Minimum receptor occupation

Threshold - physiological effect only begins at finite minimum of receptor occupancy... 

Fig. 23. Fraction of cells responding and calcium response latency. Both the fraction of cells responding and the calcium response latency depend on the equilibrium receptor occupation by phenylephrine (PhE), Ceq, which can be calculated from Eq. (7). Data without prazosin (Pz) pretreatment are shown as solid circles. Equilibrium receptor occupation for these data ranges from 2 to 90%. Open circles represent data for cells pretreated with prazosin to block cell receptors. For the three cases shown, prazosin treatment inactivates between 13 and 60% of the receptors, (a) The calcium response latency reaches a minimum at high equilibrium receptor occupation. For equal equilibrium receptor occupation, pretreatment with receptor blocker slows the calcium response, (b) The fraction of cells responding increases to a maximum at high equilibrium receptor occupation. As the fraction of receptors blocked by prazosin increases, the fraction of cells responding drops sharply.

---