TRAINING-INDUCED ALTERATIONS IN RENAL FUNCTION IN HORSES

Abstract

1621 Exercise training-induced hypervolemia in the horse is associated with renal mechanisms resulting in a decrease in urine output and net water retention (McKeever et al. M.S.S.E. 19:21-27, 1987 ). Unfortunately, the observed increase in plasma sodium content was not associated with increased sodium intake or a decrease in excretion. Renal function was measured at 7 d intervals; however, mechanisms associated with the retention of sodium and other electrolytes are most active in the first 1-3 days of training. Therefore, the purpose of the present study was to test the hypothesis that repeated exercise would alter renal excretion of electrolytes in horses within the first 3 d of training. Five, healthy, unfit, Standardbred mares (450-500 kg, 4-8 yrs) were used for this study. The experiment consisted of a 2 wk equilibration period followed by a 1 wk control period and a 3 d exercise training period (30 min/d at 60% VO2max). During control, renal fluid and electrolyte losses were measured for 24 hrs on 3 days. Renal function (urine volume, 24 hr Na+, K+, and Cl− excretion, clearance of Na+, K+, and Cl−, creatinine, osmotic substances, and free water, and the fractional excretion of Na+, K+, and Cl−) was measured for an additional 3 d during the training period. There were no differences (p > 0.05) in any variable during the control period. During training, there were no significant changes in plasma osmolality or electrolyte concentrations or creatinine clearance. Training resulted in decreases (p < 0.05) in urine volume (−30%), Na+ excretion (−73%), K+ excretion (−55%), and Cl− excretion (−70%). Training also caused decreases (p < 0.05) in osmotic clearance (−30%), through decreases in the clearance of Na+ (−60%), K+ (−60%), and Cl− (−66%). Interestingly, free water clearance increased (+30%, p < 0.05). Significant decreases in the fractional excretion of Na+ (−59%), K+ (−48%), and Cl− (−60%) suggest that mechanisms affecting tubular conservation of electrolytes rather than reabsorption of solute free water predominate in the effects of repeated exercise on renal function.