Similarity between mu-opioid receptors in mouse vas deferens and guinea-pig ileum

In vivo pharmacological resultant analysis reveals noncompetitive interactions between opioid antagonists in the rat tail-withdrawal assay

BACKGROUND AND PURPOSE

Pharmacological resultant analysis is a technique that can detect secondary effects of competitive antagonists in vitro. The utility of pharmacological resultant analysis as a potential tool for the investigation of antagonist interactions in vivo was examined in the present study using two opioid antagonists, naltrexone and CTAP.

EXPERIMENTAL APPROACH

Using the experimental design of pharmacological resultant analysis, the well-characterized opioid antagonist naltrexone was examined in the presence of multiple doses of CTAP to block the antinociceptive effects of morphine in the rat warm-water (55(o)C), tail-withdrawal assay.

KEY RESULTS

Alone, all doses of naltrexone, CTAP, and CTOP examined blocked the antinociceptive effects of morphine. In the presence of fixed doses of 1 or 10 microg CTAP, increasing doses of naltrexone produced dose-dependent shifts to the right in the morphine dose-response curve. However, a lower dose of naltrexone in combination with 1 or 10 mug CTAP failed to alter the morphine dose-response curve. In the presence of a fixed dose of 0.1 mg kg(-1) naltrexone, CTAP doses produced irregular shifts to the right in the morphine dose-response curves.

CONCLUSIONS AND IMPLICATIONS

Resultant analysis was applied and an apparent pK(C) value for CTAP was found to be one log unit higher than the apparent pA(2) value for CTAP, evidence that CTAP may have secondary actions or that a signal transducer function may be altered by the combinations of these antagonists. Taken together, these data suggest pharmacological resultant analysis can reveal novel interactions between antagonists in vivo.

Application of an allosteric ternary complex model to the technique of pharmacological resultant analysis

A simple ternary complex model of drug-receptor interaction has been used to extend the procedure of pharmacological resultant analysis, enabling the quantitation of interactions between allosteric modulators and orthosteric antagonists. Equations derived in the theoretical treatment were used to analyse functional data for the interaction between the allosteric modulator gallamine and the orthosteric antagonist scopolamine, with oxotremorine as the agonist, at rat tracheal muscarinic acetylcholine receptors. Quantitative estimates of the affinity of gallamine for the allosteric site (pKz = 4.7) and the extent of negative, heterotropic co-operativity between gallamine and scopolamine (alpha' = 13.1) were obtained. Furthermore, an alternative direct, model-fitting approach, that does not rely on the determination of concentration ratios, was also developed, and yielded similar results. It is suggested that the approach presented in this paper is useful for quantifying interactions between orthosteric antagonists and allosteric modulators, particularly when the extent of co-operativity is low or the modulators possess multiple pharmacological properties, or both.

Analysis of competitive antagonism when this property occurs as part of a pharmacological resultant

In this paper, pharmacological resultant is defined as the net effect of a single compound resulting from the simultaneous expression of two or more specific actions. The principles of concentration-ratio analysis are extended to develop a method for detecting and quantifying competitive antagonism when this property is a component of a pharmacological resultant. The method is general to the extent that it allows analysis of competitive antagonism in combination with all types of post-receptor intervention. Essentially it depends on the altered expression of competition by a reference antagonist. It incorporates tests for validating its application and it is independent of agonist concentration-effect curve shape: in these respects the method is analogous to Schild plot-analysis of simple competition. The methodology for the practical application of the analysis is exemplified by studying the net effect of a combination of a phosphodiesterase inhibitor (isobutylmethylxanthine) and histamine H2-receptor antagonist (metiamide) on histamine-stimulated tachycardia in guinea-pig, isolated, right atrium. Cimetidine was used as the reference antagonist. The equation used in this analysis is similar in form to one recently described by Hughes & Mackay (1985) to elucidate the situation when competitive antagonism occurs in combination with functional interactions. The relation between their method and the present analysis is discussed.

Quantification of the actions of agonists that simultaneously act on a particular type of receptor and have separate functional interactant properties

Null equations have been derived which allow quantification of the agonist properties of a compound that is able to modify the state of a tissue simultaneously by interacting with a particular type of receptor and by other means. Parameters can be estimated which separately characterize the agonist properties of the compound and its functional interactant effect. The null equations have been tested in a model system by using a mixture of papaverine (5 microM) and hexyltrimethylammonium bromide (30 microM) to mimic an agonist which also has functional antagonist properties. The values obtained for the various parameters measured directly and indirectly are in good general agreement, confirming the validity of the model.

Meptazinol has a similar agonist action on opioid receptors in field-stimulated mouse vas deferens and guinea-pig ileum

The effects of the opioid receptor agonist RX783006 and of the opioid receptor partial agonist (+)-meptazinol have been examined on electrically induced twitch responses of the guinea-pig isolated ileum and of the mouse isolated vas deferens. Log10 concentration-tissue state curves were determined for (+)-meptazinol and RX783006, alone, in combination and in the presence of naloxone (30 nM). Analysis of these log10 concentration-tissue state curves using the null equations derived and tested in the preceding paper indicates that the opioid agonist action of (+)-meptazinol on mouse vas deferens is quantitatively similar to that on guinea-pig ileum. The results also suggest that (+)-meptazinol acts as a functional antagonist on the guinea-pig ileum as well as on the mouse vas deferens. The potency of (+)-meptazinol relative to RX783006 has been measured by an indirect method which should eliminate any functional antagonistic action of (+)-meptazinol. This method gives a relative potency of (+)-meptazinol in both tissues which is three to six times greater than that measured directly on guinea-pig ileum. This discrepancy may be due to experimental error but it may also indicate that direct measurements on guinea-pig ileum underestimate the agonist potency of this compound on opioid receptors.