[Dioxin-l] RE: References about threshold.

[Henshel, Diane S.]

 I haven't read the paper.  It looks interesting, but I am not sure I fully
agree with his interpretation.  I started as a receptor biochemist (well, in
endocrinological pharmacology) and switched to neurophysiology.  Now I am an
anatomist primarily, incorporating biochemistry and molecular into
evaluation of where (and how)the changes occur.  In my experience, and in my
musings about how this all fits together, threshold at the whole organism
level and threshold at the molecular level varies.  As Lucier showed, when
you get one molecule binding to a single receptor, you can ensure a
detectable response (in the absence of a non-competitive antagonist), in
this case induction of P4501A mRNA. Other detectable responses might be
activation of adenyl cyclase or movement of Ca or Cl or Na across a
now-opened receptor-linked channel. So at the molecular level, threshold
doesn't really exist.

What Poland is referring to, as I think was David Bell in an email from
maybe yesterday, is one level of organization up: within any given system
containing x number of receptors, what is the likelihood that y number of
receptors are bound when y number of receptors bound are required to produce
enough of a change in the second messenger to elicit the desired cellular
response.  Thus, he refers to stimulation of an action potential requiring
the summed depolarizing responses of a number of NT receptors being bound in
order to open enough channels in order to bring in enough Na in order to
depolarize enough of the membrane that the axon hillock depolarizes enough
to set off (initiate) the action potential.)  That is the question of
threshold at the cellular rather than the molecular level.

There is yet another level of response, of course (even two, but we'll skip
organ level for now) the integration at the level of the body.  My favorite
example here is the developing organism and responses within the developing
organism.  Plasticity is inherent in almost all organs when the organism is
still developing.  It's a safety mechanism, to ensure that the organism can
compensate for damage (as much as possible) during development.  Here, the
threshold is produces as much by receptor affinity and cellular response
requirements as by the ability of the embryo to compensate for the induced
changes, sometimes immediately, sometimes over time (increased mitosis, for
example, to reproduce killed cells).  A second factor that affects threshold
is what is happening in and how other systems (nervous, endocrine, etc) are
affected by the inducing chemical.  Sometimes, or even not so sometimes but
an awful lot of the time, either more than one system or tissue or organ is
affected, or another system is geared to compensate for changes in the
first.  Thyroid hormone, for example.  THere is a more active and less
active form of TH.  If the induced changes in the produced level of the
active form also induces transformation of inactive to active form, than you
are more likely to induce net changes in the inactive form and less likely
to produce net changes in the active form.  Given that, and given a
measurement endpoint at the level of function (maybe behavior, inner ear
development, brain development), the threshold for that integrated effect
will be higher than the threshold for a biochemical assessment of T3, T4,
etc.

Thus threshold is relative, depending on the level of organisation at which
you are doing your measurement.  What is important to you, the biomarker or
the integrated level of the organism?  At the biomarker level there may well
be no threshold.  If regulations or decision are being made based on that
biomarker, a non-threshold response is probably a good idea not to assume.
If the decision should be based on information about the whole organism or
the whole population, perhaps a threshold should always be assumed, since we
have so many compensatory mechanisms that are the basis for that threshold.

-----Original Message-----
From: Oram, Louise
To: 'Henshel, Diane S.'
Cc: dioxin-l@venice.essential.org
Sent: 2/16/00 11:11 AM
Subject: References about threshold.

Have you read the following

My favorite summary of the problems of TEQ and risk assessment of
dioxins at the AHR.

Reflections on Risk Assessment of Receptor-Acting Xenobiotics
Alan Poland (discovered, isolated and characterized the AHR) 

Regulatory Toxicology and Pharmacology Vol 26 41-43 1997

For those of you who may not have access to this I'll quote a few choice
sections.

Threshold and Receptor theory.

The EPA has used a linear multistage model to extrapolate from high-dose
animal data (e.g. 1-20% incidence in the chronic bioassay) to low dose
in humans, less that or equal to 1x10(6) incidence, and for
carcinogenesis no threshold is permitted. Various arguments have been
advanced to rationalize this conservative assumption. It must be noted,
that for practical purposes, it is impossible to determine the effects
of very low dose levels which effect 1 in 10 (6) human beings i.e it is
not verifiable; hence it is not really science, but a policy.

While linear extrapolation without a threshold may be an untestable
policy at EPA, in receptor pharmacology, threshold is a bedrock concept
rooted in empiricism. Neurons have a resting potential, which must be
raised to a certain threshold before the nerve will fire and conduct the
impulse. If a nerve cell responded in a linear fashion to every small
quantity of neurotransmitter leaked across the synapse, it would not
make physiological sense. Since the dawn of studying receptors-nearly
100 years ago- I am unaware of a single instance of a drug i.e any
biologically active substance) acting upon a receptor which upon
sufficient decrease in concentration or dose does not display a
no-effect level i.e a threshold. For receptors, threshold is a universal
empiricism; for risk assessment, the absence of threshold (i.e low dose
linearity) is a useful statistical approach, a policy without
verification.

On TEQ-

Cumulative effects of multiple agonists acting on the same receptor.
Linear occupancy vs efficacy model. 

A major consideration in estimating the risk of receptor acting
xenobiotics is how to calculate the cumulative effects of multiple
agonists acting on the same receptor. The approach adopted in the dioxin
reassessment was to estimate the potency of each congener relative to
TCDD (i.e the Toxic equivalency factor) by multiplying the TEF by the
amount of congener and then summing the products. This approach assumes
linear a linear occupancy model, i.e for a given response each congener
occupies the same fraction of receptor. This is a simplifying
assumption, but incorrect. We know that among xenobiotics acting on the
AH receptor (for dioxins) and on the estrogen receptor, there are
partial agonists, compounds which never produce the maximal response and
which can antagonize full agonists. For each congener, the biological
activity is a function of the intrinsic efficacy of the drug, it's
concentration, and the receptor concentration. Receptor concentration
varies two to three orders of magnitude among cell types.....efficacy
has been determined for the beta adrenergic agonist but never for
soluble receptors. Thus the additivity TEF approach is not some
approximation estimate; it is a sheer guess into the unknown.

..............................

My favorite paper for destroying the threshold theory is George Lucier's
paper using PCR- enhanced analysis of expression of P450 mRNA.  I recall
(I
think) that the paper was EHP (Env Health Perspec) but I don't recal the
year, maybe 3 - 5 years ago at a guess.