Endocrinology, lecture on Target tissue and Receptors
III. Target Tissues and Receptors
A. Target tissue
1. Any tissue for which there is a response stimulated by a specific endocrine
a. requires receptor mediation
i. each target tissue is a target tissue only if it has receptors
for the specific hormone
ii. e.g. FSH stimulates follicular growth in the ovaries; but if
the ovaries had no FSH receptors, or blocked FSH receptors,
FSH would have no effect
B. Receptors
1. location
a. membrane receptors
i. membrane proteins which are receptors for peptide hormones
(1) + a few steroids
b. cytosolic/nuclear receptors
i. receptors for steroids may bind in the cytosol travel to
the nucleus and bind to the DNA
(1) some types (e.g. glucocorticoid) are usually bound
and \ usually in the nucleus
ii. thyroid and some steroid (e.g. E2) receptors
are always nuclear
iii. bind to and mediate genetic activation
2. Hormone receptors are specific binding proteins
a. a receptor can bind a specific hormone, but not others
i. agonists and antagonist of this hormone also bind to the receptor
ii. binds to the receiving or recognition site of the receptor
(usually 1 site/receptor)
(1) receptor also has effector site ® modifies cellular function
b. a hormone may have several distinct specific receptors
i. most neurohormones do
ii. hormones have one (or many) 1o receptor, but may
bind, under certain conditions to the receptor of another hormone
(1) eg. similar hormones at [high]
(2) hormonal specificity is insured physiologically by
[hormone] (+ affinity)
iii. function of some hormones depends upon binding to the
same receptors
(1) NE and Epi
3. Receptors intitiate change in cellular function
a. bound receptors change intracellular function in 4 ways
i. membrane receptors may be enzymes themselves
ii. membrane receptors coupled with G proteins stimulate
enzymatic action via 2nd messengers
iii. the receptor may be connected to or comprise an ion channel
iv. cytosolic/nuclear receptors and 2nd messengers interact
with genes
b. new hormonal molecules and reactivated receptors are necessary
for hormone action to continue
i. binding is reversible (non-covalent)
ii. receptor may be endocytosed and degraded or recycled
iii. hormone is degraded enzymatically
(1) enzyme may be in the blood, intercellular fluid or on the membrane
c. Affinity
i. tendency to bind and stay bound to the receptor
ii. KD (dissociation constant) = 1/KA (affininty constant)
(1) [hormonal] leading to 1/2-maximal receptor occupancy
(2) ED50 is often lower ([hormonal] leading to 1/2-maximal response)
(3) different for different tissues
iii. receptor specificity is functionally derived from high affinity
(1) due to slow dissociation rate
iv. non-specific binding