Sarah Margareta Ibrahim▯ Monday, January 7th 2012
PHGY 210 - Mammalian Physiology II
Lecture 1 - Endocrinology (Part 1 of 6)
Introduction to Endocrinology
Endocrinology - a molecular signal (a hormone) that is detected by a molecular detector (a
receptor) that leads to a metabolic change in a cell and taken at an organismal level, this leads
to a physiological change.
• Classically endocrinology is one of the primary means that the body uses to coordinate
physiological processes over long distances.
• So you can have an endocrine signal in the form of a hormone released by the brain for
example which travels to the far extremities of the body to illicit itʼs physiological
• It can also take place over very short distances or within the same cell talking to itself as
we will see.
• So the endocrine system along with the nervous system is one of the primary systems
for long distance coordination of physiological events within the body.
Coordination of Physiological Processes:
1) In a living organism there must be coordination of number of physiological activities taking
place simultaneously such as: movement, respiration, circulation, digestion, excretion and
2) The central nervous system and the endocrine system represent the two major means by
which these functions are coordinated.
Long Distance Communication:
1) Communication between cells that are not in contact is achieved through a number of
chemical substances, which are secreted by releasing cells and interact with speciﬁc
receptors on distant target cells.
2) Signaling through these receptors leads to a speciﬁc physiological effect.
Endocrine signaling always involves a hormone that is secreted into the
blood stream by an endocrine gland. As we will see, there are glands in
the body that are really dedicated endocrine structures. However,
essentially any cell in the body can send out a signal that effectively, is
hormonal in nature. So you have a hormonal signal that is released and
is transported through the bloodstream to itʼs target (distant target site).
Itʼs target is whatever cell happens to have the receptor for that particular
Figure on the right: The anterior pituitary is like a central clearing
house for hormonal signals (weʼll see it many times) but in this
case the APG is giving rise to luteinizing hormone (LH) or follicle
stimulating hormone (FSH) going to the gonads and then the
gonads release steroid hormones. This ﬁgure also shows
cascades of hormonal signalling - ie. youʼll have a primary
endocrine organ which signals to a secondary endocrine organ which will sometimes
even signal to a tertiary endocrine organ.
▯ 1 Sarah Margareta Ibrahim▯ Monday, January 7th 2012
➡ Neuroendocrine Signaling - a variation on any type of endocrine
signal except the hormone releasing cell is a neuron. The
hormone released is called a neurohormone. In the case of the
ﬁgure to the right, the hypothalamus is talking to the pituitary.
The blood system here is a portal blood system that connects
the hypothalamus directly to the pituitary so there is travel at a
distance but itʼs a relatively short distance in this case.
➡ Two types of (short-distance) signaling:
▯ (see ﬁgure below)
1) Paracrine signaling - when you have two cells in close
juxtaposition, a hormone is released from one cell and it
detects a receptor on an adjacent cell. The adjacent cell can
be a different cell type or it can be a different cell (can have
epithelial cells talking to epithelial cells for example).
2) Autocrine signaling - cell actually talks to itself.
➡ Communication by hormones (or neurohormones) can involve six steps:
*Each of these steps is a potential point of regulation! (Very important!!)
1. Synthesis of the hormone by endocrine cells (or neurons in case of neurohormone).
• Example. To regulate synthesis of insulin - control the rate of transcription of the
gene that produces insulin (so the rate of production of a mature insulin molecule)
2. Release of the hormone by the endocrine cells (or the neurohormones by the neurons).
• Example. What happens during reproductive endocrinology - LH levels build up in
the anterior pituiary without being released and all of a sudden there is a tipping
point and thereʼs all this LH hormone that is released and LH levels in the blood
spike and when they get to the developing follicle - you have ovulation!
3. Transport of the hormone or neurohormone to the target site by the blood stream.
• Most hormones donʼt travel through the bloodstream naked - they travel through
the blood stream bound to various carrier proteins
4. Detection of the hormone or neurohormone by a speciﬁc receptor protein on the target
• The essence of endocrinology is that although lots of proteins travel through the
blood stream (albumin for example), the hormone has a speciﬁc receptor on a
speciﬁc cell type which is capable of detecting the presence of that hormone in the
▯ 2 Sarah Margareta Ibrahim▯ Monday, January 7th 2012
5. A change in cellular metabolism triggered by the hormone‐ receptor interactions
• Cells have lots of proteins on their surface which detect speciﬁc molecules - for
example youʼll have proteins that act as transporters for amino acids but thatʼs all
that they do, they increase uptake of the amino acid, cʼest tout. The important thing
in the hormone receptor relationship is that the binding of the hormone to the
receptor illicits a series of biochemical responses within the cell which bears the
receptor. These biochemical responses, taken at the level of the tissue or the
organism correspond to the physiological response to that particular hormonal
6. Removal of the hormone, which often terminates the cellular response
• The circulating half life of a number of hormones in the blood stream is extremely
short. Why so short? Because you want the signal to be around when you want it
around but you donʼt want it to hang around for hours after youʼve ﬁnished needing
the signal. Itʼs an excellent way to regulate by having a tight regulation on how long
the hormone hangs around in the circulation.
Classic Endocrine Organs (Think: HAPpy BOT)
1. Brain: in particular the hypothalamus and the pituitary -
sort of one continuous structure which is part neuronal,
2. Thyroid and parathyroid glands: basically what you see
in the ﬁgure - the upside-down bowtie - those are your
thyroid glands connected by the isthmus and the
parathyroid are four tiny little guys which you canʼt see
here because theyʼre like little dots stuck two on each
side of the thyroid gland. *Note: DONʼT GET
CONFUSED by the names- the parathyroid hormone
is produced by the parathyroid glands and is called
that way because the parathyroid glands are stuck to
the thyroid. Parathyroid physiology is a distinct
signaling system from thyroid physiology so itʼs not like
the parathyroid is a cofactor or the thyroid or
3. Heart: a neoclassical endocrine organ in the sense
that atrial natriuretic peptides (ANPs) were discovered
in the 1950s. Wonʼt really talk about this.
4. Adrenal glands: sit on top of your kidneys and are a
major source of a number of hormones - particularly
the non-sex steroid hormones and to a lesser degree
the sex steroid hormones
5. Pancreas: Not that small of an organ but less than 1%
of it is endocrine (most of it is a digestive organ). Is
famous for producing insulin from the Islets of
6. Ovaries/Testes: will deal with these in detail when we
look at reproductive physiology.
▯ 3 Sarah Margareta Ibrahim▯ Monday, January 7th 2012