Water dynamics in the digestive tract of the freshwater rainbow trout during the processing of a single meal.

The temporal effects of feeding and digestion on chyme composition, specifically water and solid content, and net fluxes across the gastrointestinal tract, as well as plasma parameters, were examined in freshwater rainbow trout. A single meal of commercial dry pellets, incorporating ballotini beads as inert reference markers, was employed. Plasma Na+ levels increased by 15-20% at 2 h post-feeding, where Cl- levels did not change. Plasma osmolality was well regulated despite an initial chyme osmolality (775 mOsm) 2.8-fold higher than that in the blood plasma. Chyme osmolality throughout the gastrointestinal tract remained significantly higher than plasma osmolality for the duration of the 72 h period. Solid material was absorbed along the entire intestinal tract, although not in the stomach, necessitating the incorporation of an inert marker. A similar temporal pattern of transit between the ballotini beads (solid phase marker) and 3[H]-PEG 4000 (fluid phase marker), provided support for the use of ballotini beads. Large additions of water to the chyme were seen in the stomach, the largest occurring within 2 h following feeding (7.1+/-1.4 ml kg(-1)), and amounted to approximately 16 ml kg(-1) over the first 12 h. As the chyme entered the anterior intestine, a further large water secretion (3.5+/-0.5 ml kg(-1)) was seen. Thereafter the water fluxes into the chyme of the anterior intestine decreased steadily over time, but remained positive, whereas the mid-intestine exhibited net absorption of water at all time points, and the posterior intestine demonstrated little water handling at any time. The endogenous water that was secreted into the anterior intestine was absorbed along the tract, which showed a net water flux close to zero. However, assuming that the water secreted into the stomach was endogenous in nature, the processing of a single meal resulted in net loss of endogenous water (0.24 ml kg(-1) h(-1)) to the environment, a beneficial consequence of the osmotic challenge offered by the food for a freshwater hyperosmotic regulator.