Membrane embedded second messenger

The neurotransmitters of the ANS and the circulating catecholamines bind to specific receptors on the cell membranes of effector tissue. Each receptor is coupled to a G protein also embedded within the plasma membrane. Receptor stimulation causes activation of the G protein and formation of an intracellular chemical, the second messenger. (The neurotransmitter molecule, which cannot enter the cell, is the first messenger.) The function of intracellular second messenger molecules is to elicit tissue-specific biochemical events within the cell that alter the cell s activity. In this way, a given neurotransmitter may stimulate the same type of receptor on two different types of tissue and cause two different responses due to the presence of different biochemical pathways within each tissue. 

The picture that has emerged from these studies is of an initial interaction of a stimulus with a matched portion of a receptor protein embedded in the cell membrane (13,65). This initial interaction causes stimulation of the linked G-protein to form cGMP. This is coupled to the reactivity of adenylate cyclase in the cells, leading to increased levels of cAMP, which opens ion channels in the cell membrane. A similar sequence can alternatively activate inositol phosphate as a second messenger. Either odorants, cAMP or cGMP can cause a potential change in the membrane (13,70,71,72). As in hormone-sensitive and neurotransmitter-... 

Fig. 5.3 The situation (A) before and (B) after the receptor, activated by the hormone, had made contact with the transducer, a heterotrimeric apyG protein. After coupling to the receptor and activation, the aPyG protein dissociates into the p-ysubunits and the a-subunit, which is now in the active. GTPTiound, on state. The active G

Cells are composed of different intracellular compartments embedded in the cytoplasm, and are bounded by the plasma membrane. One of this compartments, the endoplasmic reticulum (ER), acts as intracellular store for Ca + ions. In a resting cell the Ca + concentration in the cytoplasm ([Ca +]c) is kept low ( 100 nM) and in the ER high ( 20 M) by active transport through the ER and plasma membrane. Binding of agonist to its receptor at the plasma membrane activates phospholipase C (PLC). This enzyme is responsible for the production of the second messenger IP3, which can freely diffuse in the cytoplasm. IP3 activates Ca " " channels... 

Membrane receptors are proteins (usually glycoproteins) embedded in the lipid bilayer of the membrane. They contain sites that interact with molecules external to the cell to initiate a number of different events, for example, transport of the molecule through the membrane or, if the molecule is a hormone or autocoid, the triggering of second messengers within the cell. The functioning of membrane receptors appears to be regulated in part by the microenvironment provided by the membrane lipids. 

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