CAMP-mediated cascade

Two other, faster-acting mechanisms produce some of the effects of steroids. Progesterone triggers a rapid drop in [cAMP], mediated by a plasma membrane receptor, and binding of progesterone to the classic soluble steroid receptor activates a MAPK cascade. 

Gomez-Diaz G et al (2004) cAMP transduction cascade mediates olfactory reception in Drosophila melanogaster. Behav Genet 34 395-406... 

A second level of amplification is illustrated by the cAMP-mediated stimulation of glycogenolysis. As we just discussed, cAMP promotes glycogen degradation via a three-stage cascade, that is, a series of reactions in which the enzyme catalyzing one step is activated (or inhibited) by the product of a previous step (see Figure 13-17a). The amplification that occurs in such a cascade depends on the number of steps in it. 

Mediator Receptor-hormone complex cAMP,cGMP,Ca k metabolites of complex phosphoinositols, kinase cascades... 

Phosphorylation of the -adrenergic receptor by PKA can be viewed as a classical example of negative feedback activation of the receptor stimulates intracellular cascades that feed back to reduce further receptor activation. Phosphorylation of the receptor by PKA could also mediate heterologous desensitization of the receptor any neu-rotransmitter-receptor system that works through cAMP would be expected to stimulate P-adrenergic receptor phosphorylation via PKA and lead to receptor desensitization. This is one mechanism by which one neurotrans-mitter-receptor system can affect another. 

Alternative second-messenger pathways may be at work in olfactory transduction. The role of cAMP in olfactory transduction is well established. Are there alternative pathways, such as those involving phospholipids and Ca2+ Several groups have reported that certain odorants can elicit an increase in the phosphoinositde second messenger inositol 1,4,5,-trisphosphate (IP3) (Ch. 20). However, there is no clear evidence that IP3 directly mediates an electrical response in OSNs, nor is there a clear rationale for two parallel excitatory odor transduction cascades. However, more recent data support the idea that phos-phoinositides or enzymes related to their metabolism may play a modulatory role, shaping the OSN output by... 

Glycogen phosphorylase isoenzymes have been isolated from liver, brain and skeletal muscle. All forms are subject to covalent control with conversion of the inactive forms (GP-b) to the active forms (GP-a) by phosphorylation on specific serine residues. This phosphorylation step, mediated by the enzyme phosphorylase kinase, is initiated by glucagon stimulation of the hepatocyte. Indeed, the same cAMP cascade which inhibits glycogen synthesis simultaneously stimulates glycogenolysis, giving us an excellent example of reciprocal control. 

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