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Re: "Non-shivering thermogenesis"?




>He speaks of "non-shivering thermogenesis". I was just entertaining myself
with Bakker's letter "Dinosaur >Bioenergtics - a reply to Bennett and
Danzell, and Feduccia" from year 1974 in the volume 28 of Evolution, >pages
497-503. In his letter, Bakker states that NSTG has something to do with the
activity of sodium pumps >of the cell membranes. He refers to a paper to
which I - surprisingly - don't have access to. I'd be grateful if >someone
could clear up this mess of mine a little.


    The cellular basis for endothermic metabolism involves "leaky membranes"
(Hulbert & Else, 1990).  Cells maintain high concentrations of potassium
ions (K+) and low concetrations of sodium ions (Na+).  This is against the
concentration gradients, so the cell has to work at pumping out sodium and
pumping in potassium, and it does this using specialized proteins called
"sodium-potassium pumps".  Hulbert & Else (1990) looked at the consumption
of an inhibator involved in sodium-potassium pump activity, and decided that
the pumps of endotherms work harder then those of ectotherms.  They also
determined that cell membranes of mammals are more permeable to potassium
and sodium then in reptiles.  So, thier explanation was that in endotherms,
the cell membranes are "leakier", meaning they let these ions cross in an
out more eaily, and this forces the pumps to work harder to maintain the
desired ion concentrations.  This consumes energy.
    Animals manufacture ATP, the energy currency of metabolism, using
mitochondria in thier cells.  Hulbet and Else (1990) found that in the
liver, kidney, heart, and brain of endotherms, mitochondrial membrane area
was about four time that of ectotherms.  In mammal muscles, they also found
that the mitochondrial membrane surface area was about 223% of that found in
reptiles.  Moreover, individual mitochondria in the muscles of mammals
produced about twice the enzymatic activity seen in those of reptiles
(although those from the organs mentioned before were about thre same).
Since mitochondria consume oxygen in making ATP, the higher resting oxygen
consumption in endotherms then ectotherms is also tied to metabolic output.
    So, the author's story goes something like this: a mutation in
endotherms makes thier cell membranes leaky, and more permeable to ions.
The ion pumps have to work harder to maintain the ion gradient.  This forces
the mitochondria to produce more ATP to keep the pumps supplied.  When ATP
breaks down in providing energy it releases heat as a byproduct.  Hulbert &
Else suggested that even though a leakier membrane seems like a problem, the
increased heat output it induced in endotherms was advantageous, and
retained as an adaptive trait.  Note that this doesn't say anything directly
about increased ENERGETICS in endotherms (although the heat may contribute
to this); the extra ATP is getting consumed in running the sodium-potassium
pumps.

    In mammals, "non-shovering thermogenesis" is a method heat production
activated by cold that is about 60% reliant on a specilized type of fat
called "brown fat".  Brown fat is mainly present in baby mammals (including
humans) and small adult mammals, which have the biggest problems with heat
loss in being small.  It is largely concentrated along the spine as I recall
(I think I also remember someone suggesting that SIDS may be related in part
to the spinal cord of babies getting too cold, which would help explain why
it is concentrated there.  Someone who knows more about it can speak up,
though hopefully the discussion won't wander too far from dinosaurs). The
brown color of brown fat comes from its heavy concentration of mitochondria
and extensive vasuclarization.   Like the endothermic cell membranes, the
mitochondrial membranes in brown fat are believed to also be leakier,
increasing thier energy consumption and therefore heat output.

    The title of the article I cited earlier is:
    Hulbert, A.J., and Else, P.L. 1990. The cellular basis of endothermic
metabolism:a role for "leaky" membranes?  The paper is a photocopy handed
out during my animal physiology course several years ago, but the citation I
can make out is: Int. Union Physiol Sci/Am. Physiol. Soc.   Volume
5/February 1990 NIPS.
Perhaps someone can provide the proper citation.

LN Jeff

Be silent always when you doubt your sense.
-Alexander Pope

Read the best books first, or you may not have the chance to read them at
all.
-Henry David Thoreau

Jeffrey W. Martz
3002 4th St., Apt. C26
Lubbock, TX 79415
(806) 747-7910
http://illustrations.homestead.com/Illustration.html