Hyperkalemic periodic paralysis, plasma lactate and exercise tolerance

Abstract

The purpose of this, trial was to determine the effect of hyperkalemic periodic paralysis (HYPP) on exercise tolerance in Quarter Horses. Five HYPP affected and five nonaffected horses were matched for age, size, gender and reproductive status. HYPP status was diagnosed by DNA analysis and potassium chloride challenge testing. Plasma lactate concentration and heart rate were used as indicators of work intensity. Serum potassium concentrations were also monitored. Two exercise experiments were conducted, the first being forty-five minutes of slow, aerobic exercise (hacking) and the other being moderate, partially anaerobic exercise (galloping). Post-exercise the horses were cooled out by randomly assigning them to either forty minutes, of standing still or forty minutes of walking. Heart rates of HYPP affected and unaffected horses were not significantly different during exercise or recovery. Plasma lactate concentrations changed slightly following slow exercise and were significantly higher for HYPP affected horses (P=0.01). At the end of exercise, values were 1.4±0.2 mmol/L and 1.0±0.1 mmol/L for HYPP affected and unaffected horses, respectively. Following moderate exercise, plasma lactate concentrations were much greater, and the difference ( P<0.001) between affected and unaffected horses was more marked: immediately following exercise concentrations were 10.6±1.8 and 6.2±1.0 mmol/L in affected and unaffected horses, respectively. The higher post-exercise plasma lactate concentrations in affected horses indicates increased anaerobic muscle metabolism. Serum potassium concentrations rose following exercise and significantly higher values were seen in horses that were walked rather than stood still post-exercise. Hyperkalemic periodic paralysis (HYPP) is a dominant autosomal genetic defect occurring in American Quarter Horses and related breeds. 1–5 The condition is widely geographically distributed and has been estimated to affect 0.4% of all Quarter Horses. 6 Signs include sporadic attacks of muscle fasciculation, muscle spasm, sweating and weakness. Oral administration of potassium chloride produces a more severe hyperkalemia in HYPP affected horses and induces clinical attacks. 1 Electromyography reveals widespread continuous, spontaneous, muscle contraction. 2 Some people believe that HYPP affected horses are suitable for riding 7; others have expressed doubts about the safety of this practice and the exercise tolerance of affected horses. HYPP affected horses have an unstable muscle membrane potential causing random muscle fiber contractions, 2,8 which could antagonize purposeful movement. This, in combination with increases in extracellular potassium concentration occurring during exercise, 9–11 may cause affected horses to stumble or collapse while being ridden, posing a danger to both horse and rider. In HYPP affected people, potassium concentration rises approximately one to two hours after exercise during which time HYPP attacks can occur. 9,10 For these reasons, the exercise tolerance of HYPP affected horses and the effects of exercise on spontaneous HYPP attacks deserves investigation. We standardized the exercise test and chose heart rate and plasma lactate concentrations as indicatoors, of exercise tolerance and energy metabolism. 12 Heart rate is the major determinant of oxygen delivery to muscle and the rate rises with exercise intensity until it reaches a plateau at high velocities. 13,14 Several lines of investigation indicate that plasma lactate concentrations reflect muscle work. Lactate release from resting muscle is minimal but large amounts are released during strenuous exercise. 15–17 Lactate release increases with increasing muscle work. 16,18 During exercise, muscle oxygen consumption increases until eventually a plateau is reached after which no further increase in oxygen consumption in response to increased work loads is possible. 18 At this point the mitochondrial electron chain transport system is operating at its maximal possible rate, energy production can only be supplemented by the use of inefficient anerobic glycolytic metabolism and the muscle starts to release lactate. If the concentrations of plasma lactate are high, the muscles are depending on anaerobic metabolism and are working closer to exhaustion 16,18–21 The objectives of this experiment were twofold. One was to determine if HYPP adversely affects exercise performance. The second was to study the effects of cooling out, by either standing still or walking, on serum potassium and plasma lactate concentrations.