ATP-induced [Ca(2+)](i) changes and depolarization in GH3 cells.

Extracellular ATP is a neurotransmitter and mediates a variety of responses. In the endocrine system, there are data suggesting a physiological role for ATP in Ca(2+) signalling and hormone secretion. However, the ATP receptor subtype involved has not been clearly elucidated in GH3 cells, a rat anterior pituitary cell line. BzATP- and ATP-induced Ca(2+) responses had EC(50) values of 18 and 651 microM, respectively. The maximal response to ATP was only 59+/-8% of that for BzATP. The BzATP-induced Ca(2+) increase was dependent upon the extracellular Ca(2+) concentration. Preincubation with oxidized ATP (oATP) nearly abolished the ATP- and BzATP-induced Ca(2+) increases. Both BzATP and ATP induced depolarization in GH3 cells, with EC(50) values of 31 microM and 1 mM, respectively. The maximal depolarization to BzATP and ATP were 152+/-21 and 146+/-16% of that elicited by 30 mM KCl. The rank order of agonist potency for Ca(2+) and depolarization responses was BzATP > > ATP >2-MeSATP and purine derivatives such as ADP, AMP, adenosine were ineffective. Neither UTP nor alpha, beta-methylene ATP showed any effect. In low-divalent conditions BzATP evoked non-desensitizing inward currents, which were reversed at approximately 0 mV. This nonselective cationic conductance was increased by repeated applications of BzATP and the cells became very permeable to NMDG. Longer applications (30 min) of BzATP stimulated ethidium bromide influx in low divalent conditions, suggesting increased permeability to larger molecules. We also identified the existence of P2X(7) mRNA on GH3 cells by using reverse transcriptase (RT)-polymerase chain reaction (PCR). These results suggest that the GH3 cells have an endogenous P2X(7) receptor and purinergic stimulation may play a potential role in neuroendocrine modulation on these cells.