Variations in the spontaneous activity of the uterine muscle in vitro and in situ and its response to oxytocin.

A study is made of the contractile and bioelectrical activities of the uterine muscle of guinea-pig in vitro and of rabbit in vitro and in situ, as well as their Oxytocin-induced changes upon different balance of the ovarial hormones. In rabbits in situ the uterine muscle contractions are longer and are recorded with a slower rhythm than in vitro. Both in situ and in vitro the sensitivity of the myometrium to Oxytocin is lower upon domination of progesteron and higher upon domination of oestrogen. In guinea pigs the myometrium is least sensitive to Oxytocin in the juvenile animals as compared with the sexually mature ones. The sensitivity of the guinea-pig myometrium to oxytocin in the various phases of the oestral cycle is weakest during dioestrus and strongest during oestrus.