CHANGES IN KIDNEY-FUNCTION AND FECAL EXCRETION OF WATER AND ELECTROLYTES WITH SODIUM-CHLORIDE LOADING IN SHEEP

The renal and faecal routes of water and electrolyte excretion in shee p were compared and changes in kidney function assessed when similar a mounts of sodium chloride (NaCl) were dosed in free and fixed ratios i n water. Sheep (n=6) either had free access to fresh drinking water co ntrol phase) or 0,9% saline drinking water, or NaCl was dosed intrarum inally with free access to fresh drinking water. In the final phase of the investigation, sheep were dosed intraruminally with NaCl while wa ter intake was restricted. Variables monitored included fresh or salin e water intake, water, sodium and potassium loss via the urine and fae ces, glomerular filtration rate (GFR), fractional excretions of sodium (FE(Na)) and potassium (FE(K)) and solute free water clearance (C(H2O )). Results indicated that where NaCl intake was high, sodium excretio n via the urine was of far greater importance than that via the faeces and that changes in kidney function which occurred in order to excret e excess sodium, included increases in GFR and FE(Na), and declines in FE(K) and C(H2O). Where significant differences were obtained in vari ables 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 chi efly maintained by increasing the amount of sodium made available to t he nephron tubule (increased GFR), by decreased tubular reabsorption o f this sodium and by increasing solute free water reabsorption.