Anterior pituitary peptide hormones

FIGURE 1 8-5 Tissue-specific processing of the pro-opiomelanocortin (POMC) precursor yields a wide array of bioactive peptide products. Processing of the POMC precursor varies in various tissues. In anterior pituitary, adrenocorticotropic hormone (ACTH (1-39)) and P-1 ipo tropin (P-LPH) are the primary products of post-translational processing. Arcuate neurons produce the potent opiate P-endorphin (P-endo (1-31)) as well as ACTIK1 -13) NIT,. Intermediate pituitary produces a-melanocyte-stimulating hormone (aMSH), acetylated P endof 1 31) and P-endo(l-27). NTS, nucleus tractus solitarius. 

The releasing hormones (liberins 60)) and the release-inhibiting hormones (statins) which stimulate the anterior pituitary into hormone production or inhibit release, are low-molecular peptides in comparison with the anterior pituitary hormones and are present in certain areas of the hypothalamus. The hypothalamus exerts an influence on many vital physiological processes in the organism. 

Concentrations of hypothalamic and anterior pituitary peptides and adrenal glucocorticoids are measured accurately and at low concentrations in body fluids using immunoassay and instrumental methods. Relying on basal hormone concentrations for confirmmg adrenocortical dysfunction, however, can be problematic because of the episodic and circadian secretion of the hormones that regulate the HPA axis. For example, some individuals have an abnormality of the HPA axis, but still maintain basal cortisol and ACTH secretion within die reference interval of the normal population. Dynamic testing of the HPA axis helps define abnormalities that are not reflected in basal hormone secretion. 

Growth hormone (somatotropin) is a polypeptide synthesized and secreted by somatotrophs in the anterior pituitary. Growth hormone synthesis and secretion is stimulated by the hypothalamic peptide growth hormone-... 

Pregnenolone is transported from the mitochondria to the ER, where a hydroxyl oxidation and migration of the double bond yield progesterone. Pregnenolone synthesis in the adrenal cortex is activated by adrenocorticotropic hormone (ACTH), a peptide of 39 amino acid residues secreted by the anterior pituitary gland. 

The secretion of anterior pituitary hormones is controlled in part by hypothalamic regulatory factors that are stored in the hypothalamus and are released into the adenohypophyseal portal vasculature. Hypothalamic regulatory factors so far identified are peptides with the exception of dopamine. Secretion of anterior pituitary hormones is also controlled by factors produced more distally that circulate in the blood. Predominant control of hormone production may be relatively simple, as with thyroid-stimulating hormone (TSH), the production of which is primarily stimulated by thyrotropin-releasing hormone (TRH) and inhibited by thyroid hormones, or it may be complex, as with prolactin, the production of which is affected by many neurotransmitters and hormones. 

Many small peptides exert their effects at very low concentrations. For example, a number of vertebrate hormones (Chapter 23) are small peptides. These include oxytocin (nine amino acid residues), which is secreted by the posterior pituitary and stimulates uterine contractions bradykinin (nine residues), which inhibits inflammation of tissues and thyrotropin-releasing factor (three residues), which is formed in the hypothalamus and stimulates the release of another hormone, thyrotropin, from the anterior pituitary gland. Some extremely toxic mushroom poisons, such as amanitin, are also small peptides, as are many antibiotics. 

Figure 30-1 (A) Median sagittal section of the human brain. From Maya Pines.15 (B) Drawing illustrating the synthesis of peptide hormones in the hypothalamus and transport via portal blood vessels into the anterior lobe of the pituitary gland or via nerve tracts into the posterior lobe.16...

The anterior lobe of the pituitary secretes a series of ten or more peptide hormones ranging in size from the 20-residue (i-melanotropin to the 200-residue growth hormone (somatotropin). Several of these contain a common heptapeptide unit, which is marked in green in the following structure ... 

Thyrotropin-releasing hormone is one of several small peptide hormones secreted by the anterior lobe of the pituitary gland. These are the master" hormones that function to stimulate hormone secretion from other endocrine glands. Thyrotropin stimulates the functioning of the thyroid gland. 

Anterior Lobe. The anterior pituitary, or adenohypophysis, secretes six important peptide hormones. The anterior pituitary releases growth hormone (GH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), and prolactin (Pr). The physiologic effects of these hormones are listed in Table 28-1. 

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