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:Both in in vivo and in cultured cells the Na+/K+ATPase activity is modulated by Ang II (Muscella et al. 1997, 2000, Bharatula et al. 1998, Hussain et al. 1998, Buhagiar et al. 1999, Yingst et al. 2000, Zhang & Mayeux 2001).
Category: