section 31.2
Pituitary Gland (Hypophysis)
737
TABLE 31-4
Somatomammotropin Family
Hormone
Site of Secretion
Chemistry
Location of
Gene
Function
Regulation of Secretin
Growth
Hormone (GH)
Anterior Pituitary
(Somatotroph)
191 aa (22kDa)
17q22-q24
Stimulates protein
synthesis in many
tissues; exerts
protein-sparing
effects; stimulates
production of IGFs.
Primarily stimulated by
GHRH and inhibited by
somatostatin.
Prolactin (PRL)
Anterior Pituitary
(Lactotroph)
198 aa (23 kDa)
6p22.2-q21.3
Stimulates growth and
protein synthesis
in breast; promotes
milk secretion
during lactation.
Primarily inhibited by
PIH (dopamine);
stimulated by PRF and
TRH.
Placental
Lactogen (hPL)
Placenta
(Syncytiotrophoblast)
191 aa(22kDa)
17q22-q24
Exerts protein-sparing
effect in maternal
tissues, promotes
fetal growth by
stimulating produc-
tion of fetal IGF-1.
Progesterone-stimulated
placental growth.
(PRL), luteinizing hormone (LH), follicle-stimulating
hormone (FSH), thyroid-stimulating hormone (TSH),
adrenocorticotropic hormone (ACTH, corticotropin), and
/1-endorphin. /3-Lipotrophic hormone (/J-LPH) is also
secreted but serves mainly as a precursor of /3-endorphin
and is not regarded as a hormone. In species that possess a
prominent intermediate lobe, the pituitary also secretes
a-
and /1-melanocyte-stimulating hormones (MSH), which
affect skin coloration.
Anterior pituitary hormones are classified into three
families: the somatomammotropin family (GH and PRL),
the glycoprotein hormones (LH, FSH, and TSH), and
the opiomelanocortin family (ACTH, /1-endorphin, and
related peptides). These three families appear to have
evolved from three separate ancestral polypeptides; homo-
logous members of each family occur in other parts of
the body. For example, human placental lactogen (hPL)
is somatomammotropic, human chorionic gonadotrophin
(hCG) is a glycoprotein hormone, and the brain and gut
produce substances related to endorphins (enkephalins and
dynorphin).
Somatomammotropin Family
Members of the somatomammotropin family are single-
chain proteins with two or three intrachain disulfide
bridges; their molecular weight is about
2 0
,
0 0 0
, and they
function mainly to promote protein synthesis (Table 31-4).
The hypothalamus produces both a stimulating and an
inhibiting hormone to regulate secretion of the adenohy-
pophyseal hormone.
Growth Hormone (GH Somatotropin)
Human growth hormone consists of 191 amino acid
residues (M. W. 22,000) and contains two disulfide bridges
(Figure 31-3). It exhibits extensive sequence homol-
ogy with prolactin (76%) and placental lactogen (94%)
(Figure 31-4). The gene encoding GH is on chromo-
some 17 q22-q24, and its expression is regulated by a
transcription factor (Pit-l/GHF-1) that is promoted by
hormones that stimulate GH synthesis (e.g., GHRH, glu-
cocorticoids, thyroid hormone). GH is the most abun-
dant hormone in the human pituitary gland, averaging
about
6
mg per adult gland. Its basal blood level av-
erages 2 ng/mL in adults and
6
ng/mL in preadoles-
cent and adolescent boys. GH is cleared from circulation
with a half-life of about 25 minutes in lean adults, but
is cleared more rapidly in obese subjects. GH is inacti-
vated mainly by the liver but also by the kidney. About
40% of the hormone is bound to “GH-binding protein”
(GHBP), a fragment of the GH receptor, which serves to
prolong the half-life of GH 10-fold. Its primary structure
is species-specific and antigenic (e.g., rat GH is antigenic
