Inositol 1,5,6-triphosphate

A critical component of the G-protein effector cascade is the hydrolysis of GTP by the activated a-subunit (GTPase). This provides not only a component of the amplification process of the G-protein cascade (63) but also serves to provide further measures of dmg efficacy. Additionally, the scheme of Figure 10 indicates that the coupling process also depends on the stoichiometry of receptors and G-proteins. A reduction in receptor number should diminish the efficacy of coupling and thus reduce dmg efficacy. This is seen in Figure 11, which indicates that the abiUty of the muscarinic dmg carbachol [51 -83-2] to inhibit cAMP formation and to stimulate inositol triphosphate, IP, formation yields different dose—response curves, and that after receptor removal by irreversible alkylation, carbachol becomes a partial agonist (68). 

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]. 

Excitation of smooth muscle via alpha-1 receptors (eg, in the utems, vascular smooth muscle) is accompanied by an increase in intraceUular-free calcium, possibly by stimulation of phosphoUpase C which accelerates the breakdown of polyphosphoinositides to form the second messengers inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 releases intracellular calcium, and DAG, by activation of protein kinase C, may also contribute to signal transduction. In addition, it is also thought that alpha-1 adrenergic receptors may be coupled to another second messenger, a pertussis toxin-sensitive G-protein that mediates the translocation of extracellular calcium. 

Ferguson, K.M., et al. Structure of the high affinity complex of inositol triphosphate with a phospholipase C pleckstrin homology domain. Celt 83 1037-1046, 1995. 

Second messenger, these are molecules produced by cellular effectors that go on to activate other biochemical processes in the cell. Some examples of second messengers are cyclic AMP, inositol triphosphate, arachidonic acid, and calcium ion (see Chapter 2.2). 

The NHR contains also the conserved Calcineurin docking site, PxlxIT, required for the physical interaction of NEAT and Calcineurin. Dephosphorylation of at least 13 serines residues in the NHR induces a conformational change that exposes the nuclear localization sequences (NLS), allowing the nuclear translocation of NEAT. Rephosphorylation of these residues unmasks the nuclear export sequences that direct transport back to the cytoplasm. Engagement of receptors such as the antigen receptors in T and B cells is coupled to phospholipase C activation and subsequent production of inositol triphosphate. Increased levels of inositol triphosphate lead to the initial release of intracellular stores of calcium. This early increase of calcium induces opening of the plasma membrane calcium-released-activated-calcium (CRAC) channels,... 

Another type of NR crosstalk, which has only recently been recognized, is the so-called nongenomic actions of several receptors that induce very rapid cellular effects. Effectively, evidence has accumulated over several decades that steroid receptors may have a role that does not require their transcriptional activation, such as modifying the activity of enzymes and ion channels. While the effects of steroids that are mediated by the modulation of gene expression do occur with a time lag of hours, steroids can induce an increase in several second messengers such as inositol triphosphate, cAMP, Ca2+, and the activation of MARK and PI3 kinase within seconds or minutes. Many mechanistic details of these nongenomic phenomena remain poorly understood. Notably, controversy still exists as to the identity of the receptors that initiate the non-genomic steroid actions. However, it now appears that at least some of the reported effects can be attributed to the same steroid receptors that are known as NRs. 

Purinergic System. Figure 2 Schematic of sympathetic cotransmission. ATP and NA released from small granular vesicles (SGV) act on P2X and a-i receptors on smooth muscle, respectively. ATP acting on inotropic P2X receptors evokes excitatory junction potentials (EJPs), increase in intracellular calcium ([Ca2+]j) and fast contraction while occupation of metabotropic ar-adrenoceptors leads to production of inositol triphosphate (IP3), increase in [Ca2+]j and slow contraction. Neuropeptide Y (NPY) stored in large granular vesicles (LGV) acts after release both as a prejunctional inhibitory modulator of release of ATP and NA and as a postjunctional modulatory potentiator of the actions of ATP and NA. Soluble nucleotidases are released from nerve varicosities, and are also present as ectonucleotidases. (Reproduced from Burnstock G (2007) Neurotransmission, neuromodulation cotransmission. In Squire LR (ed) New encyclopaedia of neuroscience. Elsevier, The Netherlands (In Press), with permission from Elsevier). 

It has been shown that inositol triphosphate (IP3) is involved in the excitation-contraction coupling in smooth muscle (Vergara et al., 1985), but presently no clear evidence has been reported for a similar involvement in skeletal muscle. If IP3 functions as a messenger for Ca release, it would bridge the gap between muscle metabolic changes and Ca release, as ATP is a prerequisite for IP3 regeneration. 

