The effect of postprandial changes in pH along the gastrointestinal tract on the distribution of ions between the solid and fluid phases of chyme in rainbow trout

Abstract

An area of emerging importance is the role that the diet can play in alleviating the demands for ion uptake in fish living in a freshwater environment, by providing a highly concentrated supply of electrolytes. The availability of ions for uptake from the diet likely requires dissolution in the fluid phase of the chyme. However, the distribution of ions between the fluid and solid phases of chyme has not been well‐characterized in fish, and little is known about the effects of location along the gastrointestinal (GI) tract, or about the pH gradients found therein, on this distribution. Hence, the pH and ionic concentrations (Na+, K+, Cl−, Ca2+ and Mg2+, in both fluid and solid phases) of the chyme in each GI tract section were measured at various time points during the digestion of a single meal of commercial pellets in freshwater rainbow trout (Oncorhynchus mykiss). Additionally, the presence of an inert reference marker (lead‐glass beads) in the diet was used to quantify the distribution of these ions between the solid and fluid phases of the chyme. The buffering capacity of the food was evident in the acidic stomach (ST), whereas the intestine provided an alkaline environment for further digestion. It appeared that pH had little influence on the distribution of the monovalent ions between the phases in all GI tract sections. However, the ST showed significant changes in the partitioning of both Ca2+ and Mg2+, with each mineral becoming highly dissolved as the gastric chyme pH decreased. This was followed by subsequent precipitation of both minerals in the alkaline environment of the intestine. The high degree of dissolution of Ca2+ and Mg2+ in the fluid phase of gastric chyme corresponded with large absorptive rates from the ST seen previously, however, this was not true of the monovalent ions.

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

“…In fact, the titration of a commercial fish meal down to pH 3 (Cooper and Wilson, 2008) required 10-fold more acid than that of a natural ragworm diet (Taylor et al, 2007). It is unlikely that titration in vitro exactly duplicates the real titration that occurs as chyme is progressively digested and diluted in vivo (Bucking and Wood, 2006a;Bucking and Wood, 2008). Nevertheless, greater acid secretion with a commercial diet may reflect the greater acid-base disturbances observed in the present study when compared with studies using natural diets (Wood et al, 2007b;Taylor et al, 2007).…”

“…A K + -stimulated, H + -ATPase is responsible for the apical H + secretion, and has been identified in oxynticopeptic cells of elasmobranchs (Smolka et al, 1994) as well as those of the rainbow trout (Sugiura et al, 2006). Although there is a vigorous secretion of gastric acid at this time, the pH of the stomach fluid actually increases substantially due to the buffering action of the ingested food (Sugiura et al, 2006;Bucking and Wood, 2008). As a Cl -/HCO 3 -exchanger is believed to be responsible for the basolateral HCO 3 -export and Cl -entry, the net transfer of HCl to the stomach can lead to a reduction in plasma [Cl -] that has been correlated with an alkaline tide in toads (Busk et al, 2000).…”

Mechanisms of transepithelial ammonia excretion and luminal alkalinization in the gut of an intestinal air-breathing fish, Misgurnus anguilliacaudatus

“…Although it was not possible to measure intestinal lumen Na + levels in the loach, values measured in low salinity (2.5p.p.t. )-acclimated toadfish Opsanus beta (McDonald and Grosell, 2006) and post-prandial freshwater tilapia (Grosell, 2007) and rainbow trout (Bucking and Wood, 2006;Bucking and Wood, 2009) were within the range of the mucosal saline Na + concentrations used in this study. Thus, a favorable apical Na + electrochemical gradient would be present to drive the exchange.…”

Tissue pH and gut ecomorphology in six freshwater teleostsoccupying different trophic levels

“…Formulated or controlled feed shapes the gastrointestinal environment. Direct involvement of GI tract pH in the partitioning of bivalent ions has been shown in rainbow trout stomach (Bucking and Wood, 2009). In turn, the characteristics of GI and blood pH are influenced by diet and altered dietary electrolyte balance (Saravanan et al, 2013).…”

“…) and the other is characterized by continuous acid secretion and the maintenance of a low, acidic pH during fasting (Papastamatiou and Lowe, 2004;Bucking and Wood, 2009). Our results demonstrated that C. argus and S. chuatsi hold a closely neutral pH in the stomach mucosa through a long period of fasting, acidification perhaps only stimulated by the ingestion of food (Nikolopoulou et al, 2011).…”