ATP-induced P2X7-associated uptake of large molecules involves distinct mechanisms for cations and anions in macrophages

Abstract

Macrophages express the P2X7 receptor and other nucleotide (P2) receptors, and display the phenomenon of extracellular ATP (ATPe)-induced P2X7-dependent membrane permeabilization, which occurs through a poorly understood mechanism. We used patch-clamp recordings, cytoplasmic Ca2+ measurements and fluorescent dye uptake assays to compare P2X7-associated transport phenomena of macrophages and HEK-293 cells transfected with P2X7 receptors (HEK-P2X7 cells). Both cell types showed inward currents, increase of free cytoplasmic Ca2+ concentration and the uptake of cationic dyes upon exposure to ATPe, as previously described. However, in contrast to the macrophages, HEK-P2X7 cells did not take up anionic dyes and did not display the 440 pS channels (Z pores) under cell-attached patch-clamping conditions. In addition, the transport mechanism of anionic dyes displayed by macrophages was also able to support dye efflux and, once activated at 37°C, it remained active at 4°C, whereas uptake of cationic dyes was temperature-dependent and unidirectional. Our results indicate that the mechanism of ATPe-induced dye uptake, usually called a `permeabilization phenomenon' and associated with a `permeabilization pore' can be ascribed to at least two distinct mechanisms in macrophages: a diffusional pathway, possibly associated with the 440 pS Z pores, and a cation uptake mechanism that is not diffusional and should be ascribed to an, as yet, unidentified transport mechanism.