Plasma aldosterone concentration and cardiovascular response to low sodium intake in horses in training.

REASON FOR PERFORMING THE STUDY: Horses in training lose large amounts of sodium but little is known about the cardiovascular response to low sodium intake.
OBJECTIVES: To investigate the effect of low sodium intake on plasma aldosterone (pAldo) concentrations and the cardiovascular system of athletic horses, and to identify markers of low sodium intake.
METHODS: Seven Standardbred geldings in training (trained twice a week) were randomly offered a standardised diet supplemented (NaS, 58 mg Na/kg bwt) and not supplemented (NaN, 3 mg Na/kg bwt) with NaCl for 5 weeks in a changeover design. Blood samples were taken once a week and in Week 5, before and following an exercise test until 22.30 h and analysed for blood sodium (bNa), total plasma protein (TPP), pAldo, troponin I and packed cell volume (PCV). Blood pressure (BP) was measured and pulse wave recorded at rest with high definition oscillometric-technique (HDO). ECG and echocardiography were recorded. Water intake was measured before and on the day of exercise and voluntary saline intake was measured for 2 days after each period. Faecal samples were taken weekly and analysed for sodium and potassium content.
RESULTS: The pAldo and the PCV was higher in NaN compared to NaS. There were no differences between diets in BP, ECG, plasma troponin I and echocardiogram but HDO pulse amplitude tended to be smaller on diet NaN. Water intake was lower on diet NaN and saline intake higher. The response to exercise in bNa, pAldo, PCV and TPP was different on the 2 diets. Faecal potassium/sodium ratio was higher on NaN than on NaS.
CONCLUSION: This study shows that 5 weeks of low sodium intake increased plasma aldosterone concentration and PCV but no alterations in heart function was observed. Faecal potassium/sodium ratio could be used to assess sodium status in horses.