POTASSIUM CONCENTRATIONS IN MUSCLE, PLASMA AND ERYTHROCYTES AND URINARY FRACTIONAL EXCRETION IN NORMAL HORSES AND THOSE WITH CHRONIC INTERMITTENT EXERCISE-ASSOCIATED RHABDOMYOLYSIS

Potassium concentrations were measured in semimembranosus muscle, plas ma and erythrocytes, and the urinary fractional excretion determined i n normal horses and those that had chronic intermittent exercise-assoc iated rhabdomyolysis. Muscle from the rhabdomyolysis horses was also e valuated microscopically. The horses with rhabdomyolysis had a lower m uscle potassium concentration on a dry weight basis. Although the wet weight potassium content was also lower, the difference was not signif icant. Urinary fractional excretion of potassium (and also sodium and chloride) did not differ significantly between the two groups although the rhabdomyolysis group had a lower percentage excretion of potassiu m. Erythrocyte potassium concentration was similar for both groups. Lo w grade to moderate degenerative myopathy or absence of lesions was se en on microscopic sections of muscle from horses with rhabdomyolysis; only one had a vacuolar myopathy and potassium content was not determi ned. These results suggest that altered muscle potassium content may b e a factor in rhabdomyolysis.