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Fleur sam Ch 7Endocrinology is the study of hormones.
7.1 Hormones
Processes that fall mostly under hormonal control include: metabolism, regulation of
internal environment (temp, water balance, ions), reproduction, growth, development.
Hormones act on their target cells in three ways:
1. They control the rates of enzymatic reactions
2. Controlling the transport of ions or molecules across cell membranes
3. Controlling gene expression and the synthesis of proteins
The classic steps for identifying an endocrine gland became:
1. Remove the suspected gland if the gland does produce hormones, the animal
should start to exhibit anatomical behavioral, or physiological abnormalities.
2. Replace the hormone placing the gland back in the animal or administering an
extract of the gland, this replacement therapy should eliminate the symptoms of
hormone deficiency.
3. Create a state of hormone excess give an extra gland and administer if more
symptoms characteristic for that hormone excess (=overmaat) appear.
Hormones identified by this technique are sometimes called classic hormones. Bv.
Hormones of the pancreas, thyroid, adrenal glands, pituitary and gonads all discrete
endocrine glands that could be easily identified and surgically removed.
The traditional definition of a hormone is a chemical secreted by a cell or a group of
cells into the blood for transport to a distant target, where it exerts its effect at very
low concentrations.
The group of cells that secrete hormones are called glands, however molecules that
act as hormones can also be secreted by isolated endocrine cells (hormones of the
diffuse endocrine system), neurons (neurohormones) or the immune system
(cytokines).
Secretion is the movement from a substance inside a cell to the ECF or the external
environment. In traditional definition, hormones are secreted into the blood. When
hormones are secreted into the external environment, they are called
ectohormones.
Pheromones are specialized ectohormones that act on other organisms of the same
species to elicit a physiological or behavioural response (also sex pheromones).
By traditional definition, a hormone must be transported by the blood to a distant
target cell. Molecules that are suspected of being hormones are called candidate
hormones and often carry the word ‘’factor’’’. Examples are growth factors, large
group of substances that influence cell growth and division, are being studied
whether they would meet the criteria for hormones, as they do operate locally as
autocrine or paracrine, but don’t seem to be widely distributed into circulation.
Similar situation exists with the lipid-derived signal molecules called eicosanoids.
Complicating the classification of signal molecules is the fact that a molecule may act
as a hormone when secreted from 1 location but as a paracrine/autocrine signal
, when secreted from a different location. Bv. Cholecystokinin (CCK) in intestine
causes contraction of gallbladder. CCK also found in neurons of brain, acts as
neurotransmitter/neuromodulator. CCK also possible role in controlling hunger.
Hormone has ability to act at conc. in nanomolar (10^-9 M) to picomolar (10^-12 M)
range. Some chemical signals transported in the blood to distance targets are not
considered hormones bc they must be present in relatively high conc. before an
effect is noticed. Bv. Histamine released during severe allergic reactions may act on
cells throughout the body, but its conc. exceeds the accepted range for hormone.
Boundary/distinction between hormones and nonhormonal signal molecules is
challenged. Many cytokines seem to meet many of the criteria of a hormone, but not
considered cytokines to be hormones bc cytokines are released on demand in
contrast to classic peptide hormones, which are made in advance and stored in
parent endocrine cell.
Cytokine is bv. Erythropoietin, molecule that controls RBC production.
All hormones bind to target cells receptors and initiate biochemical responses. These
are called cellular mechanisms of action of the hormone.
One hormone may act on multiple tissues. The effects may vary in different tissues at
different stages of development. Bv. Insulin has varied effects. In muscle & adipose
tissue, insulin modifies glucose transport proteins & enzymes for glucose
metabolism. In liver, it modulates enzyme activity but has no direct effect on glucose
transport proteins. In brain and certain other tissues, glucose metabolism is totally
independent of insulin.
Signal activity by hormones and other chemical signals must be of limited duration if
the body is to respond to changes in its internal state. Bv. Insulin is secreted when
blood glucose conc. increase following a meal. As long as insulin is present, glucose
leaves the blood and enters cell. body avoids that this goes on too long by limiting
insulin secretion by removing or inactivating insulin circulating in the blood and by
terminating insulin activity in target cells.
In general, hormones in the blood are degraded into inactive metabolites by
enzymes found primarily in the liver and kidneys. The metabolites are then secreted
in either bile or the urine. The rate of this hormone breakdown is indicated by the
half-life in the circulation, which is an indicator of how long a hormone is active in the
body.
Hormones bound to target membrane receptors have their activity terminated in
several ways. Enzymes that are always present in the plasma can degrade peptide
hormones bound to cell membrane receptors. The receptor-hormone complex is
brought into the cell by endocytosis, and the hormone is then digested in the
lysosomes. Intracellular enzymes metabolize hormones that enter cells.
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