The role of the kidney in compensating the alkaline tide, electrolyte load, and fluid balance disturbance associated with feeding in the freshwater rainbow trout, Oncorhynchus mykiss

The effect in freshwater rainbow trout of digesting a commercial pellet meal on the renal handling of water, ions and acid–base equivalents was investigated through urine collection over a 48 h period following meal ingestion. The glomerular filtration rate (GFR) and urine flow rate (UFR) were reduced in fed fish between 12 and 24 h following the meal, likely reflecting a loss of endogenous water across the gastric epithelium as a result of ingesting dry, ion-rich food pellets. The kidney was also responsible for the excretion of some excess dietary Ca2+, and, to a much lesser extent, Na+ and Cl−, while the urinary excretion of K+ was unaffected. The most dramatic effect of feeding was the elevation of renal Mg2+ excretion, with the kidney transitioning from net Mg2+ reabsorption to net Mg2+ secretion during digestion. The renal handling of dietary ions accounted for 3–27% of the total ions absorbed from the diet, indicating that a majority of the ions are excreted extra-renally or incorporated into growth. However this does highlight the underestimation of renal ion handling when using unfed fish models. The metabolic alkalosis created by digestion (the alkaline tide) resulted in an increase in urine pH as well as a transition from net acidic equivalent excretion in the urine to net basic equivalent excretion. This was due to a decrease in the titratable acidity minus bicarbonate component of urine as well as a decrease in ammonia secretion. Additionally, the experimental separation of the urinary component of acid–base excretion from that of the gills highlighted the substantially larger contribution of the latter. During the alkaline tide, renal excretion accounted for ∼ 5% of the total basic equivalent excretion to the external water.