Specific features of smooth muscle cells recorded from the placental region of the myometrium of pregnant rats.

The muscle membrane and contractile properties of placental and non-placental regions of pregnant and postpartum myometria of the rat were investigated.1. The membrane potential of the myometrium varied during the progress of gestation, but the membrane potential in the placental region was consistently higher than that in the non-placental region up to the last stage of gestation. The maximum difference in the membrane potential recorded from both regions was observed at 17-21 days gestation.2. The conduction velocity of the excitation measured from muscle cells of the non-placental region was consistently larger than that of the placental region at any stage of the gestation (7-22 days). At 7 days gestation, the conduction velocity in the placental region could not be measured due to widely scattered values of the latencies for the spike generation. During the progress of gestation, the conduction velocity increased, and the value was consistently larger (about 10 times) in the nonplacental region than that in the placental region at any stage of gestation up to the last stage.3. The length constant of the tissue measured from the non-placental region was larger (2.3 mm) than that in the placental region (1.1 mm) at 14 days gestation. The propagation of the excitation seems to be blocked from the cells of the non-placental region to the placental region, but not vice versa.4. The shape of the spike in the non-placental region changed markedly during the progress of gestation. The spike could be recorded from the cells of the placental region but not consistently. However, the electrical activity of muscle cells recorded at the full term was much the same in both regions.5. Various concentrations of K or Na modified the membrane potential. The differences in the membrane response to the above ionic environments appeared more significantly at the different stages of gestation, rather than topical differences.6. Sensitivity of the muscle cell membrane to oxytocin in the placental region was higher than that in the non-placental region at any stage of gestation, while that to prostaglandin F(2alpha) was much the same in the cells recorded from both regions.7. At 8 days gestation, the circular muscle layer is lost from the area of implantation and only the longitudinal muscle layer remains. The width of the placenta and the thickness of the muscle layer increased during the progress of gestation.8. From the above results, the role of smooth muscle cells in the placental region is discussed in relation to hormonal controls in the myometrium, i.e. the activity of the muscle cell membrane in the placental region was markedly suppressed up to the last stage of gestation, however, there was no apparent direct relation between the inhibitory responses and the actions of progesterone.