Lecture 2: Absorption and Distriution Flashcards Preview

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•The toxicity of a substance depends on the dose.
•However, it is ultimately not the dose but the concentration of
a toxicant at the site of action (target organ or tissue) that
determines toxicity.
•The concentration of a chemical at the site of action is often
proportional to the dose, but the same dose of two or more
chemicals may lead to vastly different concentrations in a
particular target organ of toxicity depending on the
disposition of the chemical.
•Disposition of a chemical depends on absorption,
distribution, biotransformation (metabolism) and


Absorption of small hydrophilic molecules

•Small hydrophilic molecules (<600 Da) can pass through aqueous pores (sieve tubes or aquaporin)


Types of Transport

1. Passive Diffusion—no ATP required; gradient driven
a. Simple Diffusion—hydrophobic molecules passively
diffuse across the lipid domain of membranes. Rate of
transport proportional to the octanol/water partition
coefficient or logP.
b. Facilitated Diffusion—saturable carrier-mediated
transport (e.g. glucose transporter)


Simple diffusion of weak organic acids and bases

•The ionized form usually has low lipid solubility and does not
permeate readily through the lipid domain of a membrane.
•The non-ionized form of weak organic acids and bases is
more lipid soluble, resulting in diffusion across the lipid domain
of the membrane.
•The pH at which a weak organic acid or base is 50 % ionized
is its pKa or pKb.


pH and ionization

The degree of ionization of a chemical depends on its
pKa and on the pH of the solution, a relationship
described by the Henderson-Hasselbalch equation.
For acids: pKa – pH= log [non-ionized]/[ionized]
For bases: pKb – pH=log [ionized]/[non-ionized]


pH Effect: Acid/Base effect on absorption

•R-CO2H would be absorbed under acidic conditions (e.g. stomach).
•R-NH2 would be absorbed at neutral pH (e.g. intestine, nasal passage).


Compounds will accumulate in total
amount where there are more binding sites

Applicable for the blood-brain barrier; toxicants with high affinity for binding proteins (e.g. albumin, hemoglobin) less likely to cross barrier.



Freebasing has been done with baking soda (sodium bicarbonate) or ammonia (NH3) to increase absorption of cocaine (crack) via
nasal installation.


2. Active Transport

a) chemicals are moved against an
electrochemical gradient;
b) the transport system is saturable;
c) requires the expenditure of energy.


Routes of Administration: 1. Oral (GI tract)

• GI tract can be viewed as a tube traversing the
•Although the GI tract is in the body, its contents
can be considered exterior to most of the body’s
•Unless the toxicant is an irritant or has caustic
properties, poisons in the GI tract do not produce
systemic injury until absorbed.
•Absorption can occur anywhere in the GI tract
including the mouth and rectum.
•Initial metabolism can
occur in gastric cells.


GI Tract Absorption

•Weak acids and bases will be absorbed by simple diffusion to a greater extent in the part of the GI tract in which they exist in the most lipid-soluble (non-ionized) form.
•Highly hydrophilic substances may be absorbed through transporters (xenobiotics with
similar structures to endogenous substrates).
•Highly hydrophobic compounds may be absorbed into the lymphatic system via chylomicrons.
•The greatest level of absorption for most ingested substances occurs in the small intestine.


Polar versus nonpolar GI absorption

Polar substances that are absorbed:
1. -go to the liver via the portal vein.
2. -may undergo first-pass metabolism
or presystemic elimination in gastric
and/or the liver cells where xenobiotics
may be biotransformed.
3. -can be excreted into the bile without
entrance into the systemic circulation or
enter the systemic circulation.
-The liver and first-pass metabolism serve as
the first-defense towards most
xenobiotics. The liver is the organ with the
highest metabolic capacity for xenobiotics.


Polar versus Non-polar GI Absorption (lipophilic)

Lipophilic non-polar substances (e.g.
polycyclic aromatic hydrocarbons)
1. Ride on the “coattails” of lipids via
micelles and follow lipid absorption to
the lymphatic system (via chylomicrons)
to the lungs.
2. Non-polar substances may by-pass first-pass metabolism. e.g. PAH have
selective toxicity in the lung, where they are metabolically activated.


Routes of Administration: 2. Inhalation (Lung)

Toxicants absorbed by the lung are:
1. Gases (e.g. carbon monoxide, nitrogen dioxide,
sulfur dioxide, phosgene)
2. Vapors or volatile liquids (e.g. benzene and carbon
3. Aerosols


Gases and Vapors

The absorption of inhaled gases and
vapors starts in the nasal cavity
which has:
1. Turbinates, which increase the surface area for increased absorption (bony projections in
the breathing passage of the nose improving smell).
2. Mucosa covered by a film of fluid.
3. The nose can act as a “scrubber” for water-soluble gases and highly reactive gases, partially protecting the lungs from potentially injurious insults (e.g. formaldehyde, SO2).
-Rats develop tumors in the nasal turbinates when exposed to formaldehyde.


