Plasma atrial natriuretic peptide during and after repeated exercise under heat exposure.

The research examines the changes in plasma atrial natriuretic peptide (ANP) levels in horses during and after repeated exercises mimicking a 3-day endurance event under warm temperatures (28°C) and moderate humidity (58%).

Experimental Design

• The study involved repeated competition exercise tests (CET), designed to simulate a 3-day endurance event, with the exercise protocol split into phases (A-D).
• The exercise bouts were repeated five times with intervals of two weeks, under a hot and moderately humid environment with an average temperature of 28 degrees Celsius and 58% relative humidity.
• Heart rate of the horses was monitored during the CET. Following the completion of the exercise, the horses were rehydrated with either isotonic glucose-electrolyte solution or water.
• Several parameters, including ANP, NT-ANP(rank of the peptide), aldosterone, arginine vasopressin (AVP) and plasma proteins (PP), were measured during the exercise and recovery phases.

Observations

• ANP levels increased with the intensity and duration of the exercise, from 8 pmol/litre at rest to a peak of 23 pmol/litre at the most intense phase (Phase D, canter).
• Following the exercise, regardless of rehydration method, ANP levels returned to baseline within 2.5 hours.
• Repeated exercise did not alter plasma ANP levels, though other adaptive changes were observed.
• Interestingly, there were no observed diurnal variations in plasma ANP levels.
• NT-ANP showed the most significant increase during Phases C-D, and unlike ANP, only showed a minor decrease for an hour after the canter phase, suggesting a different manner of release or elimination.
• There was no observed correlation between ANP and AVP or the heart rate.

Conclusions

• Exercise alone impacts equine plasma ANP, yet the potential effect of heat stress could not be demonstrated in the study.
• The distinct responses between ANP and NT-ANP suggest differences in their release/clearance, implying that the cleavage of the cardiac prohormone (from which both of these substances derive) might occur within cells or simultaneously with secretion, rather than in the circulation.