Contribution of Calcium Ions to P2X Channel Responses

• Published in: The Journal of neuroscience 2004-03, Vol.24 (13), p.3413-3420
• Main Authors: Egan, Terrance M, Khakh, Baljit S
• Format: Article
• Language: English
• Subjects: Adenosine Triphosphate - metabolism; Adenosine Triphosphate - pharmacology; Amino Acid Substitution - genetics; Animals; Calcium - metabolism; Cell Line; Cellular/Molecular; Glutamic Acid - metabolism; Humans; Ion Channel Gating - drug effects; Ion Channel Gating - genetics; Ion Channel Gating - physiology; Ion Channels - drug effects; Ion Channels - genetics; Ion Channels - metabolism; Kidney - cytology; Kidney - metabolism; Membrane Potentials - physiology; Mutagenesis, Site-Directed; Neurotransmitter Agents - metabolism; Neurotransmitter Agents - pharmacology; Patch-Clamp Techniques; Rats; Receptors, Purinergic P2 - genetics; Receptors, Purinergic P2 - metabolism; Receptors, Purinergic P2X; Receptors, Purinergic P2X2; Receptors, Purinergic P2X4; Recombinant Proteins - genetics; Recombinant Proteins - metabolism
• ISSN: 0270-6474; 1529-2401
• DOI: 10.1523/JNEUROSCI.5429-03.2004

Ca2+ entry through transmitter-gated cation channels, including ATP-gated P2X channels, contributes to an array of physiological processes in excitable and non-excitable cells, but the absolute amount of Ca2+ flowing through P2X channels is unknown. Here we address the issue of precisely how much Ca2+ flows through P2X channels and report the finding that the ATP-gated P2X channel family has remarkably high Ca2+ flux compared with other channels gated by the transmitters ACh, serotonin, protons, and glutamate. Several homomeric and heteromeric P2X channels display fractional Ca2+ currents equivalent to NMDA channels, which hitherto have been thought of as the largest source of transmitter-activated Ca2+ flux. We further suggest that NMDA and P2X channels may use different mechanisms to promote Ca2+ flux across membranes. We find that mutating three critical polar amino acids decreases the Ca2+ flux of P2X2 receptors, suggesting that these residues cluster to form a novel type of Ca2+ selectivity region within the pore. Overall, our data identify P2X channels as a large source of transmitter-activated Ca2+ influx at resting membrane potentials and support the hypothesis that polar amino acids contribute to Ca2+ selection in an ATP-gated ion channel.