Hormones may act locally following secretion; either on a neighboring cell (paracrine effect), on the secretory cell itself (autocrine effect), or without actually being released from the secretory cell (intracrine effect)

They signal through a variety of general (eg, nuclear vs cell surface) and specific (eg, tyrosine kinase vs phosphoinositide turnover) mechanisms in target cells.

Membrane-Associated Receptors

1.  
   Membrane-associated receptors bind hydrophilic hormones (which penetrate the plasma membrane poorly) outside the cell. Examples of hydrophilic hormones include insulin and adrenocorticotropic hormone (ACTH).
2.  
   Membrane-associated receptors transmit signals into the cell by a variety of second messenger mechanisms, including the following:
    
   1.  
      Changes in cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate caused by changes in the activity of cyclases.
   2.  
      Increased phosphoinositide turnover via increased phosphoinositide kinase activity.
   3.  
      Changes in intracellular calcium (Ca2+) by acting on intracellular stores or membrane Ca2+ channels.
   4.  
      Changes in intracellular ions by action on specific channels. These include sodium (Na+), Ca2+, potassium (K+), and chloride (Cl−).
   5.  
      Increased tyrosine phosphorylation on specific proteins by the action of tyrosine kinases.

Intracellular Receptors

1.  
   Intracellular receptors bind hydrophobic hormones (which penetrate the plasma membrane easily) inside the cell—either in the cytoplasm or in the nucleus. Examples of hydrophobic hormones include cortisol, retinol, and estrogen.
2.  
   Intracellular receptors modulate the transcription rate of specific target genes to change the levels of cellular proteins.

 

 

 

Finally, they can access target cells through the circulation (endocrine). Neurotransmitters that access the extracellular compartment, including circulating plasma, can act as paracrine or endocrine regulators of target cell activity (H, hormone; N, neurotransmitter; R, receptor).

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Identification of a tissue as a target for a particular hormone requires the presence of receptors for the hormone in cells of the target tissue. These receptors, in turn, are linked to effector mechanisms that lead to the physiological effects associated with the hormone.

 

 

 

 

 

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