The biological meaning of prostaglandin-levels

Two groups of term pregnant rats, one pretreated with progesterone (P) and another with vehicle only, were induced to deliver with PGF2α. Through an implanted intrauterine (extraovular) catheter 5 μg PGF2α was injected into each animal every 15 minutes until delivery occurred. If induction failed the fetuses were removed by hysterotomy. All vehicle controls started delivery within 38.3±11.7 minutes. In contrast, none of the P-treated rats responded to PGF2α. In all animals uterine vein blood and uterine tissue were collected during labor.Measurements of the P and PGF in uterine vein plasma and uterine tissue showed that the P-treated rats had significantly higher P-levels than the controls (P<0.001). In contrast, the PGF-levels were similar in the two groups, but significantly elevated (P<0.001) beyond spontaneous labor values. Evidently, the massive elevation of PGF-levels only induces labor when the myometrial action of P is critically reduced and is ineffective when P-action is sustained. Thus, PGF-concentrations cannot be presumed to predict PGF-effect, because the latter is controlled by P.