The effect of ambient temperature and saline loading on changes in plasma and urine electrolytes (Na+ and K+) following exercise

Summary In this study 4 Standardbred geldings (age 3–8 years, weight 431–531 kg) were used. The horses were fed a hay and oat diet and the total sodium intake was about 32 mg/kg bwt (690 mmol/day). An exercise test (ET) which contained 3 phases was performed. Phase 1 consisted of 23.5 min of mainly submaximal exercise, Phase 2 of 2 h of box rest and Phase 3 of 26 min of exercise including an intensive trot over 2600 m at 90% of VO2max. The ET was repeated 3 times: the first at 20°C (30–40% RH), the second at 35°C (30–40% RH) and the third at 35°C (30–40% RH) after a nasogastric administration of 10 litres of 0.9% NaCl solution (35°C and saline load [+ F]). Blood samples were taken before, during and after exercise. To measure fluid loss, horses were weighed before and after the ETs. Total urine output was determined 2 days before the ET (control), throughout the exercise day and for 2 days after (recovery days). There were an increase in blood and rectal temperatures after both exercise phases and a significant higher blood temperature was observed after exercise at 35°C compared to 20°C. The horses lost about 2% of their bodyweight (bwt) during the ETs. The plasma protein concentration increased during the exercise phases and remained elevated 2 h after exercise at both 20°C and 35°C, even though the horses had free access to water. The plasma protein concentration had returned to pre-exercise levels 26 h post exercise. After the saline load, total plasma protein concentration fell and increased only at the end of each exercise phase. The major mechanism regulating fluid balance after exercise seemed to be a lowered urinary excretion since water intake did not increase significantly. Urinary potassium excretion was positive throughout the experiment. During control days there was a positive sodium balance, shown by a urinary sodium excretion of 260 mmol/day. Post exercise urinary sodium excretion fell and remained very low until the second day of recovery, except after saline loading. In addition, plasma sodium was lowered 26 h after exercise at 35°C. This study shows that with a daily salt intake of 38 g it will take several days to compensate for a sodium loss caused by sweating. Therefore, it is recommended that extra salt be given during the exercise day. In the experimental situation, pre-exercise saline supplementation was beneficial since the recovery time was shortened.