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RE: [HAPS-L] End-product inhibition
One view of the human body is as a big bag of chemicals.
Does it matter in this view how complex the reactions
are?
On a recent essay exam
I asked my students to give me an example of negative feedback. Most
used
parameters such as glucose concentration, temperature, even calcium
concentrtion. Two students however
used "end-product inhibition" of
enzymes (with specific examples). I''m just curious, do most of
you
consider this negative feedback in the classical
sense?
Thanks
Chet Harbut, Ph.D.
I
think this is an excellent example of negative feedback, and I think it's
fallacious to assume that negative feedback has to involve any sort of
integrating center or control system. Sometimes it does, sometimes it doesn't.
All that is necessary for negative feedback, in my opinion, is that a chain of
reactions or processes is set into action and the end-product or final result of
these processes has the effect of reversing (negating) the original change that
started the whole thing.
Another example of negative feedback without an
integrating or control system in the usual sense is seen in edema (or its
prevention). The buildup of pressure in the interstitial fluid forces the valves
of the lymphatic capillaries open, the lymphatics absorb more fluid, and this
brings down the interstitial fluid pressure -- thus reversing the change that
started the process. Thereby, the homeostatic regulation of interstitial fluid
volume is achieved.
In The Wisdom of the Body (p. 290), Walter Cannon
referred to shifts in tissue fluid as "storage by inundation" and remarked "It
appears to have no specially developed control other than the relative
concentration of the substances concerned...." If we construe negative
feedback as a physiological process that maintains homeostasis by reversing an
initial change, then all sorts of processes can qualify that have neither neural
nor endocrine sensors or integrators.
Aside from end-product inhibition,
another good case of negative feedback where the only "sensor-integrator" is a
molecule is the Bohr effect in oxyhemoglobin dissociation.
But, if we
consider "end-product inhibition" to be negative feedback then we must also
consider chain reactions and cascades to be positive feedback
No we don't, because positive feedback means the
end-result or end-product feeds back and accelerates the change that initiated
the process. That's not so in cascades. For example, an activated kinase does
not (to my knowledge) feed back and accelerate the activation of adenylate
cyclase in the cAMP second-messenger system. Fibrin does not feed back and
accelerate the activation of factor XII in blood clotting. These cascades
certainly have an amplification effect (enzyme amplification), but they lack the
self-accelerating feedback aspect.
Ken