Estimation of apparent agonist affinity constants using desensitization of the ileal muscarinic receptor

The effects of short-term desensitization of muscarinic receptors and decreasing the extracellular calcium concentration have been studied using the isolated guinea pig ileum. Desensitization required stimulation of the receptor and was more marked against agonists of low intrinsic efficacy. The process was non-specific in that the responses to histamine were also affected. Application of null methods of analysis to pre- and postdesensitization concentration-response curves enabled an affinity constant to be derived. The values were consistently higher than those estimated using the irreversible antagonist, phenoxybenzamine. In contrast application of null methods to concentration-response curves in normal and low calcium concentrations enabled an affinity constant for pilocarpine to be calculated, which was very similar to that obtained using the irreversible antagonist. It is concluded that desensitization cannot be used to determine agonist affinity constants inasmuch as it results in overestimations.