receptor reserve

Receptor reserve, or 'spare receptors', is the idea that a tissue can hit its maximal response while many of its receptors sit empty. Imagine a stadium with far more exit doors than it needs: you can empty the crowd at full speed even if half the doors stay shut. The extra doors are the reserve — surplus capacity that is there but not all required at once.

Mechanistically, this happens when the signaling machinery downstream of the receptor amplifies strongly, so engaging only a fraction of receptors already saturates the response. A practical consequence is that the dose giving half-maximal effect (EC50) can sit well to the left of the dose giving half-maximal receptor occupancy — the system reaches full effect before it reaches full occupancy. Reserve effectively makes a partial agonist behave more like a full agonist in that tissue, and makes the tissue more sensitive (more potent-looking) to its agonists.

Receptor reserve is a property of the particular tissue and signaling context, not of the drug alone, so the same agonist may show large reserve in one organ and little in another. It also explains why blocking or losing some receptors (or some signaling capacity) may have no visible effect until the reserve is used up — only then does further loss start to cut into the maximal response. This buffering is clinically important when receptor numbers fall in disease or are reduced by antagonists.