Pharmacology Basics for Pharmacy
On this page
Direct answer
Pharmacology divides into pharmacokinetics (what the body does to the drug) and pharmacodynamics (what the drug does to the body), and the dynamic half rests on the receptor: proteins that recognise a ligand and transduce recognition into response. Four superfamilies cover nearly every target — ligand-gated ion channels (nicotinic, GABA-A), G-protein-coupled receptors with seven transmembrane domains (adrenoceptors, muscarinic, opioids), kinase-linked receptors (insulin) and intracellular nuclear receptors acting as transcription factors (steroids, thyroid). Drugs engage these as full agonists, partial agonists, inverse agonists or antagonists, and their behaviour is captured in two curves: the graded dose-response curve, from which potency (EC50, position) and efficacy (Emax, ceiling) are read, and the quantal curve, from which ED50 and TD50 generate the therapeutic index. Competitive antagonism shifts the agonist curve rightward in parallel and is surmountable; non-competitive antagonism lowers the ceiling — the most examinable graph in the subject.
What you must remember
- Superfamilies with prototypes: ligand-gated ion channels (nAChR, GABA-A — milliseconds), GPCRs (adrenoceptors, opioid — seconds), kinase-linked (insulin — minutes), nuclear receptors (glucocorticoid — hours of gene transcription).
- Agonist ladder: full agonist reaches Emax; partial agonist has lower intrinsic activity; inverse agonist stabilises the inactive conformation of a constitutively active receptor.
- Antagonism types: competitive reversible (parallel rightward shift, surmountable), irreversible or non-competitive (depressed Emax), chemical (chelation), physiological (adrenaline opposing histamine in anaphylaxis).
- Potency versus efficacy: potency is dose position (EC50) — how little drug is needed; efficacy is ceiling (Emax) — how much effect is possible; a potent drug with a low ceiling can be clinically useless.
- Spare receptors: maximal response achievable while occupying only a fraction, which is why early irreversible blockade shifts ED50 without lowering Emax.
- Graded versus quantal: graded curves chart effect intensity; quantal curves chart the percentage of a population responding, yielding ED50 and TD50.
- Therapeutic index: LD50/ED50 in animals or TD50/ED50 clinically; narrow-index drugs — digoxin, phenytoin, lithium, warfarin, theophylline — are the ones monitored by plasma levels.
- Transduction vocabulary: second messengers (cAMP, IP3-DAG, calcium), desensitisation and down-regulation — the basis of tolerance.
Reading the two graphs that run the subject
Plot log dose against effect for three opioids on one axis. Morphine's curve rises to full analgesia; buprenorphine's rises to a lower ceiling no dose can lift — partial agonism drawn as fact. Add naloxone at increasing concentrations and the morphine curve steps rightward in parallel, slope and ceiling intact: competitive, surmountable antagonism. Replace naloxone with an irreversible blocker and the first doses only shift the curve right (spare receptors absorbing the loss), then higher occupancy drags Emax down — the two-phase signature proving spare receptors exist. Move to quantal curves for safety: analgesia in half the patients defines ED50, respiratory depression in half defines TD50, and their ratio is the clinical therapeutic index — for morphine uncomfortably narrow, which is why anaesthesia counts respirations. Every prescribing controversy — why buprenorphine precipitates withdrawal in morphine dependence (higher affinity, lower efficacy, displacing the full agonist), why naloxone needs re-dosing (short half-life) — resolves on these two graphs.
Where students slip in pharmacodynamics
The potency-efficacy swap is the commonest error: calling one opioid "more efficacious" when the claim was about dose, or ranking drugs by EC50 as if a leftward curve meant a stronger drug — potency is economy, efficacy is outcome, and the question usually hinges on which is asked. The second slip forgets physiological antagonism: in anaphylaxis adrenaline does not block histamine receptors, it opposes histamine's effects through its own vascular receptors — a one-mark distinction from pharmacological antagonism. Third, partial agonism described as "weak agonist" obscures its clinical behaviour as antagonist at a receptor carrying a full agonist and agonist when alone. Finally, connect dynamics to practice: narrow therapeutic index is why level monitoring, cautious generic substitution and adherence counselling cluster around the same short list of drugs — an integration answer that scores above pure definitions.
Frequently asked questions
Name the four receptor superfamilies with one drug target each.
Ligand-gated ion channels (nicotinic receptor), G-protein-coupled receptors (beta-1 adrenoceptor), kinase-linked receptors (insulin receptor) and intracellular nuclear receptors (glucocorticoid receptor).
How do competitive and non-competitive antagonism differ on a dose-response curve?
Competitive antagonism shifts the agonist curve rightward in parallel with the same maximum (surmountable), while non-competitive antagonism depresses the maximum response itself.
What does the therapeutic index measure and what is its limitation?
The ratio of toxic (or lethal) dose to effective dose; its limitation is that population medians say nothing about the individual whose toxicity threshold overlaps the effective range in narrow-index drugs.
Why can a partial agonist act as an antagonist?
Occupying receptors while producing less maximal effect, it displaces a full agonist and lowers the net response — buprenorphine precipitating withdrawal is the classic example.
What are spare receptors?
Receptors present in excess of the number needed for maximal response, so eliminating a fraction by irreversible antagonist lowers Emax only after a rightward shift — experimental proof of receptor reserve.