The sensitivity of the longitudinal muscle of the guinea-pig ileum to muscarinic drugs producing contraction depends on optimum concentrations of calcium and magnesium. It can also be reduced by changes in sodium concentration and osmolarity.2. The rubidium efflux response to these same drugs is insensitive to any of these changes in the external medium.3. Raised calcium or magnesium concentration has the effect of largely annulling the differences in structure-activity relationships of the two responses as they exist in optimal media.4. The effects are explained in terms of a labile coupling process between a single receptor and the contractile process compared with a stable coupling process of the efflux process.
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Burgen et al. (1970) studied this question.
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