Heat acclimation improves regulation of plasma volume and plasma Na(+) content during exercise in horses.

The research article examines the impact of heat acclimatization on plasma volume regulation and plasma sodium content in horses during physical exercise. In essence, the study finds that as horses get accustomed to heat conditions, their plasma volume increases and their body becomes more efficient at sodium conservation.

Design and Methodology

• In this study, six trained Thoroughbred horses were exposed to heat and high humidity (33 degrees Celsius and 83% relative humidity) for four hours each day over a period of 21 days.
• In the course of this period (on the 2nd hour of days 0, 3, 7, 14, and 21), the horses performed a standard treadmill test. They ran at 50% of their peak oxygen uptake until their pulmonary artery temperature reached 41.5 degrees Celsius.

Findings

• Heat acclimation resulted in an increase in plasma volume from 21.3 liters on day zero to 24.3 liters on day 14, and then down to 22.6 liters on day 21.
• This change in plasma volume was accompanied by direct changes in plasma protein and osmolyte contents. Specifically, total plasma protein contents were 1273 g, 1455 g, and 1377 g, respectively, on days 0, 14, and 21. Meanwhile, increases in total plasma sodium plus chloride content were recorded as 5145 mmol, 5749 mmol, and 5394 mmol, respectively.
• The exercise led to a decrease in plasma volume by 7.1% at the five-minute mark, and remained decreased (-6.7%) five minutes into the recovery period. However, by day 21, the decrease in plasma volume was much less significant at 5.2% during early exercise, and only decreased by 2% five minutes into recovery, and was fully restored 15 minutes into recovery.
• Plasma sodium concentration also increased temporarily (about 3 meq/l) during the initial five minutes of exercise. However, it was back to normal by the 5-minute recovery period on day 0, and by the end of the exercise on day 21.

Conclusion

• The study concludes that heat acclimatization in horses leads to an improved ability to regulate plasma volume during physical exercise.
• This improved regulation is connected to an increase in plasma volume and an improved ability to conserve sodium, showing that the body becomes more efficient in its physiological management.