Postpartum changes in the mechanical responses of the circular muscle of rat uterus

The circular muscle strips were isolated from rat uterus 10-60 h after parturition, and the electrical and mechanical responses were investigated. The muscle strips exhibited a variety of contractile activity ranging from frequent spontaneous contractions generated on an elevated muscle tone to a sustained contracture, when incubated with Mg-free Krebs solution. The muscle tone was lowered, and phasic contractions were abolished when the external Ca was removed. Muscle tone was also lowered by addition of 1.2 mM Mg, 1 mM spermidine, or 1 mM tetracaine. The membrane potential was about -25 mV in a muscle which exhibited a contracture, and the membrane was hyperpolarized by 20-25 mV by the (1.5-4.5 x 10(-5) M) caused a gradual decrease in muscle tone, and the that Ca-influx was increased, for which prostaglandin(s) was membrane was hyperpolarized. In view of the above results, it is hypothesized that Ca-influx was increased, for which prostaglandin(s) was probably responsible, in the circular muscle of postpartum rat uterus, thereby elevating the muscle tone in Mg-free solution.