1. 1. The addition of ATP to cultured bovine aortic endothelial cells induced the increase in intracellular free calcium concentration ([Ca2+]i) and thereby activated the sodium/proton exchanger and the prostacyclin production in a similar dose-dependent manner, as observed by the addition of ATP. 2. 2. Other nucleoside triphosphates also activated the cells and the potency orders of the nucleotides were ATP > UTP > ITP > CTP > GTP for all the responses. 3. 3. Pretreatment of the cells with UTP desensitized the response to ATP and the pretreatment of ATP desensitized the response to UTP. 4. 4. The responses to ATP and UTP were inhibited by neither pertussis nor cholera toxin. 5. 5. The receptor for UTP, however, may be a distinct pyrimidinoceptor different from the purinoceptor of the cells for ATP, because the 50% effective concentration of UDP was much larger than that of UTP, while ATP and ADP were essentially equipotent ligands to the endothelial cells.
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