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:To examine whether the histamine-stimulated and MST4-mediated parietal cell secretion depends on proton pump H,K-ATPase, aliquots of parietal cells were treated with omeprazole (Sigma, 100 nM), a H,K-ATPase inhibitor, as previously described (11).
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