The anti-oestrogen tamoxifen is a calcium antagonist in perfused rat mesentery.

The anti-oestrogen tamoxifen in the range 10(-7)-10(-5) M induced concentration-related inhibition of potassium-stimulated vasospasm in the isolated perfused rat mesentery vascular bed. In contrast, responses to noradrenaline did not fall below control levels until the tamoxifen concentration exceeded 4 X 10(-6) M. Recovery of the potassium-stimulated responses was also concentration-related. The most likely explanation is that while tamoxifen can obstruct the entry of extracellular calcium, it is unable to prevent the intracellular release of the ion by noradrenaline.