There is evidence for immunosuppressive effects of PAHs in rodents (Davila et al. 1997). For example, strong immunosuppressive effects were reported in mice that had been dosed with benzo[fl]pyrene and 3-methyl cholanthrene, effects that persisted for up to 18 months (Environmental Health Criteria 202). Multiple immu-notoxic effects have been reported in rodents, and there is evidence that these result from disturbance of calcium homeostasis (Davila et al. 1997). PAHs can activate protein tyrosine kinases in T cells that initiate the activation of a form of phospholipase C. Consequently, release of inositol triphosphate—a molecule that immobilizes Ca + from storage pools in the endoplasmic reticulum—is enhanced. 

Berridge M Inositol triphosphate and calcium signalling. Nature 1993 361 315. 

Figure 3.1 Schematic representation of a generic excitatory synapse in the brain. The presynaptic terminal releases the transmitter glutamate by fusion of transmitter vesicles with the nerve terminal membrane. Glutamate diffuses rapidly across the synaptic cleft to bind to and activate AMPA and NMDA receptors. In addition, glutamate may bind to metabotropic G-protein-coupled glutamate receptors located perisynaptically to cause initiation of intracellular signalling via the G-protein, Gq, to activate the enzyme phospholipase and hence produce inositol triphosphate (IP3) which can release Ca from intracellular calcium stores...

Myo-inositol is one of the most biologically active forms of inositol. It exists in several isomeric forms, the most common being the constituent of phospholipids in biological cell membranes. It also occurs as free inositol and as inositol hexaphosphate (IP6) also known as phytate which is a major source from food. Rice bran is one of the richest sources of IP6 as well as free inositol. Inositol is considered to belong to the B-complex vitamins. It is released in the gastrointestinal tract of humans and animals by the dephosphorylation of IP6 (phytate) by the intestinal enzyme phytase. Phytase also releases intermediate products as inositol triphosphate and inositol pentaphosphate. Inositol triphosphate in cellular membrane functions as an important intra- and intercellular messenger, that merits its value as a nutritional therapy for cancer. 

The intracellular hgand-gated Ca " channels include the channels in endoplasmic and sarcoplasmic reticulum (SR) membranes that are opened upon binding of the second messenger, inositol triphosphate (IP3). These are intracellular Ca release channels that allow Ca to exit from intracellular stores, and consequently to increase the concentration of cytoplasmic Ca [5]. A second type of intracellular Ca release channel is the Ca - and ryanodine-sensitive channel that was originally characterized and isolated from cardiac and skeletal muscle [5-7] but appears to exist in many types of cells. It has become evident that IP3-gated channels and ryanodine-sensitive channels are structurally related but distinct proteins [8] that are present in many cell types [9]. While very interesting, time and space will not allow for further discussion of these channels. 

Berridge, M.J. (1984). Inositol triphosphate and diacylglycerol as second messengers. Biochem. J. 220, 345-360. 

The intracellular processes which precede membrane activation appear to differ from those of MOE neurones, in that cyclic nucleotide gating may not occur. The transduction process which induces current flow in snake VN neurones, utilises as a putative second-messenger the modulator compound inositol triphosphate — Ins. (1,4,5) P3 = IP3 (Liu et al, 1999 Taniguichi et al, 2000). The proposed channel component associated with the microvillous membrane is one of the transient receptor potential family (TRPC-2 Heading Fig., pp. 94), the p-splice... 

Another popular assay for GPCR activation is to measure the increase in intracellular Ca2+ that occurs upon activation. GPCRs on the cell surface produce inositol triphosphate (IP3) via the action of Phospholipase C (PLC). IP3 stimulates calcium channels called IP3 receptors on the endoplasmic reticulum, which raise... 

In contrast, functional assays that look at post-receptor events, such as cAMP stimulation (Gs), cAMP inhibition (G ), inositol triphosphate (IP3)/mono-phosphate (IP1) increase (Gq), or intracellular calcium mobilization (Gq), are homogeneous, for the most part nonradioactive, and easy to automate (with... 

Liquid Chromatography. - Diasteriomeric phosphonodipeptides have been separated by ion exchange column chromatography.267 H.p.l.c. has been used for the analysis of a variety of biologically active phosphorus compounds, such as aminoacid phosphate esters,26 phosphinothrycin,269 inositol triphosphate,270 fructose diphosphate,271 pyridoxal phosphate,272 and ATP.273... 

Berridge M. J. Inositol triphosphate and diacy(glycerol TWo interacting second messengers. Annu Rev Biochem 1987 56, 159-93. 

Inositol triphosphate (IP3)-gated channels are also associated with membrane-bound receptors for hormones and neurotransmitters. In this case, binding of a given substance to its receptor causes activation of another membrane-bound protein, phospholipase C. This enzyme promotes hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) to IP3. The IP3 then diffuses to the sarcoplasmic reticulum and opens its calcium channels to release Ca++ ions from this intracellular storage site. 

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