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:Further studies are still needed to exclude other purines such as ATP and related nucleotides, due to their significant release from bladder urothelium [8,9], and since ATP in some species can inhibit or relax the bladder [33–35].
Category: