EXTRACELLULAR ATP ACTIVATES CALCIUM-ENTRY AND MOBILIZATION VIA P-2U-PURINOCEPTORS IN RAT LACTOTROPHS

Extracellular ATP has been previously shown to activate calcium signal ling in pituitary cell populations [1] but the particular cell types i nvolved have not been identified. We used video imaging of Fura-2 load ed into single rat pituitary cells and identified as lactotrophs to st udy the effects of extracellular ATP on [Ca2+](i). ATP does not permea bilize the cells as shown by exclusion of propidium iodide. ATP causes two types of calcium transients in lactotrophs. The most common respo nse is a rapid increase in [Ca2+](i) that decays slowly and is termina ted by washout of ATP. This type of response is also seen in calcium-f ree medium, demonstrating mobilization of calcium stores dependent upo n the presence of the agonist. ATP also stimulates calcium entry as de tected by Mn2+ of Fura-2. ATP in Mg2+-free medium and ATP gamma F are effective agonists suggesting ATP(4-) is the active form. The presence of P-2-purinoceptors is apparent because ATP, ADP and AMP increase [C a2+](i) in decreasing order of potency and adenosine has no effect. AT P-induced calcium transients are reduced by the P-2-purinoceptor antag onists suramin and quinidine. UTP is equipotent with ATP and defines t he receptor subtype as P-2U We conclude that ATP(4-) acts on rat lacto trophs via P-2U-purinoceptors to elevate [Ca2+](i) from intracellular and extracellular sources.