The Metabolic Cost of Digestion in Nile Tilapia, Oreochromis niloticus

The stomach is a key vertebrate innovation that has been lost in numerous fish lineages. To understand the driving factors of this phenotypic stomach loss, the cost of maintaining the low-pH environment in the gut was explored using Nile tilapia (Oreochromis niloticus) as a model. As Nile tilapia maintain strong acidity in the stomach, omeprazole, a proton pump inhibitor, was used to pharmacologically knock-down acid production by irreversibly blocking the pumping action of the gastric proton pump, H+/K+-ATPase (HKA). Omeprazole treatment showed a 90% drop in acid production indicating an effective knock-down of gastric acid secretion. Respirometry trials indicated a significant decrease in magnitude and duration of post-prandial response in the knock-down group, indicating that acid production prolongs the digestion process. This decrease in duration of the post-prandial response is supported by gastric evacuation data showing reduced latency time in the stomach in the knock-down group. Furthermore, the knock-down animals had significantly lower specific and absolute growth rates, indicating a reduction in the ability of these animals to utilize nutrients from their feed. Further investigation into the impact of gastric acid on nutrient digestibility was undertaken, with digestibility trials indicating decreased digestibility of calcium and magnesium in the knock-down group. Additionally, carcass composition was significantly impacted, with the knock-down group having higher moisture content, as well as higher magnesium, calcium, and phosphorus levels. Most digestive enzyme activity levels remained unchanged with omeprazole treatment, except alkaline phosphatase which was significantly decreased. Finally, omeprazole treatment effects on the expression of the HKA subunits Atp4a and Atp4b were determined using Western Blotting, with knock-down animals showing lower expression levels of Atp4b. The in vitro measurement of HKA activities in stomach tissues were significantly lower in the knock-downs, indicating the effectiveness of the omeprazole knock-down once again. Overall, this thesis lays the groundwork for future research into stomach loss, establishing a method for further investigations into gastric acid and its purpose in the gut.