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

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.