Actions of ADP, but not ATP, on cytosolic free Ca2+ in single rat hepatocytes mimicked by 2-methylthioATP

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Abstract
1 Aequorin-injected, single rat hepatocytes generate series of repetitive transients in cytosolic free calcium concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway, including ADP and ATP. We have previously described differences in the [Ca2+]i responses of aequorin-injected hepatocytes to ADP and ATP. 2 The effects of the phosphorothioate analogue of ATP, 2-methylthioATP (2-meSATP), have been examined on single rat hepatocytes. This analogue is believed to be the most potent agonist at the P2Y1 subclass of purinoceptor. 3 The [Ca2+]i transients induced by 2-meSATP were indistinguishable from those induced by ADP, and in contrast to those induced by ATP. 4 At high concentrations, 2-meSATP and ADP both induced transients at high frequency. In contrast, hepatocytes responded to high concentrations of ATP with an initial rapid rise in [Ca2+]i, followed by a slowly decaying fall. 5 The modulatory effects of elevated intracellular cyclic AMP concentration were the same on both 2-meSATP- and ADP-induced [Ca2+]i transients; the peak height and frequency of transients were enhanced. ATP-induced transients, however, underwent either an increase in duration or conversion into a sustained rise in [Ca2+]i. 6 ATP-induced transients were specifically potentiated by the co-addition of α,β-methyleneATP, whereas 2-meSATP- and ADP-induced transients were unaffected by this treatment. 7 We conclude that 2-meSATP acts at the same receptor as ADP on rat hepatocytes, and that this is distinct from the receptor(s) mediating the effects of ATP.