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

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 m&g bwt (690 mmouday). An  exercise test  (ET) which contained 3 phases was performed. Phase Z consisted of 23.5 min of mainly submaximal exercise, Phase 2 of 2 h of box rest and Phuse 3 of 26 min of exercise including an intensive trot over 2600 m at 90% of VO,, . The ET was repeated 3 times: the  first at 20°C (3040% %H), the second at 35°C (3040% RH) and the third at 35°C (3040% 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 mmoyday. 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.