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We have assessed the relative contribution of Ca²⁺ entry and Ca²⁺ release from internal stores to the [Ca²⁺]_i transients evoked by purinergic receptor activation in bovine adrenal chromaffin cells. The [Ca²⁺]_i was recorded from single cells using ratiometric fura-2 microfluorometry. Two discrete groups of ATP-sensitive cells could be distinguished on the basis of their relative capacity to respond to ATP in the virtual absence of extracellular Ca²⁺. One group of cells (group I) failed to respond to ATP in the absence of Ca²⁺, was completely insensitive to UTP, and displayed suramin-blockable [Ca²⁺]_i transients when challenged with ATP in the presence of external Ca²⁺. ATP activated a prominent and rapidly inactivating Mn²⁺ influx pathway in group I cells, as assessed by monitoring Mn²⁺ quenching of fura-2 fluorescence. In contrast, a second group of ATP-sensitive cells (group II) exhibited pronounced [Ca²⁺]_i …