Fluid administration attenuates the haemodynamic effect of frusemide in running horses.

The effect of blood volume repletion after frusemide administration on the right atrial and pulmonary artery pressure responses of horses to exercise has not been reported. We examined right atrial and pulmonary artery pressure and plasma atrial natriuretic peptide concentration (ANP) responses to an incremental exercise test in 6 Standardbred mares. Horses were treated, in a 3 way cross over design, with isotonic saline, frusemide (1 mg/kg bwt, i.v.), and frusemide followed 3 h later by lactated Ringer's solution (12 ml/kg bwt, i.v.). Three and a half hours after saline or frusemide administration the horses completed a standard exercise test. Frusemide significantly affected the right atrial and pulmonary artery pressure and ANP responses to exercise. Fluid administration decreased plasma total protein concentrations at rest and during running and abolished the effects of frusemide on the haemodynamic and ANP responses to exercise. These results suggest that the haemodynamic effect of frusemide in running horses is mediated, in large part, by a reduction in plasma and blood volume.