Absorption of Gases

•Absorption of gases differs from intestinal
and percutaneous absorption of compounds because:
1. Ionized molecules are of very low volatility, so their ambient air concentration is insignificant.
2. Epithelial cells lining the alveoli (type I pneumocytes) are very thin and the capillaries are in close contact with the pneumocytes, so diffusion distance is very short.
3. Chemicals absorbed by the lungs are rapidly removed by the blood (3-4 seconds for blood to go through lung capillary network).


Area for diffusion of gas

-total area for diffusion of gases is LARGE in humans ~50-100m^2
-diffusion path length is very SMALL <1um


When gas is inhaled through the lungs...

• When a gas is inhaled into the lungs, gas molecules diffuse from the alveolar
space into the blood and then dissolve.
• The gas molecules partition between the air and blood during the absorptive
phase, and between blood and other tissues during the distributive phase.
•Note that inhalation by-passes first-pass metabolism.


the gas continues to dissolve...

• The gas continues to dissolve into the blood until the gas molecules in blood are in equilibrium with the gas molecules in the
alveolar space.
•Once equilibrium is reached, the gas may not have reached saturation with the blood.
• At equilibrium, the ratio of the concentration of chemical in the blood and chemical in the gas phase is constant. The solubility ratio is called the blood-to-gas partition coefficient.


blood/gas phase solubility ratio

-Each gas has its own blood/gas phase solubility ratio. For example, chloroform has a high ratio of 15 and ethylene has a low ratio of 0.14.
-This ratio does not change but the concentrations can change.
-Thus, increased inhaled concentration leads to increased gas concentration in blood.
-However, the ratio does not change until saturation has occurred.


equilibrium of solubility ratio

Once equilibrium is reached, gas absorption equals
the rate of removal by blood from the alveolar space ,
so further absorption is determined by:
1. Rate of blood flow
2. Rate or volume of respiration


Gases with Low versus High Solubility

•For a substance with a low solubility ratio (e.g. ethylene = 0.14), only a small % of gas in the lungs is removed by blood during each circulation since the blood becomes quickly saturated.
-Increasing the respiratory rate or volume does not affect gas absorption.
-Increasing the rate of blood flow significantly increases the rate of uptake because of more rapid removal from the site of equilibrium.
•For a substance with high solubility ratio (e.g. chloroform = 15), most of the gas is absorbed into blood with each respiratory cycle, so that the alveolar space concentration is very low.
-The rate-limiting factor is rate and volume of respiration.
-Since the blood is quickly removing all of the gas from the lungs, increasing blood flow rate does not affect absorption.


carbon monoxide (solubility)

-Carbon monoxide has an extremely high absorptive capacity because of its extremely high affinity to hemoglobin (230 x higher
affinity than O2).
-Equilibrium will only occur AFTER saturation of hemoglobin with CO (which would be lethal).


Aerosols and Particles

Size Location of Absorption
>5 μm Deposited in nasopharyngeal region (or mouth).
1. Removed by nose wiping, blowing or sneezing.
2. The mucous blanket of the ciliated nasal surface can propel insoluble particles by movement of cilia and be swallowed.
3. Soluble particles can dissolve in mucus and be carried to the pharynx or nasal epithelia and into blood. (asbestos-lung cancer)
2-5 μm Deposited in tracheobronchiolar regions of the lungs.
1. Cleared by retrograde movement of mucus layer in ciliated portion of respiratory tract.
2. Coughing can increase expulsion rate.
3. Particles can be swallowed and absorbed from the GI tract. (asbestosis)
<1 μm Penetrate to alveolar sacs of lungs and is absorbed into blood or cleared through lymphatic system after being scavenged by alveolar macrophages. (silicosis)


Pulmonary Clearance

- Particles trapped in fluid layer of conducting
airway removed by mucociliary escalator.
- Particles phagocytized by macrophages
removed by escalator.
- Particles phagocytized by alveolar macrophages removed by lymph.
- Substances dissolved from particle surface
removed in blood.
-Small particles directly penetrate epithelial


Routes of Administration: 3. Dermal (skin)

Human skin comes into contact with many toxic agents. Fortunately, the skin is not very permeable and is a good barrier for separating organisms from their environment.


