Effects of nonsteroidal antiestrogens in the in vitro rat uterus.

We have studied the effect of nonsteroidal antiestrogens on rat uterine contractions induced by oxytocin (8 nmol/l), methacholine (10 mumol/l), prostaglandin F2 alpha (1 mumol/l), KCl (60 mmol/l) and CaCl2 (6 mmol/l). In a concentration-dependent way, the antiestrogens tamoxifen, clomiphene, nafoxidine and ethamoxytriphetol inhibited the amplitude and frequency of the oxytocin-induced contractions and the contraction produced by CaCl2. At a concentration of 30 mumol/l the four drugs inhibited the contractions induced by methacholine and prostaglandin F2 alpha. They also relaxed the tonic contraction to KCl in a concentration-dependent way. This action was partially counteracted by CaCl2 (0.1-10 mmol/l). Bay k 8644 (0.3 nmol/l to 3 mumol/l) only partially reversed the inhibition by ethamoxytriphetol (0.1 mmol/l) of CaCl2 (6 mmol/l)-induced contractions. The steroidal antiestrogen, ICI 164,384, which lacks agonist activity, had an inhibitory effect (44 +/- 4%, n = 7) on KCl-induced contractions only at a concentration of 0.1 mmol/l. However, the quaternary analogue of tamoxifen (tamoxifen ethyl bromide) produced 86 +/- 3% relaxation of the KCl-induced contracture (IC50 1.52 +/- 0.1 mumol/l, n = 10) and this effect was counteracted by addition of CaCl2. Taken together the results indicate that the inhibitory effects of nonsteroidal antiestrogens on rat uterine contractions could be mediated by an action to block Ca2+ entry through an agonist action on extracellular estrogen receptors.