[Basic studies of the effects of various kinds of tocolytics on uterine contraction in pregnant rats]

The effects of various substances involved in the inhibition of uterine contraction on the uterine muscle of pregnant rats were studied in vitro in terms of uterine contraction and alterations in cyclic nucleotides. 1) The inhibitory effect of 4-aminoantipyrine (4AA) on uterine contraction showed a pattern of decline, being less potent that tranylcypromine. The effect of nifedipine was the most potent, followed by ritodrine, tranylcypromine, RU 486 and 4AA, in that order. The inhibitory effects were diminished by administration of ONO-802.PGF2 alpha. 2) Cyclic nucleotide levels changed according to the potency of each agent in the process of inhibition. Although the level of c-AMP tended to increase and that of c-GMP tended to decrease, there were no changes in their levels when nifedipine was administered. 3) Blood and uterine muscle levels of c-AMP were increased in pregnant rats that had been given ritodrine prior to hysterectomy, and there was no inhibitory effect of ritodrine in comparison with pregnant rats that had not been given the agent. The pregnant rats that had been given ritodrine prior to hysterectomy also showed no significant change in the level of c-AMP in the bath-medium.