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:These studies have generally found that cells from immature ventricles, compared with adult myocytes, display 1) a reduction in the density of outward K currents (28, 51), 2) greater activity and expression of Na /Ca(2) exchange (NCX) (2), and 3) intracellular Ca(2) transients that depend less on sarcoplasmic reticulum (SR) Ca(2) release and more on transmembrane Ca(2) influx (21, 32).
Category: