The inhibitory effects of oxytocin on distal colonic contractile activity in rabbits are enhanced by ovarian steroids.
OBJECTIVE
To study the effects of oxytocin on isolated rabbit distal colon and the regulation of ovarian steroids by its action.


METHODS
Muscle strips parallel to either the circular or the longitudinal fibres were excised and suspended in tissue chambers containing 5 mL Krebs solution (37 degrees C) and bubbled continuously with 95% O(2) and 5% CO(2). The effects of oxytocin on isometric spontaneous contractile responses were recorded. The effects of atosiban, tetrodotoxin, Mg(2+), progesterone and oestradiol on the oxytocin-induced response were also examined.


RESULTS
Oxytocin (1, 10 and 100 nmol L(-1)) dose dependently decreased the area under the contraction curve of distal colonic smooth muscle strips. The oxytocin receptor antagonist atosiban blocked the oxytocin (10 nmol L(-1))-caused responses in a dose-dependent manner. Tetrodotoxin (10 micromol L(-1)) had no effect on the oxytocin-induced response. Mg(2+)-free Krebs solution attenuated the oxytocin-induced response, but oestradiol (0.1 micromol L(-1)) or progesterone (0.1 micromol L(-1)) increased the oxytocin-induced response.


CONCLUSIONS
These results suggest that oxytocin inhibits the contractile motility of the distal colon, which is regulated by Mg(2+) and ovarian steroids.
Institute of Physiology, School of Medicine, Shandong University, Jinan, Shandong, China. xiedping@sdu.edu.cn
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D000818:Animals; D003106:Colon / Q000187:drug effects* / Q000502:physiology; D015194:Diffusion Chambers, Culture; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004958:Estradiol / Q000494:pharmacology; D005260:Female; D005769:Gastrointestinal Motility / Q000187:drug effects; D006727:Hormone Antagonists / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects* / Q000502:physiology; D010121:Oxytocin / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D011374:Progesterone / Q000494:pharmacology; D011817:Rabbits; D018045:Receptors, Oxytocin / Q000037:antagonists & inhibitors; D013779:Tetrodotoxin / Q000494:pharmacology; D014668:Vasotocin / Q000031:analogs & derivatives / Q000494:pharmacology
D006727:Hormone Antagonists; D018045:Receptors, Oxytocin; C047046:atosiban; D013779:Tetrodotoxin; D011374:Progesterone; D004958:Estradiol; D010121:Oxytocin; D008274:Magnesium; D014668:Vasotocin