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

Extracellular ATP has been previously shown to activate calcium signalling in pituitary cell populations [1] but the particular cell types involved have not been identified. We used video imaging of Fura-2 loaded into single rat pituitary cells and identified as lactotrophs to study the effects of extracellular ATP on [Ca2+](i). ATP does not permeabilize the cells as shown by exclusion of propidium iodide. ATP causes two types of calcium transients in lactotrophs. The most common response is a rapid increase in [Ca2+](i) that decays slowly and is terminated by washout of ATP. This type of response is also seen in calcium-free medium, demonstrating mobilization of calcium stores dependent upon the presence of the agonist. ATP also stimulates calcium entry as detected 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 [Ca2+](i) in decreasing order of potency and adenosine has no effect. ATP-induced calcium transients are reduced by the P-2-purinoceptor antagonists suramin and quinidine. UTP is equipotent with ATP and defines the receptor subtype as P-2U We conclude that ATP(4-) acts on rat lactotrophs via P-2U-purinoceptors to elevate [Ca2+](i) from intracellular and extracellular sources.