ABSTRACT
An often encountered example of such disproportionality is that of spare receptors. Consider a cell membrane that has receptor-gated calcium channels in it. In order for the calcium to interact with its receptors inside the cell its concentration must be elevated sufficiently to put it in the range where it can have a significant effect on fractional receptor occupancy. If more of the calcium channels are opened by the addition of more ligand no further effect will be produced. All the receptor-gated calcium channels over 50%, in this example, are spares. Receptor function also may be modulated by inhibitors or stimulators, which are ligands other than the agonist and which bind at sites other than the agonist binding site. Heterotropic cooperativity can, under certain circumstances, cause disproportionalities between receptor occupancy and receptor stimulation. The nicotinic acetyl choline receptor of the mammalian neuromuscular junction is an example of a receptor system that automatically desensitizes after the stimulated receptor-ligand complex has formed.
