RESUMO
Stimulation of the P2X7 receptor by ATP induces cell membrane depolarization, increase in intracellular Ca2+ concentration, and, in most cases, permeabilization of the cell membrane to molecules up to 900 Da. After the activation of P2X7, at least two phenomena occur: the opening of low-conductance (8 pS) cationic channels and pore formation. At least two conflicting hypotheses have been postulated to reconcile these findings: 1) the P2X7 pore is formed as a result of gradual permeability increase (dilation) of cationic channels, and 2) the P2X7 pore represents a distinct channel, possibly activated by a second messenger and not directly by extracellular nucleotides. In this study, we investigated whether second messengers are necessary to open the pore associated with the P2X7 receptor in cells that expressed the pore activity by using the patch-clamp technique in whole cell and cell-attached configurations in conjunction with fluorescent imaging. In peritoneal macrophages and 2BH4 cells, we detected permeabilization and single-channel currents in the cell-attached configuration when ATP was applied outside the membrane patch in a condition in which oxidized ATP and Lucifer yellow were maintained within the pipette. Our data support Ca2+ as a second messenger associated with pore formation because the permeabilization depended on the presence of intracellular Ca2+ and was blocked by BAPTA-AM. In addition, MAPK inhibitors (SB-203580 and PD-98059) blocked the permeabilization and single-channel currents in these cells. Together our data indicate that the P2X7 pore depends on second messengers such as Ca2+ and MAP kinases.
Assuntos
Cálcio/metabolismo , Membrana Celular/fisiologia , Receptores Purinérgicos P2/metabolismo , Sistemas do Segundo Mensageiro/fisiologia , Trifosfato de Adenosina/metabolismo , Animais , Linhagem Celular , Inibidores Enzimáticos/farmacologia , Corantes Fluorescentes , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Camundongos , Proteínas Quinases Ativadas por Mitógeno/metabolismo , Técnicas de Patch-Clamp , Receptores Purinérgicos P2/efeitos dos fármacos , Receptores Purinérgicos P2X7 , Sistemas do Segundo Mensageiro/efeitos dos fármacosRESUMO
The tremorgenic alkaloids, paxilline and paspalitrem-C (0.1-10 microM), increased the spontaneous contractility of guinea-pig and rat urinary bladder, and rat duodenum, and induced tension in guinea-pig trachea. These effects are ascribed to blockade of high-conductance, Ca(2+)-activated K+ (BKCa) channels. Paxilline potentiated the charybdotoxin-induced stimulation of guinea-pig detrusor muscle; this is consistent with the alkaloid's ability to allosterically enhance the binding of charybdotoxin to smooth muscle membranes (Knaus et al., 1994). Paspalitrem-C and paxilline did not affect the myogenic activity of isolated portal vein from guinea-pig, which is insensitive to charybdotoxin, or of that from rat which is stimulated by charybdotoxin. Paxilline and paspalitrem-C also differed from charybdotoxin in that the alkaloids did not consistently elicit tension in guinea-pig aortic rings. These discrepancies are attributed to differences in relative potency, sites and/or mechanisms of action of the indole alkaloids vs. peptidyl blockers of the BKCa channel.