Gastrointestinal transport of Ca2+ and Mg2+ during the digestion of a single meal in the freshwater rainbow trout

A diet containing an inert marker (ballotini beads, quantified by X-radiography) was used to quantify the transport of two essential minerals, Ca2+ and Mg2+ from the diet during the digestion and absorption of a single meal of commercial trout food (3% ration). Initially, net uptake of Ca2+ was observed in the stomach followed by subsequent Ca2+ fluxes along the intestine which were variable, but for the most part secretory. This indicated a net secretion of Ca2+ along the intestinal tract resulting in a net assimilation of dietary Ca2+ of 28%. Similar handling of Ca2+ and Mg2+ was observed along the gastrointestinal tract (GI), although net assimilation differed substantially between the cations, with Mg2+ assimilation being close to 60%, mostly a result of greater uptake by the stomach. The stomach displayed the highest net uptake rates for both cations (1.5 and 1.3 mmol kg−1 fish body mass for Ca2+ and Mg2+, respectively), occurring within 2 h following ingestion of the meal. Substantial secretions of both Ca2+ and Mg2+ were observed in the anterior intestine, which were attributed to bile and other intestinal secretions, while fluxes in the mid and posterior intestine were small and variable. The overall patterns of Ca2+ and Mg2+ handling in the GI tract were similar to those observed for Na+ and K+ (but not Cl−) in a previous study. Overall, these results emphasize the importance of dietary electrolytes in ionoregulatory homeostasis.