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.