Interaction between oxytocin and 'sidaverin' on the gravid and non-gravid rat uterus.

Sidaverin, a crystalline compound extracted from a polar fraction of Sida veronicaefolia (Lam), elicited oxytocin-like contractions in the non-gravid rat isolated uterus preparation with a concentration-response relationship. Equipotent concentrations of oxytocin and sidaverin, using matched responses, were approximately 0.16 U and 0.4 micrograms ml-1, respectively. Sidaverin-induced contractile response was atropine reversible. The concentration-response curves for sidaverin and oxytocin were parallel, and both responses were inhibited by the specific oxytocin antagonist, Atosiban, indicating possible involvement of oxytocin receptors in the action of sidaverin. There were potentiation of action of one drug to that of the other, irrespective of the order of administration and even after washing off the first before introducing the second drug. In the gravid uterus, sidaverin produced contractions in preparations from day 1 to day 6 or 7, caused relaxation in days 7-11, and elicited contractions in day 11 through term, the sensitivity of the preparations increasing exponentially toward term with strong sustained contractions. With the exception of days 7-11, when sidaverin antagonized oxytocin action, it potentiated action of oxytocin on the gravid uterus.