Parameters of drug antagonism: re-examination of two modes of functional competitive drug antagonism on intraocular muscles.
There are two distinct kinetic functional pharmacological procedures by which the equilibrium affinity constant, KB, of a competitive reversible blocker is obtained. The classical method on an organ system requires the study of the parallel displacement of the agonist concentration-response curve in the presence of the blocker. In the second method, the agonist-evoked functional mechanical response is reduced to half by the blocker IC50 (the concentration required for 50% inhibition). In relation to these parameters the role of the ionization constant pKa and liposolubility log Pc or log D of blockers was examined. On the ciliary muscle from human eye, IC50/KB ratios for (+/-)-atropine, its quaternary analogue (+/-)-methylatropine, (-)-scopolamine, (+/-)-cyclopentolate, (-)-tropicamide, (+/-)-oxybutynin and pirenzepine were 15, 23, 4.4, 2.6, 1.66, 1.46 and 1.71, respectively. The ratios on the iris sphincter were comparable with those of ciliary muscle. When compared with large proportions of ionized molecules with water soluble properties of (+/-)-atropine and (+/-)-methylatropine, relatively high amounts of un-ionized and/or with greater partitioning of all other blockers in the lipoid barrier correlated well to low IC50/KB ratios, as predicted by the classical theory of competitive drug antagonism. It was hypothesized that due to receptor biophase access, the reduction of the mechanical response of the agonist by the highly ionized water-soluble antagonist at IC50 represented time-distorted "pseudoequilibrium" estimation, where a higher concentration of the blocker was needed. On the other cholinergic effectors, like that of rat anococcygeus muscle or frog rectus abdominus muscle, IC50/KB ratios of respective blockers atropine or (+)-tubocurarine and hexamethonium were close to 1. Thus physicochemical properties, which affect the distribution coefficient log D and the tissue morphology (where asymmetric distribution of receptors may occur), appeared to be a critical factor in the analysis of the affinity parameters of the competitive reversible blocker. On the intraocular muscles, two functional pharmacological procedures for obtaining KB and IC50 values were not kinetically equivalent.