Factors for Dermal Absorption

•To be absorbed through the skin, a toxicant must pass through the epidermis or the appendages (sweat and sebaceous glands
and hair follicles).
•Once absorbed through the skin, toxicants must pass through several tissue layers before entering the small blood and lymph capillaries in the dermis.


Passing through dermal layer

•The rate-determining barrier in the dermal absorption of chemicals is the epidermis—especially the stratum corneum (horny layer), the upper most layer of the epidermis.
•The cell walls are chemically resistant, two-times thicker than for other cells
and dry, and in a keratinous semisolid state with much lower permeability for toxicants by diffusion—the stratum corneum cells have lost their nuclei and are biologically inactive.
•Once a toxicant is absorbed through the stratum corneum, absorption through
the other epidermal layers is rapid.


All toxicants move across the stratum corneum by passive diffusion

•Polar substances diffuse through the outer surface of protein filaments of the hydrated stratum corneum.
•Non-polar molecules dissolve and diffuse through the lipid matrix between protein filaments.
•The rate of diffusion is proportional to lipid solubility and inversely proportional to molecular weight.
-Once absorbed, the toxicant enters the systemic circulation by-passing first-pass metabolism.


Factors that Affect Stratum Corneum
Absorption of Toxicants

1. Hydration of the Stratum Corneum
• The stratum corneum is normally 7% hydrated which greatly increases permeability of toxicants. (10-fold better than completely dry skin)
• On additional contact with water, toxicant absorption can increase by 2- to 3-fold.
2. Damage to the stratum corneum
• Acids, alkalis and mustard gases injure the epidermis and increase absorption of toxicants.
• Burns and skin diseases can increase permeability to toxicants.
3. Solvent Administration
• Carrier solvents and creams can aid in increased absorption of toxicants
(e.g. DMSO).


Special Routes of Administration

Toxicants usually enter the bloodstream after absorption through the skin, lungs or GI tract. However, the following special routes may be used, as well.
1. Intraperitoneal injection (IP) (into the peritoneal cavity)—quick absorption due to high vascularization and large surface area
-absorbed primarily into the portal circulation (to liver—first-pass metabolism) as well as directly into the systemic circulation.
2. Subcutaneous injection (SC) (under the skin)
-by-passes the epidermal barrier, slow absorption but directly into systemic circulation; affected by blood flow
3. Intramuscular injection (IM) (into muscle)
-slower absorption than IP but steady and directly into systemic circulation; affected by blood flow
4. Intravenous injection (IV) (into blood stream)—directly into systemic circulation


Toxicity is Dependent on Route of Administration

-Often, if a toxicant undergoes first-pass metabolism, it will be less toxic if administered orally than IV.
-Caveat: This does not apply for toxicants that have selective toxicity towards the GI tract or the liver, or for toxicants that become selectively bioactivated in the liver.


Summary on Absorption

•The degree of ionization and the lipid solubility of chemicals are very important for oral and percutaneous exposures.
•Water solubility, tissue reactivity, and blood to gas phase partition coefficients are important after exposure to gases and vapors.
•For exposure to aerosols and particles, the size and water solubility are important.
•For dermal absorption, polarity, molecular weight and carrier solvent of the toxicant and hydration of the epidermis are important.



After a toxicant enters the blood or lymph, it is available for distribution (translocation) throughout the body, which usually occurs very rapidly. The rate of distribution to organs or tissues is determined by:
1. Blood flow
2. Rate of diffusion out of the capillary bed into the cells of a organ or tissue.
3. Affinity of a xenobiotic for various tissues.
Penetration of toxicants into cells occurs by passive diffusion or active transport (as discussed earlier).


Distribution of toxicants

-The distribution of toxicants is usually complex and cannot be equated with distribution into one of the water compartments of the body.
-Distribution can be highly localized, restricted or disperse
depending on:
1. Binding and dissolution into various storage sites (fat, liver, bone).
2. Permeability through membranes.
3. Protein binding.
4. Active transport.
-If the toxicant accumulates at a site away from a toxic site of action, it is considered as a protective storage site.


Storage of Toxicants in Tissues (plasma protein)

1. Plasma Proteins as Storage Depot:
a. Albumin can bind to a very large number of different compounds—the most abundant protein in plasma (e.g. bilirubin, Ca2+, Cu2+, Zn2+, vitamin C, fatty acids, digitonin, penicillin,
sulfonamides, histamine, barbiturates, thyroxine, etc.)
1. There appear to be 6 binding regions on the protein.
2. Protein-ligand interactions occur primarily through hydrophobic forces, hydrogen bonding, and van der Waals forces.
3. Examples: Bilirubin is neurotoxic at high levels, but is normally bound to albumin to make it less toxic.
-sulfonamides reverse binding of bilirubin to albumin and can cause bilirubin toxicity.
b. Transferrin, a β-globulin, is important for iron transport.
c. Cerruloplasmin carries most of the absorbed copper.


2. Liver and Kidney as Storage Depots—have the highest capacity for binding chemicals.

1. Ligandin: this cytoplasmic protein in the liver is a highaffinity
binding protein for many organic acids.
2. Metallothionein: found in the kidney and liver has high affinity for cadmium and zinc.
-in the liver, metallothionein binds Pb and
concentrates it to 50-fold more than plasma.


3. Fat as a Storage Depot

Many highly lipophilic toxicants are distributed and concentrated in fat (e.g. dioxin, DDT, polychlorinated biphenyls)
-Toxicity of a fat-stored compound will be
less in an obese person.
-However, a quick weight-loss can result in
large release of toxicant and toxic effect.


4. Bone as Storage Depot

1. Compounds such as fluoride, lead, and strontium may be incorporated and stored in bone matrix.
2. 90% of lead in the body is eventually found in the skeleton.
3. Mechanism of storage is through exchange of bone components for the toxicant (e.g. F- may displace –OH; Pb2+ and Sr2+ may substitute for Ca2+ in the hydroxyapatite lattice


Effects of Storage on Toxicity

1. Reduces toxicity of some substances by taking toxic substances out of the sites of action.
2. Increases toxicity if: a)toxicity at storage site, b)
displacement of one substance by another (e.g.
bilirubin), loss of storage site.
3. Can produce chronic toxicity from prolonged


Blood-Brain Barrier

-The blood-brain barrier serves to restrict access to many toxicants. It is not an
absolute barrier.
-It is a site that is less permeable to more hydrophilic substances than are most
other areas of the body.


There are four major anatomic and physiologic reasons why some toxicants do not readily enter the CNS.

1. Capillary endothelial cells of the CNS are tightly joined, leaving few or no pores between cells.
2. Brain capillary endothelial cells contain an ATP-dependent transporter, the multi-drug-resistant (mdr) protein that transports
some chemicals back into the blood.
3. Capillaries in the CNS are surrounded by glial cells (astrocytes) to further restrict access.
4. The protein concentration in the interstitial fluid of the CNS is much lower than that in other body fluids.


Blood Brain Barrier (water soluble molecules)

•For water-soluble molecules, the tighter junctions of the capillary endothelium
and the lipid membranes of the glial cells represent the major barrier.
•Many lipid soluble compounds are restricted due to the many lipid membranes to be crossed (capillary and glial cell membranes) and low protein content.
•The blood-brain barrier is more effective against water soluble substances.


Children are more susceptible to

•The blood-brain barrier is not fully developed at birth, and this is one reason why some chemicals are more toxic in newborns than adults.
•Morphine is 3-10 times more toxic to newborns than adult rats because of higher permeability into the brain.
•Lead produces encephalomyelopathy in newborn rats but not in adults, also apparently because of differences in the stages of development of the blood brain barrier.


Family of ATP Binding Cassette (ABC) Proteins

-The ATP-binding cassette (ABC) transporters form a large family of membrane proteins that transport a variety of compounds through
the membrane against a concentration gradient at the cost of ATP hydrolysis.
-Substrates include lipids, bile acids, xenobiotics, and peptides for antigen presentation. As they transport exogenous and endogenous compounds, they reduce the body load of toxicants.
-One by-product of such protective function is that they also eliminate various useful drugs from the body, causing drug resistance (e.g. many types of cancer cells can up-regulate MDR).
-MRP1 was originally cloned from a lung tumor cell.


Three subfamilies of the human ABC family:

1. ABCB1 (MDR1/P-glycoprotein of subfamily ABCB)
2. ABCC (MRPs)
3. ABCG2 (BCRP of subfamily ABCG)


Xenobiotic substrates:

1. Alkaloids (bases)
2. Metals—arsenic, oxidized GSH (GSSG)
3. Conjugates of glutathione, glucuronic acid, and sulfates
4. Neutral compounds (e.g. PAH)


ABC Transporter Family Structural Features

-A typical ABC protein is embedded in the membrane.
-The protein contains two transmembrane domains (TMD) and two ATP binding domains (NBD).
-The NBD of ABC transporters contains the Walker A and Walker B motifs found in all ATP-binding proteins.
-Motif C is a signature motif specific for the ABC family.


Structural model for mammalian MDR1 protein

-Each transmembrane domain contains six 6 helices. The two halves of this 1280-aa protein have similar amino acid sequences.
-A variety of lipid-soluble molecules that diffuse across the plasma membrane into the cell are transported outward by MDR1.