EFFECTS OF SODIUM-BICARBONATE ON FLUID, ELECTROLYTE AND ACID-BASE-BALANCE IN RACEHORSES

Sodium bicarbonate given by nasogastric tube has been used by some tra iners as the key ingredient in a 'milkshake'. It has been suggested th at such treatment given 3-5 h prior to racing may enhance a horse's ra cing performance by increasing the blood buffering capacity and enhanc ing lactate clearance from skeletal muscle, thereby delaying the onset of fatigue. Several experiments were conducted to examine the effects on fluid, electrolyte and acid-base values of 0.5 g kg(-1) dose of so dium bicarbonate, were examined. The effects of fasting, the simultane ous administration of glucose (0.5 g kg(-1)) or the withholding of wat er were also examined to determine whether they influenced the uptake and elimination of sodium bicarbonate. Six Thoroughbred horses were us ed, each wearing a urine and faecal collection harness. Prior to sodiu m bicarbonate administration, venous blood, urine and faecal samples w ere collected for 24 h to establish control values. After administrati on of sodium bicarbonate (0.5 g kg(-1)) in 21 of water, samples were c ollected at various times for up to 46 h. There were significant incre ases in water consumption, from 0.5-2.31 h(-1) at 2 h post-administrat ion. Urine output increased by approximately three fold and did not re turn to control levels until 18 h post-administration. Urinary sodium concentration increased from 95 +/- 16 mmol l(-1) (mean +/- SEM) to pe ak values of 349 +/- 12 mmol l(-1) at 12 h. In the 24 h after sodium b icarbonate administration, approximately 80% of the sodium intake (NaH CO3+feed) was excreted in the urine. There tvas no significant change in the total urinary potassium and chloride excretion. Faecal water co ntent did not change following sodium bicarbonate administration, but there was an increase in faecal sodium content. The mean increase in v enous blood bicarbonate concentration was 7.6 +/- 0.4 mmol l(-1) after the 0.5 kg(-1) dose. Water deprivation for 6 h after sodium bicarbona te administration, fasting or the co-administration of glucose did not affect the peak blood bicarbonate concentration or the time to peak c oncentration. However, the withholding of water did result in a faster rate of decrease in blood bicarbonate concentration when water was re supplied.