Hormon Part3

8/7/2019 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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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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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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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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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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Hormon Part3