title: The use of drug-receptor affinity measures in the differentiation of receptors.
abstract: The drug-receptor dissociation constant (K), determined by pharmacological procedures, is measured from a dose-effect relation and the modification of that relation by the presence of a second compound (antagonist) that competes for the same receptor. These procedures differ from those that use radiolabeled compounds and do not measure an effect but instead determine the K of the compound by quantitating its displacement of the labeled substance. In several examples the dissociation constants determined from pharmacological procedures are shown to be unique for a particular drug-receptor interaction and can therefore discriminate among receptors. A further demonstration is that K does not equal D50, the potency index. For competitive antagonists the Schild plot is commonly used to measure K. This plot, theoretically linear with a slope of -1, often leads to slopes different from -1 (using conventional linear regression), in which case its x intercept, pA2, and y intercept, -log KB, differ by an amount that depends on the slope. Even when this differences is small, the 95% confidence limits of pA2 and -log KB may be very different and are invariably larger for -log KB, the value that is presumably sought from this analysis. A Schild plot constrained to slope -1 (and, accordingly, different regression equations) should therefore be used in cases of known competitive antagonism such as that between naloxone and morphine. The special problems associated with measuring K for agonists are discussed and illustrated with recent work done in our laboratories in which we employed the new narcotic analgesic buprenorphine to determine by pharmacological methods the dissociation constant for morphine, a value previously undetermined.
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000378:metabolism; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004336:Drug Antagonism; D004983:Ethanolamines / Q000494:pharmacology; D007545:Isoproterenol / Q000037:antagonists & inhibitors; D008433:Mathematics; D009020:Morphine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism; D011955:Receptors, Drug / Q000378:metabolism*; D011957:Receptors, Opioid / Q000378:metabolism
chemical_list: D000316:Adrenergic alpha-Agonists; D004983:Ethanolamines; D011942:Receptors, Adrenergic, alpha; D011955:Receptors, Drug; D011957:Receptors, Opioid; D009270:Naloxone; D009020:Morphine; D007545:Isoproterenol