Changes in kidney function and faecal excretion of water and electrolytes with sodium chloride loading in sheep

The renal and faecal routes of water and electrolyte excretion in sheep were compared and changes in kidney function assessed when similar amounts of sodium chloride (NaCl) were dosed in free and fixed ratios in water. Sheep (n = 6) either had free access to fresh drinking water (control phase) or 0.9% saline drinking water, or NaCl was dosed intraruminally with free access to fresh drinking water. In the final phase of the investigation, sheep were dosed intraruminally with NaCl while water intake was restricted. Variables monitored included fresh or saline water intake, water, sodium and potassium loss via the urine and faeces, glomerular filtration rate (GFR), fractional excretions of sodium (FENa) and potassium (FEK) and solute free water clearance (CH2O). Results indicated that where NaCl intake was high, sodium excretion via the urine was of far greater importance than that via the faeces and that changes in kidney function which occurred in order to excrete excess sodium, included increases in GFR and FENa, and declines in FEK and CH2O. Where significant differences were obtained in variables between the phases of high salt intake, these were attributed to differences in sodium absorption from the gastrointestinal tract. It was concluded that when NaCl intake is high, sodium homeostasis is chiefly maintained by increasing the amount of sodium made available to the nephron tubule (increased GFR), by decreased tubular reabsorption of this sodium and by increasing solute free water reabsorption.