Furosemide magnifies the exercise-induced elevation of plasma vasopressin concentration in horses

The purpose of this study was to test the hypothesis that furosemide administration before exercise would cause greater increases in plasma arginine vasopressin (AVP) concentration in exercising horses than exercise alone. Six adult, clinically normal, unfit mares underwent three randomly ordered 60 minute standard exercise tests on an equine treadmill to examine the effect of furosemide administration on plasma AVP concentration. In one trial, furosemide (1 mg kg-1) was infused four hours before exercise (FUR-4) and a placebo (10 ml saline) was infused two minutes before exercise; in another trial the placebo was infused four hours before exercise and drug was infused two minutes before exercise (FUR-2); in a third trial a placebo was infused four hours and two minutes before exercise (CON). During the treadmill test each mare ran up a fixed 4 degrees incline for one hour at a speed previously determined to produce a heart rate of 65 per cent of each horse's maximum heart rate. Venous blood samples were collected at rest in the stall, immediately before exercise while standing on the treadmill, and at 15 minute intervals during the treadmill test. Plasma AVP concentration was measured by radioimmunoassay. In the CON trial, plasma AVP concentration increased 561 per cent (P < 0.05) from 6.3 +/- 1.0 pg ml-1 (mean +/- SE) at rest to 38.8 +/- 12.8 pg ml-1 at the end of the 60 minute run. During the FUR-2 trial, AVP increased 1185 per cent (P < 0.05) from 5.9 +/- 1.7 pg ml-1 to 75.8 +/- 17.7 pg ml-1.(ABSTRACT TRUNCATED AT 250 WORDS)