Progesterone enhances the tocolytic effect of ritodrine in isolated pregnant human myometrium.
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.
Chanrachakul
Boonsri
B
Academic Division of Obstetrics and Gynaecology, Derby City General Hospital, University of Nottingham, Derby, DE22 3DT, United Kingdom.
Broughton Pipkin
Fiona
F
Warren
Averil Y
AY
Arulkumaran
Sabaratnam
S
Khan
Raheela N
RN
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*
D000318:Adrenergic beta-Agonists; D018343:Receptors, Adrenergic, beta-2; D015149:Tocolytic Agents; D011374:Progesterone; D012312:Ritodrine