Ionotropic (P2X) receptor dynamics at single autonomic varicosities

Exocytotic release of ATP and activation of P2X receptors in dissociated guinea pig stellate neurons

Activation of P2X receptors by a Ca(2+)- and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein-dependent release of ATP was measured using patch-clamp recordings from dissociated guinea pig stellate neurons. Asynchronous transient inward currents (ASTICs) were activated by depolarization or treatment with the Ca(2+) ionophore ionomycin (1.5 and 3 microM). During superfusion with a HEPES-buffered salt solution containing 2.5 mM Ca(2+), depolarizing voltage steps (-60 to 0 mV, 500 ms) evoked ASTICs on the decaying phase of a larger, transient inward current. Equimolar substitution of Ba(2+) for Ca(2+) augmented the postdepolarization frequency of ASTICs, while eliminating the larger transient current. Perfusion with an ionomycin-containing solution elicited a sustained activation of ASTICs, allowing quantitative analysis over a range of holding potentials. Under these conditions, increasing extracellular [Ca(2+)] to 5 mM increased ASTIC frequency, whereas no events were observed following replacement of Ca(2+) with Mg(2+), demonstrating a Ca(2+) requirement. ASTICs were Na(+) dependent, inwardly rectifying, and reversed near 0 mV. Treatment with the nonselective purinergic receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (10 microM) blocked all events under both conditions, whereas the ganglionic nicotinic antagonist hexamethonium (100 microM and 1 mM) had no effect. PPADS also blocked the macroscopic inward current evoked by exogenously applied ATP (300 microM). The presence of botulinum neurotoxin E (BoNT/E) in the whole-cell recording electrode significantly attenuated the ionomycin-induced ASTIC activity, whereas phorbol ester treatment potentiated this activity. These results suggest that ASTICs are mediated by vesicular release of ATP and activation of P2X receptors.

P2X1 receptor currents after disruption of the PKC site and its surroundings by dominant negative mutations in HEK293 cells

It has been suggested that phosphorylation at the T18P19R20 PKC sites of the P2X1 receptor regulates its functions. Here, we show that mutation at T18 (T18A and T18N) almost abolishes P2X1 current in response to ATP and that mutations of R20T but not of P19V also decrease the P2X1 current. Immunoblotting with anti-Thr(P)-Pro monoclonal antibody of membrane proteins from HEK293 cells transfected with P2X1R20T indicate the absence of Thr(P)18 which is present in HEK293 cells transfected with WT P2X1. We conclude that T18P19R20 is phosphorylated following P2X1 binding of ligand but that the three PKC sites function to different degree.