Interaction between Calcium Chelators and the Activity of P2X7 Receptors in Mouse Motor Synapses.

The ability of P2X7 receptors to potentiate rhythmically evoked acetylcholine (ACh) release through Ca entry via P2X7 receptors and via L-type voltage-dependent Ca channels (VDCCs) was compared by loading Ca chelators into motor nerve terminals. Neuromuscular preparations of the diaphragms of wild-type (WT) mice and pannexin-1 knockout (Panx1) mice, in which ACh release is potentiated by the disinhibition of the L-type VDCCs upon the activation of P2X7 receptors, were used. Miniature end-plate potentials (MEPPs) and evoked end-plate potentials (EPPs) were recorded when the motor terminals were loaded with slow or fast Ca chelators (EGTA-AM or BAPTA-AM, respectively, 50 μM). In WT and Panx1 mice, EGTA-AM did not change either spontaneous or evoked ACh release, while BAPTA-AM inhibited synaptic transmission by suppressing the quantal content of EPPs throughout the course of the short rhythmic train (50 Hz, 1 s). In the motor synapses of either WT or Panx1 mice in the presence of BAPTA-AM, the activation of P2X7 receptors by BzATP (30 μM) returned the EPP quantal content to the control level. In the neuromuscular junctions (NMJs) of Panx1 mice, EGTA-AM completely prevented the BzATP-induced increase in EPP quantal content. After Panx1 NMJs were treated with BAPTA-AM, BzATP lost its ability to enhance the EPP quantal content to above the control level. Nitrendipine (1 μM), an inhibitor of L-type VDCCs, was unable to prevent this BzATP-induced enhancement of EPP quantal content to the control level. We propose that the activation of P2X7 receptors may provide additional Ca entry into motor nerve terminals, which, independent of the modulation of L-type VDCC activity, can partially reduce the buffering capacity of Ca chelators, thereby providing sufficient Ca signals for ACh secretion at the control level. However, the activity of both Ca chelators was sufficient to eliminate Ca entry via L-type VDCCs activated by P2X7 receptors and increase the EPP quantal content in the NMJs of Panx1 mice to above the control level.