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:Aortic segments were used for immunoblotting as described above and for the determination of vascular nitrate levels while aortic rings were used to measure vascular reactivity as described previously.(28,29) Immunoblotting in aortic homogenates was performed following a pull-down with eNOS as described in an available protocol (Cell Signaling); therefore, IgG was used as a loading control and corresponded directly with eNOS levels.
Category: