BMS 123 · Common Core (Tronc commun)Section 5.2 · Pharmacodynamics: mechanisms and receptors
Section 19 of 27
Drugs that act without receptors
Explain why a few drugs act through physical or chemical properties, and give named examples of each group.
New to this course? How each section works
Learn gives you one short idea at a time. Each idea starts with a single sentence, then explains it, gives an example, and asks you a small question you can answer straight away.
Explore shows a diagram of the same ideas. Click the numbered parts to read what each one is.
Check asks a few questions. Answer them all correctly to finish the section and unlock the next one.
“Go deeper” is optional. It adds mechanism and extra detail for when you are ready — nothing you need for the checks is hidden there.
Why this matters
If you assume that every drug needs a receptor, you will look for the wrong mechanism in a whole group of familiar medicines — laxatives, antacids, charcoal, chelating agents and anticancer alkylating drugs. You will also misread their dose–effect behaviour, because with these agents the amount given, rather than an affinity for a binding site, usually decides the size of the effect.
What you need to know first
You need the distinction from 5.1 between what the body does to a drug and what the drug does to the body, plus the everyday ideas of osmosis, adsorption and pH. No knowledge of receptors is required yet; section 5.3 introduces them.
Words to know (5)
Physically reactive agent
A drug whose action comes from a physical property such as mass, osmolarity, lubrication or adsorbing surface. Mannitol acts by osmosis and promotes diuresis.
Adsorbent
A substance that binds other molecules onto its large surface so that they can be carried out of the gut. Activated charcoal binds poisons and toxins in the gut.
Chelating agent
A molecule that forms a stable ring-shaped complex with a metal ion and so removes it from the body. EDTA and dimercaprol chelate mercury and other heavy metals.
Osmotic diuretic
A substance filtered by the kidney that holds water in the urine by its osmotic effect, increasing urine volume. Mannitol is an osmotic diuretic.
Alkylating agent
An anticancer drug that binds covalently to DNA and stops the cell from copying it, leading to cell death. Alkylating antineoplastics bind covalently to cancer cells.
One idea at a timeIdea 1 / 5
In one sentence A few drugs act through their own physical and chemical properties rather than by binding a receptor, and they fall into two arbitrary groups.
Not every drug needs a receptor
Drugs act by several mechanisms, grouped into two broad categories: non-receptor-mediated mechanisms and receptor-mediated mechanisms. The action of some drugs is attributed to their physical and chemical properties, and very few drugs fall into this category. These drugs are grouped arbitrarily into two categories: physically reactive agents and chemically reactive agents. The distinction is a teaching convenience rather than a law of nature: an agent may act by a physical route and, at the same time, irritate tissue chemically. It matters because the language of receptors, affinity and competitive blockade simply does not apply to them.
For example
Activated charcoal does not fit a receptor; it presents a vast surface that holds poison molecules. Magnesium sulfate does not fit a receptor either; it holds water in the gut by osmotic force until the colon distends and empties.
Think of it like
These drugs work like a sponge, a magnet or a sieve rather than like a key in a lock. The limit of the image is that a sponge and a sieve do not interact with the body at all, whereas these agents still provoke a physiological response — the intestine moves, the kidney excretes, the stomach stops burning.
Pause and check yourself
Do most clinically useful drugs act without a receptor?
Show the answer
No. Only a small minority acts by physical or chemical properties; the great majority of clinically useful drugs produce their effects by binding biomolecules at the molecular level.
Read detailed explanations
Why covalent attack on DNA is a different kind of drug action
An alkylating agent does not occupy a site and wait to be displaced. It forms a covalent bond with the bases of DNA, often cross-linking the two strands of the double helix. The consequence is mechanical: the helix can no longer be opened, so the replication fork stops and the cell cannot divide. Because the bond is covalent, the effect does not disappear when the drug is cleared from the plasma — it persists until the cell repairs the damage or dies. The same chemistry explains the toxicity. A malignant cell and a healthy bone marrow cell contain the same DNA bases, so an alkylating agent has no way of recognising which one it is attacking. Tissues that divide rapidly — bone marrow, gut lining, hair follicles — therefore suffer alongside the tumour. Antimetabolites are subtler: they are analogues of pyrimidine and purine bases, so the copying enzymes accept them as substrates and build them into the new strand. The strand looks normal at a glance but carries a false block, and the next round of copying fails. Both strategies show that a drug can produce a pharmacological effect without any receptor, and that the price of that power is a narrow margin between the tumour and the patient.
What “dose” means when there is no receptor
With a receptor-mediated drug, increasing the dose increases receptor occupancy until the response saturates, and the shape of that saturation is predictable from affinity. Non-receptor agents behave differently, and the difference has practical consequences. An antacid neutralises acid in a fixed chemical proportion: one molecule of base consumes a defined amount of acid, so the useful quantity depends on how much acid is present rather than on a binding constant. A chelator has a fixed capacity for metal ions, and once its binding groups are occupied, more chelator cannot remove more metal than the metal available. An adsorbent has a finite surface, and once that surface is covered, further charcoal adsorbs nothing. In all three cases the relationship between amount and effect is closer to a stoichiometric limit than to a graded response curve. This explains why a patient who has taken a very large overdose may need repeated doses of charcoal, why chelation is continued until exposure stops, and why an excessive dose of these agents produces its own harm — diarrhoea, electrolyte loss, irritation — rather than a stronger therapeutic effect.
Explore the diagram
Look at the whole drawing first. Then select one numbered part and connect it to the idea you just read.
Drugs that act without a receptor
A minority of drugs works through their own physical or chemical properties rather than by binding a receptor: by mass, osmosis, lubrication, adsorption, neutralisation, chelation or covalent attack.
New to diagrams? Read this first
Look at the whole drawing once without clicking anything. Ask yourself only: what kind of picture is this (a comparison, a route, a sequence, a table)?
Press Play, or use the arrows, to move through the numbered parts in order. The drawing stays still; only the highlight moves.
Click a number, or its name in the list below, to read what that part is and why it matters. Use “Hide labels” when you want to test your memory.
More controls
01 / 5Physically reactive agents
If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.
Physically reactive agents
Drugs whose effect comes from a physical property — the mass they add, the water they hold by osmosis, the film of oil they spread, the surface that adsorbs, or the radiation they emit. No receptor is involved at any step.
Schematic, not to scale. Only a small minority of drugs acts this way; the great majority of clinically useful drugs work through the targets shown in the next plate. Physical and chemical mechanisms still produce unwanted effects and still depend on dose.
Key points to remember
✓Only a few drugs act by physical or chemical properties; most act through receptor-mediated mechanisms.
✓Physically reactive agents work by bulk, osmosis, lubrication, adsorption, radio-opacity or effervescence.
✓Chemically reactive agents neutralise, oxidise, precipitate, chelate or modify body fluids and urine pH.
✓Alkylating agents and antimetabolites attack macromolecules, binding DNA covalently or entering nucleic acids as false blocks.
✓With these drugs the amount given, not an affinity, usually decides the size of the effect.
Go deeper
Worked example
Two intoxicated patients, two non-receptor mechanisms. Two patients arrive after an overdose. The first has swallowed a large amount of tablets an hour earlier. The second has been exposed to a heavy metal over several weeks. Both need treatment that does not work through a receptor at all.
For the first patient, the drug is still in the gut. Activated charcoal offers a huge surface that adsorbs the poison before it can be absorbed — a physical mechanism with no target molecule.
The charcoal is not displaced by giving more of the poison, because there is no binding site to compete for; what matters is how much surface is available to adsorb what remains.
For the second patient, the metal is already in the tissues. A chelating agent such as dimercaprol or EDTA forms a stable coordination complex with the metal ion — a chemical mechanism.
The complex is water-soluble and is carried to the kidney, so the metal leaves the body in the urine instead of continuing to damage enzymes and membranes.
Neither treatment requires affinity, intrinsic activity or a receptor. Both nevertheless have a dose, a limit and a toxicity of their own, which is why they are still drugs and not simple antidotes.
The same clinical problem — remove a poison — is solved by a physical mechanism in one patient and a chemical mechanism in the other. Recognising which kind of mechanism is at work tells you what to expect when the dose is increased, and what cannot be overcome by adding more of the offending substance.
Common misconception
Beginners often assume that “drug action” always means a drug fitting a receptor, so they invent a receptor for activated charcoal, for mannitol or for an antacid. Most drugs are indeed receptor-mediated, but these agents act by mass, osmosis, adsorption or a direct chemical reaction, and forcing them into the receptor model leads to wrong predictions about their dose and their toxicity.
Check your reasoning
Could a competitive antagonist ever reverse the effect of activated charcoal or of a chelating agent? Explain your reasoning in terms of binding sites.
Reveal an answer
No, because competitive antagonism requires two molecules competing for the same binding site. Neither charcoal nor a chelator acts at a receptor site: charcoal adsorbs onto its own surface and a chelator forms a chemical complex with a free ion. Adding an antagonist would simply add another substance to the mixture without releasing the captured poison.
Where this comes from
BMS 123 General Pharmacology II lecture notes · Part Two, Basic concepts of drug action, §2.1.1 Non-receptor-mediated mechanisms
Lesson progress is saved on this device for the current learner. Practice answers stay separate and are not stored.
About this course and its sources
Bilingual lessons organised from the supplied BMS 123 General Pharmacology II lecture notes (Principles of Pharmacokinetics and Principles of Pharmacodynamics). Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.