Agonist receptor proportion with

If a drug undergoes a covalent reaction with its receptor, the receptor molecules affected will be irreversibly blocked and thus altogether removed from the total receptor pool available for the interaction with the agonist. Thus, the agonist-receptor equilibrium now plays out in that reduced total pool. The number of occupied receptors will therefore be proportionally reduced (Figure 3.4). 

The first idea to consider is the effect of receptor density on sensitivity of a functional system to agonists. Clearly, if quanta of stimulus are delivered to the stimulus-response mechanism of a cell per activated receptor the amount of the total stimulus will be directly proportional to the number of receptors activated. Figure 5.8 shows Gi-protein-mediated responses of melanophores transiently transfected with cDNA for human neuropeptide Y-l receptors. As can be seen from this figure, increasing receptor expression (transfection with increasing concentrations of receptor cDNA) causes an increased potency and maximal response to the neuropeptide Y agonist PYY. 

It is worth examining the possible magnitudes of the error with various scenarios. The maximal value for [A]/Ka can be approximated, assuming a system where response is directly proportional to receptor occupancy. Under these circumstances, Response = 0.3 = [A]/Ka/([A]/Ka + 1), which in this case is [A]/Ka = 0.5. Therefore, the pA2 is pKB + Log (2) (i.e., the pA2 will overestimate the affinity of the antagonist by a maximal factor of 2). If the insurmountable antagonist is allosteric antagonist that reduces the affinity of the receptor for agonist (a < 1), then the error will be <2. However, if the modulator... 

If L combines only with R, then the presence of L will reduce the proportion of active receptors. L is said to be an inverse agonist or negative antagonist. If L combines with R there will be an increase in active receptors and so L will behave as a conventional agonist. Where L has equal affinity for R and R, it will not affect the fraction of receptors in the active state. However, it will reduce the binding of either a conventional or an inverse agonist, and so will behave as an antagonist. 

Suppose that L combines only with the inactive, R, form. Then the presence of L, by promoting the formation of LR at the expense of the other species, will reduce the proportion of receptors in the active, R, state. L is said to be an inverse agonist or negative antagonist and to possess negative efficacy. If, in contrast, L combines with the R form alone, it will act as a conventional or positive agonist of very high intrinsic efficacy. 

Though this looks complicated, it still predicts a simple hyperbolic relationship (as with the Hill-Langmuir equation see Figure 1.1 and the accompanying text) between agonist concentration and the proportion of receptors in the state (AR G ) that leads to a response. If a very large concentration of A is applied, so that all the receptors are occupied, the value of pAR.G. asymptotes to ... 

Our task is to work out how the proportion of receptors occupied by the agonist varies with the concentrations of the agonist and the antagonist. Equilibrium is assumed. Applying the law of mass action gives ... 

We will use B to denote the competitive antagonist being investigated and C to represent the substance with some competitive blocking action that is present in all the bathing solutions used in the experiment. When the control curve is determined, the tissue is exposed to both the agonist A and the substance C at concentrations [A] and [C], respectively. Assuming equilibrium, the proportion of receptors in the active state is then ... 

Fig. 21.11. (Opposite) (A) Representative vesicle-attached patch recordings from SENS, PYRR and LEVR isolates 30 pM levamisole in the patch-pipette, -50 mV patch potential. Note that the SENS isolate recording contains more openings than the other two patches and so will carry more current across the membrane. (B) Log10 plot of the proportion of open-time against the isolate type. Open circles represent individual receptor channels from different preparations at -50 mV with 30 pM levamisole as the agonist. Note that there is a wide spread of greater than tenfold difference between the maximum and minimum values observed for each of the isolates. There is overlap between values of the isolates but the mean (closed square) for SENS is greater than for LEVR and PYRR.

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