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Toxicity testing' Doseresponse and timeresponse

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Colinus (northern bobwhite) Daphnia spp. Freshwater crustacean ... Colinus virginianus (northern bobwhite quail) often used to test toxicity of pesticides ... – PowerPoint PPT presentation

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Title: Toxicity testing' Doseresponse and timeresponse


1
Toxicity testing. Dose-response and time-response
  • Topic 7

2
Learning goals
  • To learn basic concepts underlying standard
    toxicology models
  • To understand and be able to apply dose-response
    and time-response models of toxicology testing
  • To learn examples of model test organisms

3
Why dose is important?
  • All substances are poisons there is none which
    is not a poison. The right dose differentiates a
    poison and a remedy.
  • Paracelsus (1493-1541)

4
Toxicity testing
  • Used to determine acute and chronic effects of a
    toxic compound or mixture of compounds
  • Acute typically 96 h or shorter
  • Chronic weeks to months (optimally ca. 10 of a
    species life-time)
  • Traditionally and most often based on lethality,
    but other endpoints can be used (e.g. growth,
    reproduction)

5
Dose-response models
Measure mortality after a certain exposure period
(e.g. after 48 h or 5 days)
6
Dose-response models are based on the normal
distribution of biological variables
7
Population response to toxicants are usually
skewed to the left
8
Individual tolerance concept basis of the
dose-response model
  • Individual effective dose (IED) a minimum dose
    required to kill an individual
  • Genetically determined for each individual

9
IED model results in log normal distribution
(sigmoidal curve)
10
Alternative models log logistic model
  • Linked to enzyme kinetics, autocatalysis, and
    adsorption
  • Probability of death for any individual is a
    consequence of random processes and is determined
    by the probability that a specific sequence of
    events leading to death will occur in a certain
    individual
  • These probabilities are equal for all individuals
    in the population
  • Which particular individual responds quickly
    (sensitive) or slowly (tolerant) is a matter
    of chance alone

11
Log-logistic model predicts similar type of
sigmoidal curve
12
So whats the difference?
1st toxic insult 30 died
2nd toxic insult expectations?
13
Determination of toxic doses from log normal and
log logistic models
  • Data need to be transformed (linearized) a
    sigmoidal curve must be converted into a straight
    line

14
Data transformation Step 1Change Dose axis to
log scale
15
Data transformation Step 2Convert response to
probit or logit
16
Probit
  • Derived from normal equivalent deviation (the
    proportion of dying expressed in terms of
    standard deviations from the mean of a normal
    curve)
  • Probit(P)NED(P)5

17
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18
Alternatively, response can be converted to
logit
19
Resulting straight lines are not very different
Probit
Probit or Transformed logit
Logit
Log concentration
20
Measures of toxicity derived from dose-response
curves
21
Using probit- or logit-linearized dose- response
curve
Probit or logit
22
Estimate LC from the linear regression line
  • YabX, where Y transformed logit, X- logC, b
    slope of the line, a intercept of the line
  • Calculate log(LC50)
  • What is the transformed logit value for 50
    mortality?
  • Calculate LC5010log(LC50)

23
Time-response models
24
Time-response LT50
  • Mathematical apparatus very similar to that used
    for LC50 determination
  • Same methods could be applied
  • Probit
  • Logit

25
Alternatives of LC50 and LD50
  • Using endpoints other than mortality
  • EC50
  • Fixed Dose Procedure (FDP)
  • Limit Test
  • 5000 mg/kg
  • The Up and Down Procedure (UDP)

26
Conducting toxicity testing
  • Now we know WHAT to measure what about HOW?

27
Experimental design for toxicity testing
28
Overcoming problems with static design Way 1.
Increasing volume
29
Overcoming problems with static design Way 2.
Static-renewal
30
Standard Toxicity Tests
  • ASTM - American Society for Testing and Materials
  • EPA - both federal state agencies
  • OECD - Organization for Economic Co-operation and
    Development

31
Test organisms are surrogates for natural
populations
  • test species not necessarily species at risk
  • but are representative of non-target populations
  • we assume responses of test species is similar to
    species at risk
  • test species chosen for ease of culture, etc.

32
Criteria for selection of test (sentinel, model)
organisms
  • Widely available - from lab culture, hatchery,
    field collections
  • Successfully maintained in the lab, sufficient
    quantities
  • Preferably known genetic composition and history
    of culture
  • Relative sensitivity to various classes of
    toxicants should be known
  • Sensitivity of organism should be representative
    of the phyla that species represents
  • In multispecies tests (microcosms field
    studies), the interactions among the component
    species should be understood

33
Common organisms used in standard toxicity tests
  • Invertebrates
  • Lumbriculus variegatus
  • (terrestrial worm)
  • Daphnia magna (water flea)
  • Daphnia pulex (water flea)
  • Ceriodaphnia dubia (water flea)
  • Chironomus tentans (blood wormmidge larvae)
  • Hyalella azteca (scud)
  • Gammarus pseudolimnaeus (scud)
  • Hexagenia (mayflies)
  • Vertebrates
  • Pimephales promelas (fathead
  • minnow)
  • Oncorhynchus mykiss (rainbow trout)
  • Lepomis macrochirus (bluegill)
  • Fundulus heteroclitus (mummichog)
  • Rana sp (frog)
  • Rattus (rat)
  • Colinus (northern bobwhite)

34
Daphnia spp.
  • Freshwater crustacean
  • Require hard water
  • Acute 48h test
  • 10 neonates at each conc.
  • no food
  • static renewal
  • endpoint immobility (mortality)
  • Chronic test (partial life cycle)
  • 10 organisms
  • 21 days exposure (3 molts)
  • no food
  • static renewal
  • endpoint survival, growth (length),
    reproduction

35
Advantages of Daphnia tests
  • easy, inexpensive to rear
  • small size rapid test
  • can perform partial life cycle tests rapidly
  • Disadvantages standard tests are heavily biased
    towards arthropods (insects and crustaceans),
    hardly anything is known about other taxa

36
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37
An example of a vertebrate model
  • Colinus virginianus (northern bobwhite quail)
  • often used to test toxicity of pesticides
  • toxicant delivered through food
  • birds feed ad libitum
  • endpoints mortality, reproduction

38
Prokaryotes
  • MICROTOX tests

39
Factors modifying toxicity
  • Acclimation/acclimatization
  • Allometry
  • Abiotic factors
  • Photo-induced toxicity
  • pH
  • Hardness
  • Temperature
  • Organic ligands
  • Physiological status
  • Age
  • Gender

40
Take-home message
  • Formulate three main things you have learned
    about toxicity testing
  • Formulate one thing you have learned from this
    topic that was most surprising or unexpected for
    you
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