HYPERKALEMIC PERIODIC PARALYSIS, PLASMA LACTATE AND EXERCISE TOLERANCE

The purpose of this trial was to determine the effect of hyperkalemic periodic paralysis (HYPP) on exercise tolerance in Quarter Horses, Fiv e HYPP affected and five nonaffected horses were matched for age, size , gender and reproductive status. HYPP status was diagnosed by DNA ana lysis and potassium chloride challenge testing. Plasma lactate concent ration and heart rate were used as indicators of work intensity. Serum potassium concentrations were also monitored. Two exercise experiment s were conducted, the first being forty-five minutes of slow, aerobic exercise (hacking) and the other being moderate, partially anaerobic e xercise (galloping), Post-exercise the horses were cooled out by rando mly 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 we re significantly higher for HYPP affected horses (P = 0.01). At the en d of exercise, values were 1.4 +/- 0.2 mmol/L and 1.0 +/- 0.1 mmol/L f or HYPP affected and unaffected horses, respectively. Following modera te exercise, plasma lactate concentrations were much greater and the d ifference (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, respective ly. The higher post-exercise plasma lactate concentrations in affected horses indicates increased anaerobic muscle metabolism. Serum potassi um concentrations rose following exercise and significantly higher val ues were seen in horses that were walked rather than stood still post- exercise. Hyperkalemic periodic paralysis (HYPP) is a dominant autosom al genetic defect occurring in American Quarter Horses and related bre eds.(1-5) The condition is widely geographically distributed and has b een estimated to affect 0.4% of all Quarter Horses.(8) Signs include s poradic attacks of muscle fasciculation, muscle spasm, sweating and we akness, Oral administration of potassium chloride produces a more seve re 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 sui table for riding(7); others have expressed doubts about the safety of this practice and the exercise tolerance of affected horses. HYPP affe cted horses have an unstable muscle membrane potential causing random muscle fiber contractions,(2,8) which could antagonize purposeful move ment. This, in combination with increases in extracellular potassium c oncentration occurring during exercise,(9-11) may cause affected horse s to stumble or collapse while being ridden, posing a danger to both h orse 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 a ttacks deserves investigation.