FUNCTIONAL ANTAGONISM AS A MEANS OF DETERMINING DISSOCIATION CONSTANTS AND RELATIVE EFFICACIES OF SYMPATHOMIMETIC AMINES IN GUINEA‐PIG ISOLATED ATRIA

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Abstract
1 The positive inotropic and chronotropic responses to sympathomimetic amines were examined in guinea-pig isolated atria. 2 The order of potency measured from EC50 values was isoprenaline > orciprenaline > salbutamol ≥ fenoterol > terbutaline. Terbutaline and salbutamol were partial agonists on rate and together with orciprenaline and fenoterol also on tension responses. 3 Functional antagonism by carbachol caused a rightwards shift of the dose-response curve and depression of the maximum response. The rate maxima for orciprenaline, fenoterol and terbutaline were above that of isoprenaline. All the tension maxima were below isoprenaline. 4 Dissociation constants (KA) and relative efficacies (er) were determined by analogy with irreversible antagonism. 5 The relative orders of affinity (KA) were isoprenaline > orciprenaline > fenoterol > salbutamol > terbutaline. Affinities were identical on rate and tension. 6 The relative efficacies were all greater than isoprenaline for rate responses. On tension they were the same or less than isoprenaline. 7 The implications of these results are discussed, in particular the fact that a partial agonist has a greater efficacy than a full agonist.

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