Transduction mechanisms of P2Z purinoceptors

N-methyl-D-glucamine and propidium dyes utilize different permeation pathways at rat P2X(7) receptors

Activation of membrane P2X(7) receptors by extracellular ATP [or its analog 2',3'-O-(4-benzoylbenzoyl)-ATP] results in the opening within several milliseconds of an integral ion channel that is permeable to small cations. If the ATP application is maintained for several seconds, two further sequelae occur: there is a gradual increase in permeability to the larger cation N-methyl-d-glucamine and the cationic propidium dye quinolinium, 4-[(3-methyl-2(3H)-benzoxazolylidene)methyl]-1-[3-(triethylammonio)propyl]diiodide (YO-PRO-1) enters the cell. The similarity in the time course of these two events has led to the widespread view that N-methyl-d-glucamine and YO-PRO-1 enter through a common permeation pathway, the "dilating" P2X(7) receptor pore. Here we provide two independent lines of evidence against this view. We studied single human embryonic kidney cells expressing rat P2X(7) receptors with patch-clamp recordings of membrane current and with fluorescence measurements of YO-PRO-1 uptake. First, we found that maintained application of the ATP analog did not cause any increase in N-methyl-d-glucamine permeability when the extracellular solution contained its normal sodium concentration, although YO-PRO-1 uptake was readily observed. Second, we deleted a cysteine-rich 18-amino acid segment in the intracellular juxtamembrane region of the P2X(7) receptor. This mutated receptor showed normal YO-PRO-1 uptake but had no permeability to N-methyl-d-glucamine. Together, the clear differential effects of extracellular sodium ions or of mutation of the receptor strongly suggest that N-methyl-d-glucamine and YO-PRO-1 do not enter the cell by the same permeation pathway.

P2X7 receptors in rat brain: presence in synaptic terminals and granule cells

ATP stimulates [Ca2+]i increases in midbrain synaptosomes via specific ionotropic receptors (P2X receptors). Previous studies have demonstrated the implication of P2X3 subunits in these responses, but additional P2X subunits must be involved. In the present study, ATP and BzATP proved to be able to induce intrasynaptosomal calcium transients in the midbrain synaptosomes, their effects being potentiated when assayed in a Mg2+-free medium. Indeed, BzATP was shown to be more potent than ATP, and their effects could be inhibited by PPADS and KN-62, but not by suramin. This activity profile is consistent with the presence of functional P2X7 receptors in the midbrain terminals. The existence of presynaptic responses to selective P2X7 agonists could be confirmed by means of a microfluorimetric technique allowing [Ca2+]i measurements in single synaptic terminals. Additionally, the P2X7 receptor protein could be identified in the midbrain synaptosomes and in axodendritic prolongations of cerebellar granule cells by immunochemical staining.

Sodium block and depolarization diminish P2Z-dependent Ca2+ entry in human B lymphocytes

Despite a high Ca2+ -permeability of the P2Z receptor in human B lymphocytes, extracellular ATP4- has only a minor effect on global [Ca2+]i. The aim of this study was to reveal the mechanisms responsible for this discrepancy. We investigated the relationship between ATP4- -application, Cai 2+ -response, membrane current and membrane potential in two human B cell lines and in human tonsillar B cells. This was achieved by a combination of FACS- and voltage clamp measurements and the usage of appropriate voltage- and Ca2 -sensitive fluorescent dyes. ATP4 -induced changes in whole-cell current and [Ca2]i were blocked by extracellular as well as intracellular Na+. Under current clamp conditions, ATP4- -induced Na+ -entry diminished the Ca2+ entry via reduction of the driving force. A substantial increase in [Ca2+]iinduced by ATP4- was only observed in Na+ -free solutions. The pathway of signal transduction activated by ATP4via P2Z receptor of human B lymphocytes under physiological conditions seems not to operate by an increase in the global intracellular Ca2+ -concentration, but rather by the depolarization of the cell membrane as a result of the Na+-influx.