Experimental and theoretical comparisons between the classical Schild analysis and a new alternative method to evaluate the pA(2) of competitive antagonists

The Schild analysis is undoubtedly the most frequently used powerful diagno stic tool to investigate the nature of an antagonist and, consequently, to evaluate its potency, often expressed as pA(2). Nevertheless, different rea sons often prevent the experimenter from applying this analysis, leading to use an inhibition curve for the antagonist and to evaluate its potency by means of several approaches, which are generally considered theoretically i nvalid. In a recent work, a new theoretical approach, mathematically analog ous to the Schild one, has been shown. By means of a simplified experimenta l protocol based on an antagonist inhibition curve (following a control con centration-response curve for the agonist), this method allows a linear reg ression analysis, giving a slope value absolutely equivalent to the Schild slope and a reliable estimation of the pA(2) of a competitive antagonist. I n this paper, this new method has been compared with the Schild analysis, t o determine the parameters of potency relative to well-known competitive an tagonists, on different in vitro isolated preparations. In strips of guinea pig isolated gastric smooth muscle, pirenzepine antagonised the effects of bethanechol. In guinea pig isolated ileum, atropine blocked the contractur ant effects of carbamylcholine, while in electrostimulated ileum segments, the inhibitory responses to alpha-methylnoradrenaline were reduced by idazo xan. Finally, in guinea pig isolated spontaneously beating atria, the negat ive inotropic effects of 5'-N-ethylcarboxamidoadenosine were antagonised by 8-cyclopentyl-1,3-dipropylxanthine. The parameters of potency, relative to all the above competitive antagonists and expressed as pA(2), resulted alm ost equivalent, when calculated by the Schild analysis or by the alternativ e method. Furthermore, when tested also for the well-known irreversible alk ylating agent dibenamine in rat aortic rings stimulated by noradrenaline, t he alternative method furnished a profile of clear nonlinearity, unmasking the nature of the antagonism. Finally, the relationships between the results calculated by the alternativ e analysis or by the Schild analysis and different levels of computer-gener ated "random noise" (affecting the shape and the position of theoretical cu rves) were also evaluated, in order to know the robustness of the new metho d. The two methods proved reliable and almost equivalent in robustness, when a pplied with different levels of "random noise". These results confirm the Schild analysis as the most accurate tool to stud y antagonists, since this analysis can furnish the highest number of inform ation and observations on the behaviour of an antagonist. Nevertheless, whe n limiting conditions prevent a classical Schild analysis and impose the us e of an inhibition curve, the new method probably represents the most prefe rable experimental approach. Indeed, it allows to calculate the antagonist potency, after the evaluation of a slope parameter giving an important info rmation about the possible nature of the antagonism.