Page 281 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

2+
Hormones cGMP, IP 3, DAG, Ca and NO function as second
messengers (and sometimes as third mes-
Hormones are messenger substances that con- sengers; ! p. 274ff.). Some peptide hormones
vey information signals relevant to cell func- like insulin, prolactin, atriopeptin and numer-
tion (! p. 266). Endocrine hormones, i.e., ous growth factors bind to cell surface recep-
those transported in the bloodstream, are pro- tors with cytosolic domains with enzymatic ac-
duced in endocrine glands such as the hy- tivity (! p. 278). Steroid hormones, on the
pothalamus, thyroid, parathyroid glands, other hand, enter the cells themselves
adrenal medulla, pancreatic islets, ovaries and (! p. 278). Once they bind to cytosolic receptor
testes. They are also synthesized in diffusely proteins, steroid hormones (as well as cal-
Hormones and Reproduction mones, i.e., those that affect nearby cells only ent receptors for different hormones (e.g., in-
citriol, T 3 and T 4) are transported to the cell nu-
scattered endocrine cells of the CNS, in C cells of
the thyroid, and in the thymus, atria, kidneys,
cleus, where they influence transcription
(genomic action). A target cell can have differ-
liver, gastrointestinal tract, etc. Paracrine hor-
sulin and glucagon) or different receptors for a
(tissue hormones or mediators; see below) are
single hormone (e.g., α 1 and " 2 adrenoceptors
secreted by cells widely distributed through-
out the body.
for epinephrine).
Hierarchy of hormones (! A). The secretion
Types of hormone.
1. Peptide hormones (! A, dark blue areas)
of hormones is often triggered by neural im-
main neurohormonal control center (! p. 280
areas) are hydrophilic hormones stored in
and 330). Hypothalamic neurons extend to the
secretory granules and released by exocytosis
11 and glycoprotein hormones (! A, light blue pulses from the CNS. The hypothalamus is the
posterior pituitary (neurohypophysis). The
as required. Multiple hormones can be pro-
duced from a single gene (! e.g., POMC gene, hormones are secreted either by the hy-
p. 280) by variable splicing and posttrans- pothalamus itself or by the posterior pituitary.
lational modification (! p. 8ff.) . Hypothalamic hormones also control hor-
2. Steroid hormones (! A, yellow areas) and mone release from the anterior pituitary (ade-
calcitriol are chemically related lipophilic nohypophysis). Anterior pituitary glandotropic
hormones metabolized from cholesterol hormones control peripheral endocrine glands
(! pp. 292ff). They are not stored, but are syn- (! A top, green areas), which release the end-
thesized as needed. hormone (! A). The original signal can be
3. Tyrosine derivatives (! A, orange areas) amplified or modulated at these relay sites
include (a) the hydrophilic catecholamines (! p. 272).
dopamine, epinephrine and norepinephrine Pituitary hormones. Hypothalamic hor-
(! p. 84) and (b) lipophilic thyroid hormones mones control anterior pituitary hormone
(T 3, T 4; ! p. 286). secretion by either stimulating or inhibiting
The lipophilic hormones in (2) and (3b) are hormone production. They are therefore called
transported in the blood while bound to releasing hormones (RH) or release-inhibiting
plasma proteins. Corticosteroids are carried hormones (IH), resp. (! A and table). Most
bound to globulin and albumin, testosterone anterior pituitary hormones are glandotropic
and estrogen to sex hormone-binding globulin (! p. 280). The posterior pituitary hormones
and T 3 and T 4 to albumin and two other plasma are released by neuronal signals and are
proteins (! p. 286). mainly aglandotropic (! p. 280).
Hormone receptors. The receptors (docking Other endocrine hormones are secreted
sites) for glycoprotein hormones, peptide hor- largely independent of the hypothalamic–
mones and catecholamines are transmem- pituitary axis, e.g., pancreatic hormones, para-
brane proteins (! p. 14) that bind to their thyroid hormone (PTH), calcitonin and cal-
specific hormone on the outer cell surface. citriol, angiotensin II, aldosterone (! p. 182ff.),
Many of these hormones induce the release of erythropoietin (! p. 88) and gastrointestinal
intracellular second messengers that transmit hormones (! p. 234). Atriopeptin is secreted
268 the hormone signal inside the cell. cAMP, from the heart atrium in response to stretch
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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