Protein hormone-producing cells exhibit an ultrastructure that includes an abundance of rough endoplasmic reticulum and Golgi membranes. These cells also contain secretory vesicles in the membrane of the endoplasmic reticulum, specifically on its cisternal surface. A signal peptidase removes the signal peptide, resulting in the generation of a hormone or prohormone. The hormone or prohormone then moves from the cisternae of the endoplasmic reticulum to the Golgi apparatus, where it undergoes packaging.

Protein/peptide hormones are soluble in body fluids, with the exception of IGFs and growth hormone. They predominantly circulate in an unbound form in the blood, which leads to their short biological half-lives. Endocytosis and lysosomal degradation of hormone-receptor complexes primarily remove protein hormones from the blood. Some protein hormones are small enough to appear in urine in an active form, such as follicle-stimulating hormone and luteinizing hormone.

The synthesis of peptide hormones involves several steps. In the nucleus, the primary gene transcript, a messenger RNA precursor molecule, undergoes excision of introns, splicing of exons, capping of the 5' end, and addition of polyadenylation at the 3' end. The resulting mature messenger RNA enters the cytoplasm and directs the synthesis of a preprohormone peptidesequence on ribosomes. During this process, the N-terminus signal is removed, and the resulting prohormone is transferred into the endoplasmic reticulum. The prohormone undergoes further processing and packaging in the Golgi apparatus. After final cleavage within the granules, the granules contain the hormone and copeptides, ready for secretion through exocytosis.

A protein hormone-producing cell displays an ultrastructure that includes the presence of secretory vesicles and rough endoplasmic reticulum.

The chemical structures of catecholamines include norepinephrine, epinephrine, and dopamine. Catecholamines are synthesized by the adrenal medulla and neurons.

Proteins/peptides are susceptible to digestion in the gastrointestinal tract if administered orally. Therefore, they need to be administered through injection or, in the case of small peptides, through a mucous membrane, such as sublingually or intranasally. Since proteins/peptides do not easily cross cell membranes, they signal through membrane receptors.

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The ultrastructure of protein hormone–producing cells is characterized by abundant rough endoplasmic reticulum and Golgi membranes and the presence of secretory vesicles •Fig. 38.3 Acircadianpacemakerdirectsnumerousendocrineand bodyfunctions,eachwithitsowndailyschedule.Thenighttimerise inplasmamelatoninmaymediatecertainothercircadianpatterns.ACTH,adrenocorticotropichormone.(DatafromSchwartzWJ.Adv Intern Med. 1994;38:81.) in the membrane of the endoplasmic reticulum located on the cisternal (i.e., inner) surface of the endoplasmic reticular membrane. Removal of the signal peptide by a signal peptidase generates a hormone or prohormone, which is then transported from the cisternae of the endoplasmic reticulum to the Golgi apparatus, where it is packaged into (
Fig. 38.6
Protein/peptide hormones are soluble in body fluids and, with the notable exceptions of IGFs and growth hormone, circulate in blood predominantly in an unbound form and therefore have short biological half-lives. Protein hormones are removed from blood primarily by endocytosis and lysosomal degradation of hormone-receptor complexes (see the section “
Cellular Responses to Hormones”). Many protein hormones are small enough to appear in urine in a physiologically active form. For example, follicle-stimulating hormone and luteinizing hormone are present in urine.
Fig. 38.5 Schematic Representation of Peptide Hormone Synthesis. Inthenucleus,theprimarygenetranscript,amessengerRNAprecursormolecule,undergoesexcisionofintrons,splicingofexons,cappingofthe5′ end,andadditionofpolyadenylation(poly-A)atthe3′ end.TheresultantmaturemessengerRNAentersthecytoplasm,whereitdirectsthesynthesisofapreprohormonepeptidesequenceonribosomes.Inthisprocess,theN-terminussignalisremoved,andtheresultantprohormoneistransferredvectoriallyintotheendoplasmicreticulum.TheprohormoneundergoesfurtherprocessingandpackagingintheGolgiapparatus.Afterfinalcleavageoftheprohormonewithinthegranules,theycontainthehormoneandcopeptidesreadyforsecretionbyexocytosis.NH2,amidogen.
Fig. 38.6 Ultrastructure of a Protein Hormone–Producing Cell. Notethepresenceofsecretoryvesiclesandroughendoplasmicreticulumintheproteinhormone–secretingcell.(FromKierszenbaumAL.Histology and Cell Biology: An Introduction to Pathology. 2nded.Philadelphia:Mosby;2007.) •Fig. 38.7 Chemicalstructuresofcatecholamines.
Proteins/peptides are readily digested in the gastrointestinal tract if administered orally. Hence they must be administered by injection or, in the case of small peptides, through a mucous membrane (sublingually or intranasally). Because proteins/peptides do not cross cell membranes readily, they signal through membrane receptors (see
Catecholamines are synthesized by the adrenal medulla and neurons and include norepinephrine, epinephrine, and dopamine (