Pharmacodynamics dissociation constant

One frequently encounters the case where the equilibrium dissociation constant (iQ, see above) is defined by microconstants with Tast rates on and off the receptor. However, any change in potency in a chemical series (affinity) must represent an increase in the on (k+i) rate or a decrease in the off rate (fe i). Occasionally, either by accident or design, the off rate is altered dramatically enough to redefine the receptor kinetics of the compound such that the rates influence the actual pharmacodynam-... 

Clark realized this, and said The action of acetylcholine depends on at least two separable factors, firstly the fixation of the drug by certain receptors, and secondly the power to produce its action after fixation (Clark, 1937). Further experimental work in Holland established these concepts firmly under the names affinity and intrinsic activity respectively (Ariens, 1954). An inhibitor (e.g. a chemotherapeutic drug, insecticide, or fungicide) requires only affinity but many pharmacodynamic drugs must have intrinsic activity as well, namely the ability to produce a natural physiological response as soon as combination between drug and receptor has occurred. The affinity is a measure of the attraction between the stimultant and the receptor numerically it is the reciprocal of the dissociation constant of the... 

---