Positive modulation of intracellular Ca2+ levels by adenosine A2b receptors, prostacyclin, and prostaglandin E1 via a cholera toxin-sensitive mechanism in human erythroleukemia cells.

Human erythroleukemia (HEL) cells express megakaryocyte/platelet membrane markers and thus have been used as a model for studying platelet membrane receptors and their coupling to cell signaling pathways. Our previous studies, however, indicated that platelets and HEL cells possess different subtypes of adenosine A2 receptors. Furthermore, we now report that, whereas adenosine inhibits intracellular Ca2+ increases in platelets, it potentiates the rise in intracellular Ca2+ produced by thrombin, prostaglandin E1, thapsigargin, and the calcium ionophore A23187 in HEL cells. Stable adenosine analogs potentiated intracellular Ca2+ increases with a rank order of potencies of 5'-N-ethylcarboxamidoadenosine (NECA) > (R)-(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) >> CGS 21680, suggesting that this effect is mediated by A2b receptors. EC50 values for NECA and R-PIA were 0.8 and 42 microM, respectively. NECA (100 microM) potentiated by 2-3-fold the increase in intracellular Ca2+ produced by 0.3 unit/ml thrombin. This effect was mimicked by cholera toxin and was shared by other Gs-coupled receptors, such as those activated by the prostacyclin analog iloprost and prostaglandin E1, indicating the involvement of Gs proteins. Adenosine analogs also increased intracellular cAMP with the same rank order of potencies. The membrane-permeable analog 8-bromo-cAMP, however, had no effect on intracellular Ca2+ levels, indicating that the potentiation of intracellular Ca2+ increases and the activation of adenylate cyclase are parallel but independent events. The increase in intracellular Ca2+ produced by adenosine is due not to an increase in phosphoinositide hydrolysis but, rather, to an increase in calcium influx, and it is lost if cells are studied in the absence of extracellular Ca2+. We conclude, therefore, that adenosine A2b receptors in HEL cells are coupled to Gs proteins and their activation leads to stimulation of adenylate cyclase and, independently, to potentiation of the rise in intracellular Ca2+. We speculate that A2b receptors in HEL cells activate a calcium channel through a cholera toxin-sensitive mechanism that requires an initial increase in intracellular Ca2+.