Hormonal signaling process

The hormonal-signaling process is summarized in Figure 7-1. Hormones are released from secretory tissues in response to metabolic signals as well as electrical or chemical signals from the nervous system. The released hormone binds to a receptor, which can either be on the cell surface or, as in the case of steroid and similar hormones, within the cell. The hormone-receptor complex starts a series of events in which the signal is converted to other chemical forms that bring about changes in the biochemical reactions within the cell. 

AKAPs are a diverse family of about 75 scaffolding proteins. They are defined by the presence of a structurally conserved protein kinase A (PKA)-binding domain. AKAPs tether PKA and other signalling proteins to cellular compartments and thereby limit and integrate cellular signalling processes at specific sites. This compartmentalization of signalling by AKAPs contributes to the specificity of a cellular response to a given external stimulus (e.g. a particular hormone or neurotransmitter). 

Calcitonin lowers serum Ca2+ and Pi levels, primarily by inhibiting the process of bone resorption, but also by decreasing resorption of Pi and Ca2+ in the kidney. Calcitonin receptors are predictably found primarily on bone cells (osteoclasts) and renal cells, and generation of cAMP via adenylate cyclase activation plays a prominent role in hormone signal transduction. 

In any signalling process, it is essential that the signal travels only in one direction (e.g. action potential in a nerve, signalling in hormone action). To do this, non-equilibrium reactions must be included in the sequence. 

Each hormone is the center of a hormonal regulation system. Specialized glandular cells synthesize the hormone from precursors, store it in many cases, and release it into the bloodstream when needed (biosynthesis). For transport, the poorly water-soluble lipophilic hormones are bound to plasma proteins known as hormone carriers. To stop the effects of the hormone again, it is inactivated by enzymatic reactions, most of which take place in the liver (metabolism). Finally, the hormone and its metabolites are expelled via the excretory system, usually in the kidney (excretion). All of these processes affect the concentration of the hormone and thus contribute to regulation of the hormonal signal. 

Beginning with the hormone-producing cell, the following processes are all contributing factors for hormonal signal transduction in higher organisms (fig. 3.10) ... 

Heterodimerization of receptor molecules is a mechanism that can increase and modulate the diversity and regulation of signal transduction pathways. Since the various members of a receptor family differ in the exact structure of the autophosphorylation sites and the other regulatory sequences, it is assumed that activity and regulation are different for the various combinations of receptor subtypes. Tissue-specific expression of receptor subtypes enables the organism to process growth hormone signals in a differential way. 

In addition to growth hormone signals, other signals such as Ca signals are also processed by the Ras switching station. 

Information processing. Stimuli external to a cell, such as hormone signals or light intensity, are detected by specific proteins that transfer a signal to the interior of the cell. A well-characterized example is the visual protein rhodopsin, located in membranes of retinal cells. 

Pre- and postmature childbirths are each estimated to occur in almost 10% of pregnancies in the United States. The physiological processes of parturition (birth) must be precisely timed and effective to avoid harm to the mother and fetus. Like pregnancy, parturition relies on the sending and receiving of numerous hormonal signals between mother and fetus and is therefore especially susceptible to toxic insult. 

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