On equilibrium dissociation constants for complexes of drug-receptor subtypes

Summary

An algebraic method is proposed to decide whether or not two classes of complexes are formed for receptors that mediate the pharmacological effects of a partial agonist. The procedure consists in determining the slope (m) of the line relating equieffective concentrations of full agonist in the absence and presence of a partial agonist (P). For the case of a single class of receptor the relation [P]m/(1−m) is independent of P concentration; for the case of more than one class of complexes, the relation [P]m/(1−m) describes a hyperbola. A plot of log [(1/m)−1] against log[P] should have a unit slope for a system with a single receptor class; deviation from unit slope is consistent with a system of more than one receptor class.

The positive chronotropic effects of the full agonist (-)-isoprenaline and of 2 partial agonists were analyzed at 32.5° C in spontaneously beating atria treated with 5 μmol/l phenoxybenzamine from reserpine-pretreated kittens. [P]m/(1−m) appeared not to change significantly with different concentrations of the partial agonist (±)-p-chloroisoprenaline, suggesting the formation of a single class of drug-β-adrenoceptor complex. A dissociation equilibrium constant of 0.13 μmol/l was estimated for the β-adrenoceptor-(±)-p-chloro-isoprenaline complex.

[P]m/(1−m) depends on the concentration of (-)-practolol, suggesting the formation of more than one class of β-adrenoceptor-(-)-practolol complex. Assuming that two β-adrenoceptor subtypes mediate positive chronotropic effects of ligands, we calculated dissociation equilibrium constants of 0.04–0.35 μmol/l and 2–19 μmol/l for a high affinity and low affinity (-)-practolol-β-adrenoceptor complex, respectively. The evidence seems to be compatible with the view that 2 sinoatrial β-adrenoceptor subtypes mediate positive chronotropic effects of ligands in kitten heart. (±)-p-Chloro-isoprenaline appears to exhibit nearly the same affinity for both receptor subtypes and can be considered to be a nonselective ligand. (-)-Practolol exhibits an approximately 50-fold higher affinity for one subtype than for the other, being therefore characterized as a selective ligand.

The concentration-effect curve for positive chronotropic effects of (-)-practolol is closely associated with the receptor occupancy curve for the high affinity receptor. (-)-Practolol appears therefore to cause its stimulant effect predominantly through the high affinity β-adrenoceptor.