title: Progesterone enhances the tocolytic effect of ritodrine in isolated pregnant human myometrium.
abstract: OBJECTIVE
  To evaluate the effect of natural progesterone on the relaxant effect of ritodrine on pregnant human oxytocin-induced myometrial contractility.
  
  
  METHODS
  Isometric tension recordings were performed under physiologic conditions on isolated myometrial strips taken from low-risk term pregnant women undergoing elective cesarean section. Cumulative effects of natural progesterone (10 (-11) to 10 (-5) mol/L) on oxytocin-induced myometrial contractility were evaluated. Contractile activity following ritodrine exposure was also investigated in myometrium pretreated with natural progesterone.
  
  
  RESULTS
  Natural progesterone alone exerted a concentration-dependent relaxant effect on myometrial contractions. The concentration-response curve for ritodrine from natural progesterone pretreated myometrium was shifted to the left with a significant reduction ( P < .01) of 50% of the maximal response, contraction amplitude ( P < .05), and frequency ( P < .05). However, there was no significant difference in the mean maximal inhibition achieved ( P = .95).
  
  
  CONCLUSIONS
  Natural progesterone increased the relaxant effect of ritodrine by reducing 50% of the maximal response, amplitude, and frequency of myometrial contraction, most likely through nongenomic actions. These results suggest that natural progesterone may be beneficial for preventing preterm birth in a low-risk population.
authors.affiliation: Academic Division of Obstetrics and Gynaecology, Derby City General Hospital, University of Nottingham, Derby, DE22 3DT, United Kingdom.
authors.forename: Boonsri
authors.initials: B
authors.lastname: Chanrachakul
authors.forename: Fiona
authors.initials: F
authors.lastname: Broughton Pipkin
authors.forename: Averil Y
authors.initials: AY
authors.lastname: Warren
authors.forename: Sabaratnam
authors.initials: S
authors.lastname: Arulkumaran
authors.forename: Raheela N
authors.initials: RN
authors.lastname: Khan
mesh_terms: D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D009126:Muscle Relaxation / Q000187:drug effects; D009215:Myometrium / Q000187:drug effects*; D011247:Pregnancy; D011374:Progesterone / Q000494:pharmacology*; D018343:Receptors, Adrenergic, beta-2 / Q000502:physiology; D012312:Ritodrine / Q000494:pharmacology*; D015149:Tocolytic Agents / Q000494:pharmacology*
chemical_list: D000318:Adrenergic beta-Agonists; D018343:Receptors, Adrenergic, beta-2; D015149:Tocolytic Agents; D011374:Progesterone; D012312:Ritodrine