Endocrinology, lecture on homeostasis
IV. Homeostasis
A. balanced physiological systems operating in the organism to maintain a
dynamic equilibrium
1. usually a constant steady state maintained within certain
tolerable limits
a. maintained by:
i. hormonal production (rate) and storage (capacity)
ii. negative feedback
(1) neuroendocrine reflex stimulates endocrine action
(2) positive feedback stimulates endocrine action
iii. receptor regulation
iv. biological rhythmicity
v. pulsatile secretion
vi. carrier proteins
B. Hormone Production
1. Rate of hormone production depends primarily on stimulation by
neural input, paracrines, or other hormones
a. and inhibition
2. after stimulation/inhibition:
G-protein transduction, 2nd messenger activation, enzyme action,
DNA transcription, messenger RNA processing, transport and
translation, transfer RNA transport and protein synthesis,vesicle
production at the golgi, enzymatic cleaving, and exocytosis
all take time
a. peptide hormones are made in the rough endoplasmic reticulum (rER)
b. steroids are made in the sER and/or mitochondria
c. thyroid hormones are made at the cell membrane
d. monoamine hormones are made primarily in the cytosol
C. Storage
1. little or no storage of hormones
a. usually less than the normal need for one day
b. especially true for thyroid and steroid hormones
D. Negative Feedback
1. Hormones may inhibit the factor (hormone, paracrine,
neurotransmitter) which stimulated it
a. directly or indirectly inhibit
2. Endocrine Axes and feedback
a. neurotransmitter stimulates neurohormone secretion
e.g.
i. 5-HT ®+CRH
b. neurohormone stimulates tropic hormone secretion
i. CRH ®+ACTH
c. tropic hormone stimulates peripheral hormone secretion
i. ACTH ®+B/F
d. hormone stimulates cellular action and inhibits
neurotransmitter, neurohormone, and tropic hormone secretion
i. B/F ®+ hippocampus ®-5-HT, CRH, ACTH
ii. long-loop feedback
3. Short-loop feedback
a. ACTH inhbits CRH secretion
4. Ultrashort-loop feedback
a. ACTH inhibits secretion of ACTH, CRH inhibits CRH secretion
E. Receptor regulation
1. hormones act only on tissues with receptors
2. receptor numbers may be up- (more) or down-regulated (less)
F. Biological Rhythmicity and Pulsatility
1. Periodicity of hormone secretion may vary over minutes or a year
2. Rhythmic patterns may determine effect at the target tissue
a. e.g. pulsatile secretion is required for GnRH function
b. many environmentally stimulated biological functions are
mediated/timed via rhythmic patterns of hormone secretion
G. Many hormones are carried/protected in the plasma by carrier proteins
1. receptors usually have a greater affinity
than carrier proteins for hormone
