Alterations in plasma volume, plasma constituents, renin activity and aldosterone induced by maximal exercise in the horse.

This research article explores the changes in plasma volume, plasma constituents, renin activity, and the aldosterone hormone observed in horses following intense exercise, particularly noting a noteworthy decrease in plasma volume, and increases in total plasma protein, osmolality, plasma renin activity, and aldosterone concentration.

Plasma Volume Reduction

• The study found that plasma volume in the horses significantly reduced by 13% after completing 1,000 meters of maximum exercise.
• This considerable reduction took place mostly within the first 10 minutes following exercise completion. This finding is consistent with previous research carried out on humans.

Total Plasma Protein Increase

• Total plasma protein increased noticeably, specifically by 23% at 2 and 5 minutes, and 21% at 10 minutes after exercise.
• Due to these significant variations, the authors state that total plasma protein levels may not provide an accurate measure of plasma volume contractions in horses.
• It suggests that protein was added to the intravascular space either during or after exercise.

Osmolality Correlation

• The changes in plasma osmolality, or concentration of the plasma, were found to be strongly correlated with changes in sodium levels. Sodium is the primary determinant of alterations in plasma tonicity or osmotic pressure.
• The increase in plasma osmolality was just as proportionate (12%) as the reduction in plasma volume (13%), leading to a conclusion that a transient, or temporary, loss of hypotonic fluid had taken place.

Plasma Renin Activity and Aldosterone Concentration

• The increase in plasma renin activity, an enzyme that plays a fundamental role in the body’s renin-angiotensin-aldosterone system, following maximum exercise was then succeeded by an increase in the aldosterone hormone concentration.
• The increase in both the plasma renin activity and aldosterone concentration occurred in both magnitude and time course.

Implications for Interpretation

• The researchers stress that, when assessing electrolyte and serum enzyme activity data collected after maximum exercise, such plasma volume changes must be taken into account.
• This is because the changes in plasma volume can strongly impact these measurements and any related interpretations.