The Endocrine System
329
Mechanisms of Hormone Action
I. The great majority of receptors for steroid and thyroid
hormones are inside the target cells; those for the peptide
hormones and catecholamines are on the plasma membrane.
II. Hormones can cause up-regulation and down-regulation
of their own receptors and those of other hormones. The
induction of one hormone’s receptors by another hormone
increases the fi rst hormone’s effectiveness and may be essential
to permit the fi rst hormone to exert its effects.
III. Receptors activated by peptide hormones and catecholamines
utilize one or more of the signal transduction pathways
available to plasma-membrane receptors; the result is altered
membrane potential or protein activity in the cell.
IV. Intracellular receptors activated by steroid and thyroid hormones
function as transcription factors, combining with DNA in the
nucleus and inducing the transcription of DNA into mRNA; the
result is increased synthesis of particular proteins.
V. In pharmacological doses, hormones can have effects not seen
under ordinary circumstances.
Inputs that Control Hormone Secretion
I. The secretion of a hormone may be controlled by the plasma
concentration of an ion or nutrient that the hormone regulates,
by neural input to the endocrine cells, and by one or more
hormones.
II. The autonomic nervous system is the neural input controlling
many hormones. Neuron endings from the sympathetic and
parasympathetic nervous systems terminate directly on cells
within some endocrine glands, thereby regulating hormone
secretion.
Types of Endocrine Disorders
I. Endocrine disorders may be classifi ed as hyposecretion,
hypersecretion, and target-cell hypo- or hyperresponsiveness.
a. Primary disorders are those in which the defect is in the
cells that secrete the hormone.
b. Secondary disorders are those in which there is too much or
too little tropic hormone.
c. Hyporesponsiveness is due to an alteration in the receptors
for the hormone, to disordered postreceptor events, or to
failure of normal metabolic activation of the hormone in
cases requiring such activation.
II. These disorders can be distinguished by measurements of the
hormone and any tropic hormones under both basal conditions
and during experimental stimulation of the hormone’s secretion.
SECTION A KEY TERMS
SECTION A CLINICAL TERMS
adrenal cortex
317
adrenal gland
317
adrenal medulla
317
aldosterone
321
amine hormone
317
androgen
322
angiotensin II
322
cholesterol esterase
320
cortisol
322
cytochrome P450
320
dopamine
317
down-regulation
324
endocrine gland
316
endocrine system
316
epinephrine
317
estradiol
323
estrogen
323
glucocorticoid
322
gonad
320
hormone
316
mineralocorticoid
321
norepinephrine
317
peptide hormone
319
permissiveness
324
progesterone
323
prohormone
319
steroid hormone
319
testosterone
322
thyroid hormone
317
tropic hormone
327
up-regulation
324
diabetes mellitus
328
hyperresponsiveness
327
hypersecretion
327
hyporesponsiveness
327
hyposecretion
327
masculinization
322
pharmacological effect
325
primary hypersecretion
328
primary hyposecretion
327
secondary hypersecretion
328
secondary hyposecretion
327
SECTION A REVIEW QUESTIONS
1. What are the three general chemical classes of hormones?
2. Which catecholamine is secreted in the largest amount by the
adrenal medulla, and why?
3. What are the major hormones produced by the adrenal cortex?
By the testes? By the ovaries?
4. Which classes of hormones are carried in the blood mainly
as unbound, dissolved hormone? Mainly bound to plasma
proteins?
5. Do protein-bound hormones diffuse out of capillaries?
6. Which organs are the major sites of hormone excretion and
metabolic transformation?
7. How do the rates of metabolism and excretion differ for the
various classes of hormones?
8. List some metabolic transformations that prohormones and
some hormones must undergo before they become biologically
active.
9. Contrast the locations of receptors for the various classes of
hormones.
10. How do hormones infl uence the concentrations of their own
receptors and those of other hormones? How does this explain
permissiveness in hormone action?
11. Describe the sequence of events when peptide or catecholamine
hormones bind to their receptors.
12. Describe the sequence of events when steroid or thyroid
hormones bind to their receptors.
13. What are the direct inputs to endocrine glands controlling
hormone secretion?
14. How does control of hormone secretion by plasma mineral
ions and nutrients achieve negative feedback control of these
substances?
15. What roles does the autonomic nervous system play in
controlling hormone secretion?
16. What groups of hormone-secreting cells receive input from
neurons located in the brain rather than in the autonomic
nervous system?
17. How would you distinguish between primary and secondary
hyposecretion of a hormone? Between hyposecretion and
hyporesponsiveness?
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