Antagonism agonist response effects

Figure 10.20. A partial alteration in the efficacy of the agonist results in a different steady state whereby the curve is partially depressed but no further dextral displacement is observed (Figure 10.19b and Figure 10.21, see Section 10.6.6). While the models used to describe allosteric alteration of both affinity and efficacy of receptors are complex and require a number of parameters, the identification of such effects (namely, incomplete antagonism of agonist response) is experimentally quite clear and straightforward.

In contrast, a low dose of the Ca2+ channel agonist BAY K 8644, a dihydropyridine derivative, antagonizes the antinociceptive effect of p-opioids. This is in agreement with results from Smith and Stevens (1995), who reported that Ca2+, when administered i.c.v., antagonizes morphine-induced antinociception in the mouse tail flick assay. The dose - response curve of morphine is shifted to the right by i.c.v. administration of calcium ions. 

FIGURE 6.1 Effects of antagonists on agonist dose-response curves, (a) Surmountable antagonism with no diminution of maxima and no limiting antagonism (competitive antagonists). 

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