Abstract
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.
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