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

Plasma atrial natriuretic peptide (ANP) responses were studied in horses during and after a competition exercise test (CET) designed to simulate the endurance test of a 3-day event and including test Phases A-D. CET was repeated 5 times at 2 week intervals at mean temperature +28 degrees C and relative humidity (RH) 58%. Heart rate (HR) was monitored during CET. After CET, the horses were rehydrated with isotonic glucose-electrolyte solution or water. ANP, NH2-terminal pro-atrial natriuretic peptide (NT-ANP), aldosterone, arginine vasopressin (AVP) and plasma proteins (PP) were measured during CET and recovery. ANP rose with intensity and duration of exercise from 8 pmol/l to 15 pmol/l at a gallop (Phase B), remained elevated at Phases C-X, and peaked (23 pmol/l) at a canter (Phase D). Thereafter, ANP decreased to the pre-exercise level within 2.5 h irrespective of rehydration. Repeated exercise bouts failed to affect plasma ANP, although other adaptive changes occurred. No diurnal variation in plasma ANP appeared. In NT-ANP, the most marked elevation occurred during Phases C-D. In contrast to ANP, the decrease was minor for 1 h after the canter. ANP did not correlate with AVP or HR. In conclusion, exercise per se affected equine plasma ANP but the possible effect of thermal stress could not be demonstrated. The differences between ANP and NT-ANP responses indicate differences in their release or elimination and suggest that the cardiac prohormone cleavage may occur intracellularly or simultaneously with secretion, not in circulation.