Pepsin-like protease activity and the gastric digestion within ex vivo Pacific bluefin tuna Thunnus orientalis stomachs, as a gastric digestion model

• Bluefin tuna isolated stomach maintained functionality ex vivo.
• Ex vivo stomachs of bluefin tuna responded to luminal stimuli.
• Ex vivo stomachs of bluefin tuna moved for the duration of incubation period (8 h).
• Gastric digestion was observed in the isolated bluefin tuna stomach.
• Results support the applicability of ex vivo gastric digestion tests.

Isolated whole stomachs of farmed Pacific bluefin tuna Thunnus orientalis were externally batch-cultured in cell culture medium at 24 °C for 8 h. This was done in order to confirm that gastric function was maintained during incubation (Exp. 1) so that the effectiveness of the ex vivo technique outlined in the present study in serving as an artificial method of gastric digestion could be determined (Exp. 2). During experiment 1, fresh or cold-stored stomachs (48 h on ice) were intra-gastrically infused with a saline solution (control group) or a skipjack tuna broth stimulant (infusion group). Pepsin-like protease activities within the gastric effluent of each study group were compared. In both control and infusion groups, fresh stomachs demonstrated higher pepsin-like protease activity. Within fresh stomachs, protease activity subsequent to stimulant infusion was significantly greater than it was in the control group. In cold-stored stomachs, however, no difference was observed in the protease activities of control and infusion groups. According to the results, fresh stomachs respond to luminal stimuli. Furthermore, fresh stomachs exhibited motility during incubation while cold-stored stomachs did not. Mucosal layers within fresh stomachs indicated histological normality subsequent to incubation. In experiment 2, both fresh and cold-stored stomachs were filled with a diet consisting of boiled fish paste. After a period of 8 h, dry matter loss of diet was significantly greater within fresh stomachs than it was in cold-stored stomachs. The level of pepsin-like protease activity within fresh stomachs was also higher than levels observed within cold-stored stomachs during the incubation period. These results suggest that fresh stomachs obtained from bluefin tuna maintained functionality ex vivo and that the technique outlined in the present study could be used effectively in the evaluation of bluefin tuna gastric digestion.