4-Aminopyridine activates calcium influx through modulation of the pore-forming purinergic receptor in human peripheral blood mononuclear cells

We investigated whether 4-aminopyridine (4AP), a drug recently linked to calcium influx and apoptosis, also affected purinergic receptor channels that are known to play an important role in the activation of T lymphocytes. The application of 4AP induced a rise in [Ca2+](i) that was sensitive to nickel. This action was also observed in cells in which calcium reserves were emptied using thapsigargin (Tg). However, it was not present in the absence of extracellular Ca2+, despite full internal reserves. Adenosine trisphosphate (ATP), a partial agonist and a physiological activator of purinergic receptors, also stimulated Ca2+ entry independently of the calcium release from internal compartments. The effects of 4AP and ATP were not additive when studied on the same population of cells. KN-62 inhibited an increase in calcium entry induced by 4AP, while brilliant blue G (BBG) prevented it, supporting the hypothesis that purinergic P2X(7) receptors are involved in this action. Furthermore, 4AP allowed entry of ethidium bromide (314 Da) but not propidium iodide (415 Da) into the cell, also corroborating the involvement of P2X(7) pores. The presented results demonstrate, for the first time in human mononuclear cells isolated from healthy volunteers, that the P2X(7) channel pore is involved in the action of 4AP and intervenes in the sustained calcium entry induced in response to 4AP.