ATP-induced [Ca2+](i) changes and depolarization in GH3 cells

1 Extracellular ATP is a neurotransmitter and mediates a variety of respons es. In the endocrine system, there are data suggesting a physiological role for ATP in Ca2+ signalling and hormone secretion. However, the ATP recepto r subtype involved has not been clearly elucidated in GH3 cells, a rat ante rior pituitary cell line. 2 BzATP- and ATP-induced [Ca2+](i) responses had EC50 Values of 18 and 651 mu M, respectively. The maximal response to ATP was only 59+/-8% of that fo r BzATP. The BzATP-induced [Ca2+](i) increase was dependent upon the extrac ellular Ca2+ concentration. Preincubation with oxidized ATP (oATP) nearly a bolished the ATP- and BzATP-induced [Ca2+](i) increases. 3 Both BzATP and ATP induced depolarization in GH3 cells, with EC50 values of 31 mu M and 1 mM, respectively. The maximal depolarization to BzATP and ATP were 152+/-21 and 146+/-16% of that elicited by 30 mM KCI. 4 The rank order of agonist potency for [Ca2+](i) and depolarization respon ses was BzATP > > ATP > 2-MeSATP and purine derivatives such as ADP, AMP, a denosine were ineffective. Neither UTP nor alpha,beta-methylene ATP showed any effect. 5 In low-divalent conditions BzATP evoked non-desensitizing inward currents , which were reversed at similar to 0 mV. This nonselective cationic conduc tance was increased by repeated applications of BzATP and the cells became very permeable to NMDG. Longer applications (30 min) of BzATP stimulated et hidium bromide influx in low divalent conditions, suggesting increased perm eability to larger molecules. We also identified the existence of P2X(7) mR NA on GH3 cells by using reverse transcriptase (RT)-polymerase chain reacti on (PCR). 6 These results suggest that the GH3 cells have an endogenous P2X(7) recept or and purinergic stimulation may play a potential role in neuroendocrine m odulation on these cells.