Prostaglandins and the initiation of labor.

A total of 96, dated pregnant, New Zealand white rabbits were studied. In 58 animals the intrauterine pressure (IUP) of the unstimulated and PGF2alpha-stimulated myometrium was recorded, by the extraovular microballoon technique, before, during and after parturition. In the remaining 38 the concentrations of PGE and PGF and progesterone (P) were measured by radioimmunoassays (RIA). The samples were collected individually or sequentially during the perinatal period from uterine tissue and uterine or peripheral vein blood. At the critical time, at around parturition, when the myometrium is converted from a suppressed and refractory muscle into a spontaneously active and reactive organ (quantitated by recording the IUP), the uterine PGE and PGF levels decreased rather than increased (quantitated by RIA). Thus, this critical regulatory and functional change of the myometrium cannot be accounted for by an increase in the intrinsic uterine stimulant: PG, but only by a decrease in the suppressor: P. These findings, 46 years after the discovery of P, demand the further exploration of Corner's legacy.