Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Basal ATP concentrations in the bulk medium of unstimulated CFs are low, but the concentrations of released nucleotides near the cell surface may be sufficient to activate adjacent P2Y receptors and contribute to basal signaling (48, 49).
Category: