hormon part3
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Hormon Part3
Pertemuan 15
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Fatty Acid Derivatives - Eicosanoids
Arachadonic acid is the most abundant precursorfor these hormones. Stores of arachadonic acid arepresent in membrane lipids and released through the
action of various lipases. The specific eicosanoidssynthesized by a cell are dictated by the battery ofprocessing enzymes expressed in that cell.
These hormones are rapidly inactivated by beingmetabolized, and are typically active for only a fewseconds.
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Fatty Acid Derivatives - Eicosanoids
Eicosanoids are a large group of molecules
derived from polyunsaturated fatty acids.
The principal groups of hormones of this
class are prostaglandins, prostacyclins,
leukotrienes and thromboxanes.
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Regulation of hormone secretion
Sensing and signaling: a biological need is sensed,
the endocrine system sends out a signal to a target
cell whose action addresses the biological need. Key
features of this stimulus response system are:y receipt of stimulus
y synthesis and secretion of hormone
y delivery of hormone to target cell
y evoking target cell response
y degradation of hormone
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Control of Endocrine Activity
The physiologic effects of hormones depend
largely on their concentration in blood andextracellular fluid.
Almost inevitably, disease results when hormone
concentrations are either too high or too low, and
precise control over circulating concentrations ofhormones is therefore crucial.
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Control of Endocrine Activity
The concentration of hormone as seen by target
cells is determined by three factors:
Rate of production
Rate of delivery
Rate of degradation and elimination
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Control of Endocrine Activity
Rate of production: Synthesis and secretion ofhormones are the most highly regulated aspect of
endocrine control. Such control is mediated by
positive and negative feedback circuits, as described
below in more detail.
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Control of Endocrine Activity
Rate of delivery: An example of this effect is
blood flow to a target organ or group of target
cells - high blood flow delivers more hormonethan low blood flow.
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Control of Endocrine Activity
Rate of degradation and elimination: Hormones,
like all biomolecules, have characteristic rates of
decay, and are metabolized and excreted from the
body through several routes.
Shutting off secretion of a hormone that has a very
short half-life causes circulating hormone
concentration to plummet, but if a hormone's
biological half-life is long, effective concentrations
persist for some time after secretion ceases.
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Feedback Control of Hormone
Production
eedback loops are used
extensively to regulate
secretion of hormones in the
hypothalamic-pituitary axis.
An important example of a
negative feedback loop is seen
in control of thyroid hormone
secretion
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Inputs to endocrine cells
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Neural control
Neural input to hypothalamus stimulates
synthesis and secretion of releasing factors
which stimulate pituitary hormone production
and release
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Chronotropic control
Endogenous neuronal rhythmicity
Diurnal rhythms, circadian rhythms (growth
hormone and cortisol),S
leep-wake cycle;seasonal rhythm
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Episodic secretion of hormones
Response-stimulus coupling enables the
endocrine system to remain responsive to
physiological demands
Secretory episodes occur with different
periodicity
Pulses can be as frequent as every 5-10
minutes
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Episodic secretion of hormones The most prominent episodes of release occur with a
frequency of about one hourreferred to ascirchoral
An episode of release longer than an hour, but lessthan 24 hours, the rhythm is referred to as ultradian
If the periodicity is approximately 24 hours, therhythm is referred to as circadian usually referred to as diurnalbecause the increase in
secretory activity happens at a defined period of the day.
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Circadian (chronotropic) control
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Circadian Clock
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Physiological importance of
pulsatile hormone release Demonstrated by GnRH infusion
If given once hourly, gonadotropin secretion and
gonadal function are maintained normally
A slower frequency wont maintain gonad function
Faster, or continuous infusion inhibits gonadotropin
secretion and blocks gonadal steroid production
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Clinical correlate
Long-acting GnRH analogs (such as leuproline)
have been applied to the treatment of
precocious puberty, to manipulate
reproductive cycles (used in IVF), for the
treatment of endometriosis, PCOS, uterine
leiomyoma etc
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eedback control
Negative feedback is most common: for example, LH
from pituitary stimulates the testis to produce
testosterone which in turn feeds back and inhibits LH
secretion
Positive feedback is less common: examples include
LH stimulation of estrogen which stimulates LH surge
at ovulation
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Negative feedback effects of cortisol
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Substrate-hormone control
Glucose and insulin: as glucose increases it
stimulates the pancreas to secrete insulin
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eedback control of insulin byglucose concentrations
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