Pharmacological investigation of ritodrine hydrochloride, a beta 2-adrenoceptor stimulant

Ritodrine hydrochloride (ritodrine) has been effectively prescribed for the prevention of premature labor. The present study was carried out to investigate the mode of action of ritodrine on the uterus and heart in comparison with those of isoxsuprine and isoproterenol. 1) Ritodrine (10(-8)-10(-6) M) suppressed the spontaneous motility of pregnant rat uterus and showed positive chronotropic action at the doses of 10(-6)-10(-4) M in guinea-pig atria. 2) In the Ca2+-free, K+-rich Tyrode solution, ritodrine suppressed the Ca2+ induced contracture of pregnant rat uterus, while it potentiated the carbachol induced contraction. 3) Ritodrine increased the amount of cyclic AMP in the uterus but not in heart. This action of ritodrine was suppressed by pretreatment with propranolol (10(-6) M). 4) These results suggest that ritodrine causes actions through activation of cyclic AMP production, as in the case of isoproterenol, and it acts more selectively on beta 2-adrenoceptors than on beta 1-adrenoceptors.