Actions of morphine, pethidine and pentazocine on the oestrus and pregnant rat uterus in vitro.

The effects of morphine, pethidine and pentazocine have been studied on the rat uterus in vitro. Morphine caused a concentration-dependent decrease in the frequency of contraction of the oestrus uterus. In contrast, both pethidine and pentazocine enhanced the contraction rate. The pregnant uterus showed little response to morphine in the concentrations studied, whereas there was an augmented response to the stimulant actions of pethidine and pentazocine. Naloxone 1-100 ng ml-1 was unable to antagonize the uterine responses to the opioids, but higher concentrations of naloxone, when administered alone, slowed the frequency of the myogenic activity.