RENAL TUBULAR FUNCTION IN HORSES DURING SUBMAXIMAL EXERCISE

Exercise-induced changes in renal function were examined during steady-state submaximal treadmill exercise in six unfit mares. Horses were randomly assigned to either an exercise or parallel control (no exercise) trial on day 1 and the alternate trial 1 wk later. The mares ran on a treadmill, set at a 6-degrees incline, for 1 h at 55-60% of maximal heart rate. Exercise significantly (P < 0.05) increased plasma osmolality, plasma [K+], urine flow (+45%), Na+ excretion (+371%), K+ excretion (+57%), osmotic clearance (+32%), Na+ clearance (+391%), K+ clearance (+33%), and fractional Na+ excretion (+320%) and significantly decreased plasma [Cl-], Cl- excretion (-46%), Cl- clearance (-41%), and fractional Cl- excretion (-47%). Glomerular filtration rate, fractional K+ excretion, and free water clearance did not change during exercise. Atrial natriuretic peptide increased during exercise from 11 +/- 1 pg/ml at rest to a peak of 40 +/- 9 pg/ml (264%, P < 0.05) at 40 min. Increases in plasma renin activity (66%, P < 0.05) were accompanied by increases in plasma aldosterone concentration (760%, P < 0.05). Vasopressin concentration increased (P < 0.05) steadily over the 60-min period of exercise. It was concluded that, in horses, submaximal exercise-induced increases in urine flow and sodium excretion are associated with a concurrent increase in the plasma concentration of atrial natriuretic peptide.