P2RX2 and embryonal carcinoma: Ca2+-imaging-based pharmacological analysis revealed that P2X4Rs or P2X4-containing heteromultimers mediate ATP-induced calcium-responses of undifferentiated P19 embryonal carcinoma cells, while P2X2Rs and possibly heteromeric P2X2/6Rs are the major mediators of calcium-responses in neuronally differentiated cells after retinoic acid treatment (Resende et al., 2008).