Quantitative analysis of the agonist and antagonist actions of some ATP analogues at P2X‐purinoceptors in the rabbit ear artery

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Abstract
The agonist and antagonist effects of a series of β,γ‐methylene dihalo‐ and 2‐methylthio‐substituted analogues of ATP at P2x‐purinoceptors have been analysed on the rabbit isolated ear artery preparation. Cumulative and sequential dosing experimental protocols were employed in the construction of agonist concentration‐effect curves in order to address the possible influence of acute receptor desensitization on subsequent analyses. Using the cumulative curve design the following results were obtained: d‐AMP‐PCBr2P, 2‐methylthio‐d‐AMP‐PCCl2P, l‐AMP‐PCF2P, l‐AMP‐PCCl2P and LAMP‐PCBr2P each behaved as partial agonists. d‐AMP‐CPP was used as a reference full agonist and these analogues were analysed by the comparative method of Barlow et al. (1967), to provide estimates of affinity and efficacy. 2‐Methylthio‐l‐AMP‐PCBr2P was virtually silent as an agonist and was analysed as a competitive antagonist by Schild analysis. Two agonists, l‐AMP‐PCCl2P and l‐AMP‐PCBr2P, were analysed by the sequential curve design, and the antagonist effects of one of the agonists, l‐AMP‐PCBr2P were also analysed using this protocol. The resulting estimates of affinity and efficacy, while similar to those obtained with the cumulative design, indicated that acute desensitization may affect curve definition and estimation of these quantities. The following structure‐activity trends emerged: d‐analogues tended to have higher efficacy but lower affinity than l‐analogues; efficacy varied markedly and inversely with the atomic weight of the halogen while affinity was only minimally affected; 2‐methylthio‐ substitution also reduced efficacy with minimal effect on affinity. The results of this analysis are discussed in terms of the utility of affinity and efficacy information in the classification of purinoceptors and the design of chemical probes for them.

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