Calcium antagonism by the antioestrogen tamoxifen.

Tamoxifen, an oestrogen receptor antagonist and an effective treatment for breast carcinoma, has recently been shown to possess spasmolytic activity in smooth muscle. Tamoxifen in vitro inhibited the contraction of smooth muscle from rat myometrium and aorta produced by exogenous calcium. At the same concentration tamoxifen did not affect the uptake of calcium into the muscle. The importance of calcium in cell proliferation suggests that some of the unexplained antitumor activity of the oestrogen antagonists may be accounted for by intracellular calcium antagonism.