The relationship between muscarinic receptor binding and ion movements in rat parotid cells.

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Abstract
The relationship of muscarinic receptor binding to ion fluxes induced by muscarinic agonists was studied in rat parotid cells. Receptor binding was measured with [3H]quinuclidinyl benzilate in living, dispersed parotid acinar cells. The same cells were used to determine concentration-effect relationships (ion fluxes) for agonists. Receptor binding of antagonists accurately reflected the pharmacologically determined affinities. In the case of agonists, apparent pharmacological affinities were greater than binding affinities by a factor of 12.5 for methacholine and 18.7 for carbachol. The reason for the discrepancy between agonist binding and effect is not known, but the existence of spare receptors is a possibility. The total number of muscarinic receptors is estimated to be 23,000/cell. If some are spare, as few as 1800/cell may suffice to fully activate the available Ca channels.