Cell Feedback mechanism

The responsiveness of a tissue to a hormone depends on the density of receptors within its component cells. The number of receptors is determined by their rate of synthesis and catabolism, which is itself controlled by complex feedback mechanisms involving hormone action. Some chemicals are known to interfere with this regulation. For example, TCDD can act to increase or decrease the expression of the oestrogen receptor. ... 

COMMENT You were showing your reticulata cell showing an increased aetivation. Some people think that they may have this feedback mechanism. Several years ago Koob and others were postulating that this was indeed part of an outflow system. And if you put GABA or GABAergic drugs into that outflow system, that you could also produce behaviors. 

Angiotensin II directly inhibits the secretion of renin from the granular cells. This negative feedback mechanism enables angiotensin II to limit its own formation. 

Perrimon, N. and McMahon, A. P. (1999), Negative feedback mechanisms and their roles during pattern formation , Cell, 97, 13-16. 

TCDD also causes thyroid tumours in male rats. This has been shown to proceed through a mechanism that involves altered thyroid hormone metabolism and consequent increases in feedback mechanisms, TSH (thyroid stimulating hormone), which results in a chronic proliferative stimulation of thyroid follicular cells. 

Once the siderophore-iron complexes are inside the bacteria, the iron is released and utilized for vital cell functions. The iron-free hydroxamate siderophores are commonly re-excreted to bring in an additional iron load (Enterobactin is at least partially degraded by a cytoplasmic esterase This cycle is repeated until specific intracellular ferric uptake regulation proteins (Fur proteins) bind iron, and signal that the intracellular iron level is satisfactory, at -which point ne-w siderophore and siderophore-receptor biosynthesis are halted and the iron-uptake process stops. This intricate feedback mechanism allows a meticulous control over iron(III) uptake and accumulation against an unfavorable concentration gradient so as to maintain the intracellular iron(III) level within the required narrow window. Several excellent reviews concerning siderophore-iron transport mechanisms have been recently published i.3,i6, is,40,45,60-62 ... 

Other photosensitisers in clinical or pre-clinical trials include zinc phthalocya-nine, aluminium sulphonated phthalocyanines, benzoporphyrins, benzochlorins and purpurin-lS-iV-alkylamides, all of which absorb strongly in the 675-700 nm region. An alternative approach to the photosensitisation in PDT involves the use of 5-aminolaevulinic acid (ALA). This compound itself is not a sensitiser but in human cells it is the key metabolic precursor in the biosynthesis of protoporphyrin IX, which can act as a photosensitiser. Normally the biosynthetic process would continue beyond protoporphyrin IX to the iron containing haem. However, by adding extra ALA and iron chelators, the ferrochelatase action is inhibited and the normal feedback mechanism by-passed resulting in a build up of protoporphyrin IX in the cell. The mechanism is illustrated in Figure 4.24. ... 

Presynaptic or prejunctional receptors are located on the presynaptic nerve endings and function to control the amount of transmitter released per nerve impulse and in some instances to affect the rate of transmitter synthesis through some as yet undetermined feedback mechanism. For instance, during repetitive nerve stimulation, when the concentration of transmitter released into the synaptic or junctional cleft is relatively high, the released transmitter may activate presynaptic receptors and thereby reduce the further release of transmitter. Such an action may prevent excessive and prolonged stimulation of the postsynaptic cell. In this case, the activation of the presynaptic receptor would be part of a negative feedback mechanism... 

Angiotensin II, the primary end product of the renin-angiotensin system, acts on the juxtaglomerular cells to inhibit the release of renin this process is therefore a negative feedback mechanism. The half-life of renin in the circulation is 10 to 30 minutes, with inactivation occurring primarily in the liver. Small amounts of renin are eliminated by the kidneys. Pure human renin... 

Receptors are also located on the presynaptic membrane. These autc-reivptors regulate the release and synthesis of the neurotransmitter and are part of a feedback mechanism that aims to keep the activation or inhibition within the synapse for a discrete time interval and to terminate the signal transfer once the information has reached the adjacent cell via removal (mactivation) of the neurotransmitter from the synaptic cleft. 

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