Effects of ritodrine hydrochloride, a beta 2-adrenoceptor stimulant, on uterine motilities in late pregnancy.

Ritodrine hydrochloride (ritodrine) is a beta 2-adrenoceptor stimulant which has been effectively prescribed for the prevention of premature labor. The present studies were carried out to investigate the effects of ritodrine on uterine motility in rats and rabbits during gestation, as compared with those of isoproterenol and isoxsuprine. The results were as follows: 1) Spontaneous movements and evoked contractile responses of isolated rat uterus (19-20th days of gestation) were suppressed by 10(-9) - 10(-6) M ritodrine. The potency of ritodrine was approximately 10 times more than that of isoxsuprine and 100 - 1,000 times less than that of isoproterenol. 2) When these drugs were administered to pregnant rats or rabbits intravenously, the tocolytic potency was in the following order: isoproterenol greater than ritodrine greater than isoxsuprine. 3) Ritodrine induced hypotension and tachycardia, but these effects were less than those of isoproterenol and isoxsuprine. 4) The effects of isoproterenol and ritodrine were almost prevented by pretreatment with propranolol, but those of isoxsuprine were only partially or not affected. These results suggest that ritodrine is effective in preventing the uterine contractions in rats and rabbits and that it has less effect on the circulatory system than isoproterenol and isoxsuprine. It is also concluded that ritodrine produces these effects through activation of beta-adrenoceptors.