The alkaline tide and ammonia excretion after voluntary feeding in freshwater rainbow trout

Carol Bucking,Chris M Wood

doi:10.1242/jeb.015610

Abstract

We investigated the potential acid-base and nitrogenous waste excretion challenges created by voluntary feeding in freshwater rainbow trout, with particular focus on the possible occurrence of an alkaline tide (a metabolic alkalosis created by gastric HCl secretion during digestion). Plasma metabolites (glucose, urea and ammonia) were measured at various time points before and after voluntary feeding to satiation (approximately 5% body mass meal of dry commercial pellets), as was the net flux of ammonia and titratable alkalinity to the water from unfed and fed fish. Arterial blood, sampled by indwelling catheter, was examined for post-prandial effects on pH, plasma bicarbonate and plasma CO2 tension. There was no significant change in plasma glucose or urea concentrations following feeding, whereas plasma ammonia transiently increased, peaking at threefold above resting values at 12 h after the meal and remaining elevated for 24 h. The increased plasma ammonia was correlated with an increase in net ammonia excretion to the water, with fed fish significantly elevating their net ammonia excretion two- to threefold between 12 and 48 h post feeding. These parameters did not change in unfed control fish. Fed fish likewise increased the net titratable base flux to the water by approximately threefold, which resulted in a transition from a small net acid flux seen in unfed fish to a large net base flux in fed fish. Over 48 h, this resulted in a net excretion of 13 867 micromol kg(-1) more base to the external water than in unfed fish. The arterial blood exhibited a corresponding rise in pH (between 6 and 12 h) and plasma bicarbonate (between 3 and 12 h) following feeding; however, no respiratory compensation was observed, as PaCO2 remained constant. Overall, there was evidence of numerous challenges created by feeding in a freshwater teleost fish, including the occurrence of an alkaline tide, and its compensation by excretion of base to the external water. The possible influence of feeding ecology and environmental salinity on these challenges, as well as discrepancies in the literature, are discussed.

Highlights

The gut is indispensable for multicellular life, and is responsible for meeting the nutritional and energy demands of an organism
As the amino acid surplus from protein-rich diets cannot be directly stored in fishes, it is deaminated and converted into energetic compounds (Ballantyne, 2001; Stone et al, 2003), resulting in post-prandial increases in plasma total ammonia levels (Kaushik and Teles, 1985) and ammonia excretion rates
HCl secretion is essential for protein digestion through the aforementioned pepsinogen activation as well as by direct acid

Summary

Introduction

The gut is indispensable for multicellular life, and is responsible for meeting the nutritional and energy demands of an organism. Digestion could potentially create challenges for fish, especially carnivorous fish such as rainbow trout, because of the formation of excess ammonia during the catabolism of dietary proteins (Handy and Poxton, 1993), as well as excess base during the formation of HCl by the stomach (reviewed by Hersey and Sachs, 1995; Niv and Fraser, 2002) In this regard, a marked systemic alkaline tide during digestion has recently been described in a carnivorous marine elasmobranch, the dogfish shark (Wood et al, 2005; Wood et al, 2007a; Wood et al, 2007b). HCl secretion is essential for protein digestion through the aforementioned pepsinogen activation as well as by direct acid

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Discussion

Conclusion