Effects of ritodrine, a beta 2-adrenoceptor agonist, on smooth muscle cells of the myometrium of pregnant rats.

1 Effects of ritodrine on the electrical and mechanical activities of the myometrium of pregnant rats were investigated in relation to cyclic adenosine 3',5'-monophosphate (cyclic AMP) production and the actions of isoprenaline. 2 In the longitudinal muscle cells of the 22nd day of gestation, ritodrine (greater than 10(-8) M) hyperpolarized the membrane, reduced the membrane resistance and suppressed the amplitude of contraction. Increased concentrations of ritodrine (greater than 10(-7) M) suppressed the generation of spikes and increased the amount of cyclic AMP produced. These actions of ritodrine were much the same as those of isoprenaline but the dose-response curves of all parameters were shifted to the right. 3 The effects of ritodrine on the circular muscle cells at the 22nd day of gestation were similar to those observed in the longitudinal muscle cells. 4 In the circular muscle cells at the 17-19th days of gestation, ritodrine produced neither a hyperpolarization nor an increase in cyclic AMP production (up to 10(-5) M) but did reduce the plateau potential and did relax the tissue (greater than 10(-7) M). 5 In the longitudinal muscle cells, during the 17-19th days of gestation, the responses of the tissues were similar to those observed in circular muscles on the 22nd day of gestation. These actions of ritodrine were suppressed by propranolol (10(-6) M). 6 These results indicate that ritodrine is a beta-adrenoceptor agonist and its mechanism of action similar to that of isoprenaline. However, the potency of ritodrine was approximately 100 times less than that of isoprenaline.