EFFECTS OF DIVALENT-CATIONS, PROTONS AND CALMIDAZOLIUM AT THE RAT P2X(7) RECEPTOR

The P2X(7) receptor is a uniquely bifunctional molecule through which ATP can open a small cationic channel typical of ionotropic receptors and also induce a large pore permeable to high molecular weight molecu les (>600 Dal. Activation of this large pore can lead to cell lysis wi thin 1-2 min. We asked whether pharmacological differences existed bet ween the cationic channel and the cell permeabilizing pore by measurin g whole-cell currents and uptake of a propidium dye (YO-PRO; M-W 629) in HEK293 cells stably expressing the rat P2X(7) receptor, and compari ng the actions of divalent cations and protons in these two assays. Cu rrents in response to 2'-3'-(0)-(4-benzoyl benzoyl) ATP (BzATP, 30 mu M) were inhibited by extracellular calcium, magnesium, zinc, copper an d protons with half-maximal inhibitory concentrations (IC50) of 2.9 mM , 0.5 mM, 11 mu M, 0.5 mu M and 0.4 mu M, respectively. The inhibition was voltage independent in each case. YO-PRO uptake induced by BzATP was also inhibited with similar IC50 values. The rank order of potency of a range of divalents was Cu2+ > Cd2+ = Zn2+ > Ni2+ much greater th an Mg2+ = Co2+ > Mn2+ > Ca2+ = Ba2+ = Sr2+. These results suggest that these divalent cations and protons all act primarily as allosteric mo dulators to alter the affinity of ATP binding to the P2X(7) receptor. In contrast, extracellular (but not intracellular) calmidazolium inhib ited the BzATP-evoked current by up to 90% (IC50 = 15 nM) but had no e ffect on YO-PRO uptake. Thus, calmidazolium can block activation of th e ionic channel but this does not prevent the formation of the large p ermeabilizing pore. (C) 1997 Elsevier Science Ltd.