Plasma renin activity and aldosterone and vasopressin concentrations during incremental treadmill exercise in horses

Six untrained mares were subjected to incremental treadmill exercise to examine exercise-induced in plasma renin activity (PRA) and plasma aldosterone (ALDO) and plasma arginine vasopressin (AVP) concentrations. Plasma renin activity, ALDO and AVP concentrations, and heart rate (HR) were measured at each step of an incremental maximal exercise test. Mares ran up a 6 degrees slope on a treadmill set at an initial speed of 4 m/s. Speed was increased 1 m/s each minute until HR reached a plateau. Plasma obtained was stored at - 80 C and later was thawed, extracted, and assayed for PRA and ALDO and AVP values by use of radioimmunoassay. Exercise caused significant increase in HR from 40 +/- 2 beats/min (mean +/- SEM) at rest to 206 +/- 4 beats/min (HRmax) at speed of 9 m/s. Plasma renin activity increased from 1.9 + /- 1.0 ng/ml/h at rest to a peak of 5.2 +/- 1.0 ng/ml/h at 9 m/s, paralleling changes in HR. Up to treadmill speed of 9 m/s, strong linear correlations were obtained between exercise intensity (and duration) and HR (r = 0.87, P < 0.05) and PRA (r = 0.93, P < 0.05). Heart rate and PRA reached a plateau and did not increase when speed was increased from 9 to 10 m/s. Plasma ALDO concentration increased from 48 +/- 16 pg/ml at rest to 191 +/- 72 pg/ml at speed of 10 m/s. Linear relation was found between exercise intensity (and duration) and ALDO concentration (r = 0.97, P < 0.05). Plasma AVP concentration increased from 4.0 +/- 3.0 pg/ml at rest to 95 +/- 5.0 pg/ml at speed of 10 m/s. The relation between AVP concentration and exercise intensity (and duration) appeared to be curvilinear, and was described by an exponential function (r = 0.92, P < 0.05). These data indicate that PRA and ALDO and AVP concentrations increase in horses during progressive treadmill exercise.