Waste nitrogen dynamics and an alkaline tide in the gut and of freshwater rainbow trout after voluntary consumption of a single satiating meal

Abstract We examined sequential changes in ammonia, urea-N, and acid-base parameters in four gut sections (stomach, anterior, mid, and posterior intestine) over a prolonged period of digestion (175 h) following a satiating meal (4.54% body weight) of commercial trout pellets given to a homogeneous batch of fasted rainbow trout (~ 75g) held in ion-poor fresh water at 10ºC. We related these to simultaneous changes in some of these same parameters in the blood plasma. Initial post-feeding concentrations of ammonia and urea-N in the stomach chyme could be explained by leaching of these pre-formed N-products from the consumed food pellets. Chyme ammonia-N concentrations dropped progressively in all parts of the intestine through to 80 h post-feeding, while urea-N concentrations increased slightly. Ammonia appeared to be scavenged from the tract, whereas urea-N appeared to be synthesized in the tract. Yet the plasma concentrations of both N-products increased simultaneously by more than 3-fold up to 54 h post-feeding, before declining thereafter. The initial very acidic pre-feeding pH of the stomach (2.0) in fasted trout increased by more than 3 pH units as the pellets were digested by elevated gastric acid secretion, reflecting the high acid-neutralizing capacity of the pellet food. A marked long-lasting (> 80 h) alkaline tide in the blood plasma was associated with this acid secretion into the stomach. There was also a distinct alkaline tide in all parts of the intestine, manifested as substantial and prolonged elevations in chyme pH and HCO 3 − concentrations. These lagged slightly behind the alkaline tide in the blood plasma, suggesting that HCO 3 − secretion into the intestine helped compensate the systemic alkaline tide. We discuss the possible transport mechanisms responsible for these responses, and the influence of the ion-poor nature of the fresh water.