Occupancy: how full are the receptors?
Receptor occupancy is simply the fraction of a tissue's receptors that have a drug bound at any moment. It rises smoothly with dose, and at a concentration equal to the Kd, occupancy is exactly 50%. The naïve assumption is that response should track occupancy one-for-one: fill 50% of receptors, get 50% of the effect. Often that is roughly true — but not always, and the exception is illuminating.
Spare receptors: a safety margin built in
In many tissues, an agonist reaches its full Emax while occupying only a *fraction* of the receptors — sometimes as little as 1–5%. The leftover, unneeded receptors are the receptor reserve, or spare receptors. They are not a manufacturing mistake; they are a feature. Because the signal saturates downstream before the receptors do, the system responds quickly and reliably even when the messenger is scarce.
Spare receptors have a sharp consequence: the dose for half-maximal *response* (the functional EC50) can sit at a lower concentration than the Kd for half-maximal *occupancy*. In other words, a receptor reserve makes an agonist look more potent than its raw binding affinity alone would predict.
Two ways to block: competitive vs non-competitive
An antagonist reduces an agonist's effect, but *how* it does so splits into two distinct stories. In competitive antagonism, the blocker binds the same site as the agonist, and the two compete. Crucially, this is a tug-of-war you can win with numbers: pile on more agonist and you out-compete the blocker, eventually reaching the same Emax as before. The cost is that you need more agonist — the dose–response curve shifts right (apparent potency drops) but its height is preserved. This is *surmountable* block.
In non-competitive antagonism, the blocker either binds irreversibly or grabs a different site, so heaping on agonist cannot dislodge it. Once enough receptors are taken out of play — including any spare ones — the ceiling drops: the Emax falls and no extra agonist can restore it. This is *insurmountable* block.
Competitive antagonist (same site, reversible): curve shifts RIGHT | Emax UNCHANGED | surmountable by more agonist Non-competitive antagonist (different site or irreversible): Emax DROPS | cannot be overcome | insurmountable Memory hook: Right-shift, same ceiling = competitive Lower ceiling = non-competitive