A model for the interaction of competitive antagonists with two receptor-subtypes characterized by a Schild-plot with apparent slope unity. Agonist-dependent enantiomeric affinity ratios for bupranolol in tracheae but not in right atria of guinea pigs.

Abstract

1. A model is presented that predicts the double log relationship between the agonist concentration ratio \t-1 against antagonist concentration for the case of two receptor subtypes mediating the same qualitative effect of an agonist and interacting with both agonist and antagonist. The relationship is a nonlinear function which is approximated for small and high antagonist concentrations by two lines of slope 1.0 united by an inflection. The function depends on the concentration of the antagonist, on the two equilibrium dissociation constants KBQ and KBR for the corresponding complexes of agonist-receptor-subtype and on the fractional stimuli σQ = 1 − σR generated by the agonist via the receptor-subtypes Q and R. 2. To test the model, the antagonism of the effects of 4 agonists was investigated with 9 antagonist concentrations in a single guinea-pig trachea. Optically pure (+)-bupranolol was used as an antagonist selective for \gb-adrenoceptor subtypes. (+)-Bupranolol blocked the relaxant effects of (\+-)-salbutamol, (\t-)-adrenaline and (\t-)-isoprenaline to the same extent whereas the effects of (\t-)-noradrenaline were blocked less effectively. Experimental points of the double-log plots were well approximated by the non-linear functions based upon the two-receptor model. 77% of the relaxant effects of (−)-noradrenaline (σQ = 0.77) appear to be mediated through a low-affinity (KBQ=45 nmol/l) β-adrenoceptor subtype; 23% (σQ = 0.23) through a high-affinity (KBR=3.1 nmol/l) β-adrenoceptor subtype for (+)-bupranolol. (−)-Adrenaline, (−)-isoprenaline and (±)-salbutamol caused most of their effects (93%, 92% and 99%, respectively) through the high-affinity R-subtype. 3. In contrast to (+)-bupranolol, the antagonism of the relaxant effects of agonists by (\t-)-bupranolol were nearly independent of the agonist used. KB-values between 0.2 and 0.4 nmol/l were estimated for the (\t-)-bupranolol-\gb-adrenoceptor complex of the trachea. The enantiomeric affinity ratio (i.e. KBQ(+)-bupranolol/KB(\t-)-bupranolol) was 102 for \gb-adrenoceptors (Q-subtype) activated predominantly by (\t-)-noradrenaline. The enantiomeric affinity ratio was 14 for \gb-adrenoceptors (R-subtype) activated mostly by (\t-)-adrenaline, (\t-)-isoprenaline and (\+-)-salbutamol. 4. In guinea-pig right atria, the blocking potencies of both (\t-)-bupranolol and (+)-bupranolol were nearly independent of the used agonist. The enantiomeric affinity ratio of bupranolol was 89 for sinoatrial \gb-adrenoceptors. The agreement of this ratio with that for Q-receptors of tracheae suggests that most sinoatrial \gb-adrenoceptors resemble the tracheal Q-receptors. 5. Tissue selectivities (i.e. tracheal/sinoatrial affinity ratios) were agonist-dependent. Against (\t-)-noradrenaline the ratio was less than 2 for both (+)-bupranolol and (\t-)-bupranolol. Against (\t-)-adrenaline, (\t-)-isoprenaline and (\+-)-salbutamol the ratio was 26 for (+)-bupranolol and 4 for (\t-)-bupranolol.