Ouabain, a sodiumpump (Na + /K + -ATPase) inhibitor, has been shown to act as a hormone and is possibly involved in the pathogenesis of hypertension. The mechanismby which ouabain may act was investigated using primary cultures of human umbilical artery endothelial cells (HUAECs), which are known to express and release the vasoconstrictive hor-mone endothelin (ET-1). Fiveminutes after application, low concentrations of ouabain induced Ca 2+ oscillations and stimulated ET-1 release from endothelial cells into the medium. To investigate whether the observed effects were due to inhibition of the sodiumpump, the effects of ouabain on the uptake of 86 Rb + by HUAECs were examined
