Transduction mechanisms

Taste-active chemicals react with receptors on the surface of sensory cells in the papillae causing electrical depolarization, ie, drop in the voltage across the sensory cell membrane. The collection of biochemical events that are involved in this process is called transduction (15,16). Not all the chemical steps involved in transduction are known however, it is clear that different transduction mechanisms are involved in different taste quaUties different transduction mechanisms exist for the same chemical in different species (15). Thus the specificity of chemosensory processes, ie, taste and smell, to different chemicals is caused by differences in the sensory cell membrane, the transduction mechanisms, and the central nervous system (14). 

An alternative view (123) is that no single model can adequately explain why any given compound is sweet. This hypothesis derives from several features. First, there is the observation that all carbohydrates having a critical ratio of OH to C are sweet tasting. In other words, there are no stmctural constraints to the sweetness of carbohydrates. Second, not all sweeteners can be fit to the same SAR model. Rather, some fit one, others fit another. Third, studies on the transduction mechanisms of sweetness suggest more than a single mechanism for sweet taste, implying multiple receptors for sweeteners. 

Transduction mechanism Inhibition of adenylyl cyclase stimulation of tyrosine phosphatase activity stimulation of MAP kinase activity activation of ERK inhibition of Ca2+ channel activation stimulation of Na+/H+ exchanger stimulation of AM PA/kainate glutamate channels Inhibition of forskol in-stimulated adenylyl cyclase activation of phos-phoinositide metabolism stimulation of tyrosine phosphatase activity inhibition of Ca2+ channel activation activation of K+ channel inhibition of AM PA/ kainate glutamate channels inhibition of MAP kinase activity inhibition of ERK stimulation of SHP-1 and SHP-2 Inhibition of adenylyl cyclase stimulation of phosphoinositide metabolism stimulation of tyrosine phosphatase activation of K+ channel inhibi-tion/stimulation of MAP kinase activity induction of p53 and Bax Inhibition of adenylyl cyclase stimulation of MAP kinase stimulation of p38 activation of tyrosine phosphatase stimulation of K+ channels and phospholipase A2 Inhibition of adenylyl cyclase activation/ inhibition of phosphoinositide metabolism inhibition of Ca2+ influx activation of K+ channels inhibition of MAP kinase stimulation of tyrosine phosphatase... 

A cell subjected to a stress of any kind can potentially exhibit a wide range of responses. Severe stress may lead to cell death and, ultimately, to cell lysis imposition of less severe conditions may result in a metabolically perturbed system, which may either revert to its initial state or adapt in some way to the imposed conditions. Figure 10 shows a hypothetical scheme, presented by Prokop and Bajpai [12], for the signal-response cascade associated with hydrodynamic shear stress. The signal reception/transduction mechanisms are, as yet, poorly understood. While Fig. 10 can be applied to any biological system, Namdev and... 

In Colpaert. F.C., and Balster, R.L., eds. Transduction Mechanisms of Drug Stimuli. Berlin Springer-Verlag, 1988. pp. 16-31. 

Schild D. and Restrepo D. (1998). Transduction mechanisms in vertebrate olfactory receptor cells. Physiol Revs 78, 429-466. 

FIGURE 5.43 Signal transduction mechanism for dendritic molecule 42, through a self-immolative reaction sequence. 

This term has increasingly come to be employed (as here) in a rather different sense from that introduced by R. F. Furchgott (Section 1.4.2). With this newer usage, intrinsic efficacy indicates the maximum receptor activation (often expressed as the fraction of receptors in the active state) that can be achieved by an agonist acting through a mechanism that can be formulated and studied at the molecular level, as in the present example. The intention of this redefinition is to focus on the receptor itself and its immediate transduction mechanism (e.g., G-protein activation), rather than on the cellular events that follow. 

G-PROTEIIM-COUPLED RECEPTORS CONSTITUTE A UNIFYING SIGNAL-TRANSDUCTION MECHANISM... 

Finally, it may be possible to obtain some limited information on the mechanisms of action of agonists from the shapes of binding curves. For example, as discussed later, the binding of some agonists is affected by guanosine triphosphate (GTP), immediately suggesting the involvement of G-proteins in the transduction mechanism. 

In Chapter 1 (Section 1.2.4.3), the Hill equation and the Hill coefficient, nH, are described. Hill coefficients greater than or less than unity are often interpreted as indicating positive or negative cooperativity, respectively, in the relationship between receptor occupancy and response. For example, positive cooperativity could arise due to amplification in a transduction mechanism mediated by G-proteins and changes in cell calcium concentration. 

Neary JT Department of Veterans Affairs, Medical Center, Miami, FL Effects of lead on nerve growth factor-induced neurite outgrowth and branching due to an interaction of lead with signal transduction mechanisms important for neuronal differentiation Department of Veterans Affairs, Research and Development... 

Roy, D. Belsham, D. D. (2002). Melatonin receptor activation regulates GnRH gene expression and secretion in GT1-7 GnRH neurons. Signal transduction mechanisms. J. Biol. Chem. 277, 251-8. 

Fig. 5 Proposed signal transduction mechanisms that stimulate the pheromone biosynthetic pathway in Helicoverpa zea and Bombyx mori. It is proposed that PBAN binds to a G protein-coupled receptor present in the cell membrane that upon PBAN binding will induce a receptor-activated calcium channel to open causing an influx of extracellular calcium. This calcium binds to calmodulin and in the case of B. mori will directly stimulate a phosphatase that will dephosphorylate and activate a reductase in the biosynthetic pathway. In H. zea the calcium-calmodulin will activate adenylate cyclase to produce cAMP that will then act through kinases and/or phosphatases to stimulate acetyl-CoA carboxylase in the biosynthetic pathway...

In concluding the section on photosynthesis, this energy transduction mechanism producing oxygen is found in one major group of bacteria, a chemotype with... 

As discussed throughout this article, there is strong evidence that the bluelight photoreceptor is a flavin (flavoprotein) anistropically bound to a membrane moiety. All potential sensory transduction mechanisms require such a secondary component. 

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