Somatic ATP release from guinea pig sympathetic neurons does not require calcium-induced calcium release from internal stores.

Prior studies indicated that a Ca(2+)-dependent release of ATP can be initiated from the soma of sympathetic neurons dissociated from guinea pig stellate ganglia. Previous studies also indicated that Ca(2+)-induced Ca(2+) release (CICR) can modulate membrane excitability in these same neurons. As Ca(2+) release from internal stores is thought to support somatodendritic transmitter release in other neurons, the present study investigated whether CICR is essential for somatic ATP release from dissociated sympathetic neurons. Caffeine increased intracellular Ca(2+) and activated two inward currents: a slow inward current (SIC) in 85% of cells, and multiple faster inward currents [asynchronous transient inward currents (ASTICs)] in 40% of cells voltage-clamped to negative potentials. Caffeine evoked both currents when cells were bathed in a Ca(2+)-deficient solution, indicating that both were initiated by Ca(2+) release from ryanodine-sensitive stores in the endoplasmic reticulum. Sodium influx contributed to generation of both SICs and ASTICs, but only ASTICs were inhibited by the presence of the P2X receptor blocker PPADs. Thus ASTICs, but not SICs, resulted from an ATP activation of P2X receptors. Ionomycin induced ASTICs in a Ca(2+)-containing solution, but not when it was applied in a Ca(2+)-deficient solution, demonstrating the key requirement for external Ca(2+) in initiating ASTICs by ionomycin. Pretreatment with drugs to deplete the internal stores of Ca(2+) did not block the ability of ionomycin or long depolarizing voltage steps to initiate ASTICs. Although a caffeine-induced release of Ca(2+) from internal stores can elicit both SICs and ASTICs in dissociated sympathetic neurons, CICR is not required for the somatic release of ATP.