The protocol below is adapted for trout from Toleikis et al. (1997). The ventricle was cut in three to four pieces which were weighed (range: 10–25 mg) and placed in 1.5 ml of ice-cold buffer (ATP: 1 mM, PCr: 2 mM, Dithiothreitol: 0.5 mM EDTA: 5.5 mM, MgCl2: 2.5 mM, Imidazole: 10.0 mM, HEPES: 20.0 mM, KCl: 70.0 mM; pH 7.4) during less than 10 min to eliminate all traces of blood. Pieces of ventricle were then moved into 1.5 ml of chilled buffer with saponin (50 mg/ml) and collagenase (1.5 mg/ml). After 30 min, tissue fragments were then rinsed twice in cold buffer (10 min per rinse).