Abstract
Effects of placental extract (and perfusate) obtained from humans or rats were studied by recording the electrical and mechanical activities of pregnant rat and mouse myometria.
1) Membrane potential of circular muscle cells of pregnant rat myometrium ranged between 50-60 mV. Slow potentials amounting 20-30 mV dominated the circular muscle cells in contrast to spike potentials in the longitudinal cells.
2) The placental extract (as well as rat uterine perfusate) reduced the magnitude of spontaneous contractions of myometrial strips of mouse and rat. The inhibitory effect on contractile response became more pronounced when Mg ions were present in the bathing media. The extracts did not cause a significant change in the membrane potential or action potential discharge, but the compound action potential recorded extracellularly was depressed.
3) The extracts potentiated the mechanical response, when the myometrial tissue from pregnant animals was treated with β-blocking agent (10-7g/ml propranolol) or with bretylium (10-5g/ml).
4) It was argued that placenta and endometrium of the pregnant animal release bioactive substances including a certain inhibitory agent which may cause the depressant effect on the conducting mechanism of excitation, possibly through an indirect action in releasing the endogenous catecholamines.
