CHANGES IN PLASMA ATRIAL-NATRIURETIC-PEPTIDE CONCENTRATION IN EXERCISING HORSES IN RELATION TO HYDRATION STATUS AND EXERCISE INTENSITY

Objective-To investigate effects of hydration status and exercise inte nsity on plasma atrial natriuretic peptide (ANP) concentration in exer cising horses. Animals-4 trained Standardbreds. Procedure-Horses perfo rmed a 40-minute exercise test at 65 to 70% maximal heart rate (HRmax; no incline) and a 12-minute test at approximately 90% HRmax (3.5 degr ees incline). The 40-minute test was performed with ad libitum access to drinking water (normohydration), after water had been withheld for 24 hours (dehydration), and 30 minutes after 12 L of water at body tem perature had been given via nasogastric tube (hyperhydration). The 12- minute test was only performed with normohydrated horses. Results-Plas ma ANP concentration was higher toward the end of, exercise during hyp erhydration than during the other 2 hydration slates, and the area und er the plasma ANP concentration curve was significantly greater during hyperhydration than during dehydration. The maximal plasma ANP value induced during the 12-minute test was similar to that of the 40-minute test. After the 12-minute test, plasma ANP concentration did not decr ease during the first 30 minutes of recovery, whereas after the 40-min ute test, plasma ANP concentration decreased by approximately 30% duri ng the same period. Conclusions-Hydration status can affect plasma ANP concentration in horses during submaximal exercise. Maximal plasma AN P concentration was similar at the 2 exercise intensities Studied, but the postexercise return to baseline values differed. Clinical Relevan ce-Changes in hydration status can cause alterations in plasma concent ration of ANP, one of the hormones involved in the control of cardiova scular functions during exercise.