Effects of naloxone and post-tetanic stimulation on isolated guinea-pig ileum followed by long exposure to morphine and bestatin.

This study was performed to compare the effects of naloxone (NLX) and post-tetanic stimulation on isolated guinea-pig ileum followed by prolonged exposure to morphine and bestatin. Morphine or bestatin alone did not induce any responses. In the presence of 1 microM morphine, the challenge with 1 microM naloxone caused quick contraction and post-tetanic contraction. A longer duration of these NLX-induced contraction and post-tetanic contractions was observed at the 6th stimulation compared to those after the 2nd stimulation. By contrast, the addition of bestatin, an aminopeptidase inhibitor, did not induce any NLX-induced contraction, although the same results for post-tetanic contraction as those of morphine were observed. These different effects of morphine and bestatin on NLX-induced contraction and post-tetanic contraction in the ileum may be due to different mechanisms of action in the opioid system. That in turn may suggest the possibility that bestatin has a physical dependence liability.