Plasma potassium and lactate concentrations in Thoroughbred horses during exercise of varying intensity

Summary To investigate the effect of moderate to high intensity exercise of up to 6 min duration on plasma potassium and lactate concentrations, 6 Thoroughbred horses were studied using a treadmill at a 5° incline. Each test consisted of an 8‐min standardised warm‐up followed by an exercise bout at 8, 9, 10 or 12 m/sec. The horses were galloped at each speed for up to a maximum of 6 min or until signs of fatigue were present. The horses were then walked at 0° incline. Carotid arterial blood samples were taken during and after the exercise. At 8, 9 and 10 m/sec there was a general pattern of an initial rise in potassium to a peak around 1.5 min of exercise with the concentration then slowly decreasing. At 12 m/sec there was a continuous rise to a peak at the end of exercise in all horses. Immediately after exercise there was a rapid return (within 3–4 min) to the potassium concentrations recorded at the end of the warm‐up period. Plasma lactate peaked around the end of exercise at all speeds. At the highest intensity of exercise the mechanisms for the re‐uptake of potassium did not appear to be able to match the rate of efflux. In contrast, at less intense work loads, the rate of re‐uptake appeared to be similar to or slightly greater than the rate of efflux. It is possible that a disturbance in this balance between efflux and re‐uptake could result in a disturbance in normal neuromuscular function during exercise.