[Effects of short- and long-term administration of ritodrine on spontaneous contractions of longitudinal muscle strips dissected from the pregnant rat uterus].

: We investigated the effects of Ritodrine (10(-10)-10(-7) g/ml) on spontaneous contractions of rat myometrium at gestational days 14, 16, 18 and 21. 1. More than 10(-8)g/ml Ritodrine obviously suppressed the spontaneous contractions more than smaller doses. The inhibitory effects of smaller doses on days 14 and 16 were different from those on days 18 and 21, the former being more effective. 2. Smaller doses of Ritodrine (10(-10)-10(-8)g/ml) caused transient excitation, which was inhibited by pretreatment with Phentolamine. 3. Rebound excitation was observed in most of the specimens. 4. Spontaneous contractions reappeared during long-term administration of Ritodrine and the patterns could be classified into three types. These results suggested that the effects of Ritodrine might vary according to the number of pregnant days and that the critical Ritodrine dose for a suppressive effect is 10(-8)g/ml, and there are a transient alpha-excitatory effect, rebound excitation and three types of suppression as a result of long-term administration.