Signaling molecules with intracellular receptors

Little  lipophilic  (lipid-soluble)  hormones  diffuse  across  the plasma  membrane and  then  interact  with  intracellular  receptors  in  the  nucleus or cytosol.  The receptors are all structurally associated, being part of the nuclear receptor super family.  The resulting hormone–receptor complex frequently binds to regions of the DNA and affects   the transcription   of certain   genes.  The Small lipophilic  hormones  with intracellular  receptors  involves  the steroid  hormones which  are  synthesized   from  cholesterol  for instance  the  female  sex hormones  estrogen and progesterone,  thyroxine  that is produced  through thyroid cells and is the principal iodinated compound in animals, retinoic acid that is derived  from  vitamin  A,  and  vitamin  D  that  is  synthesized  in  the  skin  in response to sunlight.

An very important  and  remarkable  instance  of  a  small  signaling  molecule  which passes  readily  across  the goal  cell  plasma  membrane  is the gas nitric  oxide that is NO. This is used in both animals and plants. Nitric oxide is synthesized through the deamination of arginine catalyzed by the enzyme nitric oxide synthase.  Nitric oxide rapidly diffuses out of the cell where it is produced and into neighboring cells it only acts locally as it has a short half-life about 5–10 s. In several target cells, nitric oxide binds to the active site of guanylyl cyclase, motivating the enzyme to produce the small intracellular mediator cGMP. Nitroglycerine, that is used to treat patients with angina pain resulting from inadequate blood flow to the heart muscle, is converted to Nitric oxide that relaxes the blood vessels while reducing the workload on the heart. The Carbon monoxide (CO) is another gas which is used as a signaling molecule, again through stimulating guanylyl cyclase.

                                              Figure:  Cellular signaling. (a) Endocrine, (b) paracrine,  (c) autocrine.