Re: querry--thyroid effects not receptor mediated?

[Jugal Bhurtel <jugal@rocketmail.com>]

Tony,

There is an excellent site on hormone mechanism. The
URL is
http://www.healthsci.utas.edu.au/weller/+docs/endocr1.htm.


It says,  Chemically, hormones may be conveniently
divided into the following classes of
organic compound:
          proteins (eg. insulin) and peptides (eg.
oxytocin) 
          amine derivatives (eg adrenaline) 
          amino acid derivatives (eg. thyroxine) 
          steroids (eg. cortisol) 

Proteins, peptides and amines are not lipid soluble,
so they cannot cross the plasma
membranes of cells. Hormones classified as these
types of compound have their
receptor on the outside of the cell membrane.
Combination of the hormone with its
membrane receptor causes the production within the
cell of a second messenger
compound which then exerts the hormone's actions
within the cell.

The second messenger activates, accelerates or
inhibits pre-programmed activities
within the cell. These activities usually involve
changing enzyme action, altering
protein synthesis (through messenger RNA) or opening
or closing membrane channels.

Examples of second messengers used by various
hormones include cyclic adenosine
monophosphate (cAMP), cyclic guanosine monophosphate
(cGMP), inositol triphosphate
(IP3) and calcium ions.

Steroid hormones and thyroid hormone are lipid
soluble compounds. This means that
they can cross cell membranes, so do not need to have
their receptors on the outside
of the plasma membrane, nor do they need to employ
second messengers.

Instead, lipid-soluble hormones gain access to
intra-cellular receptors within the
cytoplasm. What is more, the hormone-receptor complex
can then gain access to the
cell nucleus. This means that this type of hormone
can re-program the activities of the
target cell by changing nuclear DNA and gene
expression.  

I hope you find it helpful.


Jugal Bhurtel
B102, The International House Ube
Faculty of Engineering, Yamaguchi University, 
2557 Tokiwadai,
Ube 755-8611, JAPAN
Lab Tel:   (81)-836-35-9111 ext. 8313 (9:00-17:00)
Home Tel:  (81)-836-32-9002 -8012




---Tony Tweedale <ttweed@wildrockies.org> wrote:
>
> davis baltz's recent excellent final EDSTAC summary
psoted to these lists
> mentions briefly: '"From what is known, thyroid
disrupting chemicals
> usually act through non-receptor mediated
mechanisms."   Not that I've ever
> been trained in any endocrinology, but I have
though of a hormone as a
> biomolecule that acts through a receptor molecule. 
Looking at a dictionary
> definition & derivation of the word, I see hormone
defined more broadly, as
> an endocrine gland substance that has an effect on
an organ.
> 
> Anyone care to share to the list any interesting
insights on the mechanisms
> or toxicol. of non-receptor hormonal action?
> 
> Tony Tweedale
> 
> Causality is a concept not subject to empirical
demonstration. -David Hume
> (1711-'76)
> 
> 
> 

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