Maximum agonist activity

This term has increasingly come to be employed (as here) in a rather different sense from that introduced by R. F. Furchgott (Section 1.4.2). With this newer usage, intrinsic efficacy indicates the maximum receptor activation (often expressed as the fraction of receptors in the active state) that can be achieved by an agonist acting through a mechanism that can be formulated and studied at the molecular level, as in the present example. The intention of this redefinition is to focus on the receptor itself and its immediate transduction mechanism (e.g., G-protein activation), rather than on the cellular events that follow. 

The term intrinsic activity (ia) was defined as a measure of the abiUty of the dmg—receptor complex to generate response. When ia = 1, a full agonist is defined when ia = 0, an antagonist is defined. Thus, values 0 < ia < 1 define partial agonists as follows, where R is the response to dmg and R is the maximum response achieved. 

D. For this simulation the equilibrium constant for isomerization between AR and the AR has been set so that few of the receptors are in the active state even in the presence of a large concentration of A on its own. However, with B also present at increasing concentrations, the equilibria shown in Figure 1.28 are shifted toward the active forms so that the maximum response to A rises to a point at which almost all of the receptors can be activated. In effect, B is acting as a co-agonist. Note that it causes little receptor activation when [A] is small. 

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