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NONCONVENTIONAL DOSE-RESPONSERELATIONSHIPS

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The dose-responseline also can be used to estimate the threshold dose. The threshold dose is defined as the lowest dose of the chemical that would be expected to elicit a response under conditions at which the assay was performed. The threshold dose is often empirically estimated as being a dose less that the lowest dose at which an effect was measured but higher than the greatest dose at which no effect was detected. Conceptually, the threshold dose is defined as the intercept of segments I and II of thedose-responsecurve (Figure 11.3a). Statistically, the threshold dose can be estimated from the linearizeddose-responsecurve as the LC05. This value will closely approximate the threshold dose and can be statistically derived from the entire data set (i.e., thedose-responseline). However, confidence in this value is greatly compromised, since it is derived from one end of the line (Figure 11.3b).

11.4NONCONVENTIONAL DOSE-RESPONSERELATIONSHIPS

The low-leveleffects of chemicals have received attention among pharmacologists for over 100 years. A current resurgence in interest among pharmacologists inlow-leveleffects stems from use of homeopathic approaches to treating disease. Proponents of homeopathy maintain that low levels of toxic materials stimulate physiological responses that can target disease without eliciting adverse effects in the individual undergoing treatment. Homeopathic principles may have application in toxicology based on the premise that exposure to some chemicals at subthreshold levels, as defined by standard acute toxicity evaluations, can elicit toxicological as well as pharmacological effects. Both pharmacological and toxicological homeopathy may be the consequence of hormesis.

Hormesis is defined as an overcompensatory response to some disruption in homeostasis. Thus hormesis is typically evident at low doses of a chemical at which gross disruptions in homeostasis do not mask the hormetic response. Further, hormesis typically presents as an effect opposite to that elicited at higher levels of the chemical. For example, a chemical that stimulates corticosteroid secretion at high dosages resulting in hyperadrenocorticism might elicit a hormetic response at low dosages resulting in corticosteroid deficiency. A hypothetical nonconventional dose-responserelationship resulting from such interactions is depicted in Figure 11.4. At the true threshold dose,

Response (%)

True threshold dose

100

 

 

I

II

III

0

 

 

Hormesis

 

Dose

 

Pseudo

 

 

 

 

threshold

 

 

dose

Figure 11.4 Nonconventionaldose-responserelationship involvinglow-doseeffects and compensation. (I) True initiation of the response followed by a compensatory response that returns the effects to the 0% level. (II) A negative response due to overcompensation (hormesis) followed by recovery to the 0% effect level. (III) The standard sigmoidaldose-responserelationship.