The isolated central ear artery of the rabbit contracts in response to adenosine 5'-triphosphate (ATP) and analogues, effects proposed to be mediated by stimulation of P2x-receptors. We have extended the characterization of the purinoceptor in this tissue by examining the effects of a series of receptor agonists. The study was designed in such a way as to avoid factors which normally limit attempts to classify receptors on the basis of agonist potency orders. 2. D-alpha, beta-methylene ATP (D-alpha, beta-meATP), D-beta, gamma-methylene ATP (D-beta, gamma-meATP), L-beta, gamma-methylene ATP (L-beta, gamma-meATP), 2-methylthio-D-ATP (2-MeSATP) and ATP produced concentration-related contractions of the ear artery with similar maximum responses, suggesting that they were full agonists. Selective desensitization of P2x-receptors abolished or greatly reduced responses to D-alpha, beta-meATP, L-beta, gamma-meATP, D-beta, gamma-meATP. Responses to ATP were inhibited by by desensitization but a significant resistant component was still apparent. 3. D-alpha, beta-meATP was the most potent agonist tested (pA50 6.47 +/- 0.04) being 2138 times more potent than ATP and approximately 9 times more potent than L-beta, gamma-meATP. The agonist potency order was: D-alpha, beta-meATP greater than L-beta, gamma-meATP greater than D-beta, gamma-meATP greater than or equal to 2-MeSATP greater than ATP. This is generally consistent with the order proposed for P2x-receptors. The relative potencies of P2x-agonists in the rabbit ear artery show both similarities to and differences from data obtained in other smooth muscle preparations.
