the production of inositol trisphosphate (IP3). In cells treated with elicitor, the release of radioactive IP3 increased with time and attained a maximum at 3 - 5 min after treatment. Phospholipase activity responsible for the degradation of phosphorylated phosphatidylinositol increased correspondingly. Several reports have shown that IP3 induces rapid release of Ca2+ from intracellular stores in animal cells [18, 19]. Studies on plant cells have also demonstrated that exogenous IP3 releases Ca2+ from microsomal preparations at micromolar concentrations, although only limited... 

Gj, G0, Gq). As a result, the inner leaflet of the plasma membrane is the source of a variety of chemical mediators that are released as a consequence of receptor activation. These mediators include inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG), which are both products of Pi-specific phospholipase C (Fig. 6-23) and arachidonic acid, which is an unsaturated fatty acid product of phospholipase A2 and phosphatidate (a product of phospholipase D). IP3 induces the release of Ca2+ ions from intracellular endoplasmic reticulum stores. DAG is a known activator of a lipid-dependent serine/threonine protein kinase (protein kinase C). 

The IP3 receptor in ASM is competitively blocked by low molecular weight heparin (Chopra etal., 1989). The sensitivity of this effect is high, half maximal inhibition of IP3-induced Ca release occurring at 0.8/tgml" low molecular weight heparin. The inhibitory effect is struc-tiually specific to low molecular weight heparin and its... 

A powerful new approach is the direct measurement of released Ca + with fluorescent Ca + indicators such as fura-2 or fluo-3, whereby the permeabilized smooth muscle has to be placed into microcuvettes (lino, 1991). Since force is no longer required as an indicator, Ca + release may be measured in the absence of MgATP, which allowed the study of adenine nucleotides on the IP3-induced Ca + release without the interference by Ca + uptake (lino, 1991 Hirose and lino, 1994). Inclusion of ryanodine allows a separate study of the IP3- and caffeine-sensitive stores (Hirose et al., 1993). Experimental protocols have been designed that allow the use of high concentrations of EGTA, thereby preventing Ca -mediated feedback regulation of the Ca2+ release (Hirose and lino, 1994). Direct measurement of Ca + release with indicators becomes even more powerful in combination with... 

Fig. 9.23. Oscillations of cytosolic Ca (in p,M, solid line) following sustained stimulation by an external signal in the extended version of the model based on Ca -induced Ca release (CICR). Also represented are the normalized concentration of IP3 (dashed line) and the fraction of active, nondesensitized IP3 receptors (dotted line), which are both driven here passively by oscillations based on CICR. In the extended model, which contains the five variables listed in table 9.1, IP3-mediated release of Ca" and CICR are supplemented with the following additional regulatory processes activation of IP3 synthesis by Ca, and desensitization of the IP3 receptor induced by IP3 (as in the present simulations) or cytosolic Ca (Dupont et al, 1991).

Malcuit C, Knott JG, He C, Wainwright T, Parys JB, Robl JM, Fissore RA. 2005. Fertilization and inositol 1,4,5-trisphosphate (IP3)-induced calcium release in type-1 inositol 1,4,5-trisphosphate receptor down-regulated bovine eggs. Biol Reprod 73(1) 2-13. 

Another mechanism in initiating the contraction is agonist-induced contraction. It results from the hydrolysis of membrane phosphatidylinositol and the formation of inositol triphosphate (IP3)- IP3 in turn triggers the release of intracellular calcium from the sarcoplasmic reticulum and the influx of more extracellular calcium. The third mechanism in triggering the smooth muscle contraction is the increase of calcium influx through the receptor-operated channels. The increased cytosolic calcium enhances the binding to the protein, calmodulin [73298-54-1]. 

IP3 Receptors. Figure 1 Interplay between Ca2+ channels. Ca2+ signals are initiated when an extracellular stimulus (red) directly opens a Ca2+ channel in the plasma membrane or indirectly, via a signalling pathway (green), opens an intracellular Ca2+ channel. Ca2+ signals may then be propagated across the cell by Ca2+-induced Ca2+ release mediated by IP3R or RyR. 

However, the total regulatory system is not so simple and linear. In skinned muscle preparations especially, it can be shown that there are calcium stores which cannot be released by IP3 but which are released by elevated levels of calcium itself That is, by the mechanism of calcium induced calcium release (CICR). The CICR... 

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