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, plasma and erythrocytes, and the urinary fractional excretion determined in normal horses and those that had chronic intermittent exercise-associated rhabdomyolysis. Muscle from the rhabdomyolysis horses was also evaluated microscopically. The horses with rhabdomyolysis had a lower muscle potassium concentration on a dry weight basis. Although the wet weight potassium content was also lower, the difference was not significant. 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 potassium. Erythrocyte potassium concentration was similar for both groups. Low grade to moderate degenerative myopathy or absence of lesions was seen on microscopic sections of muscle from horses with rhabdomyolysis; only one had a vacuolar myopathy and potassium content was not determined. These results suggest that altered muscle potassium content may be a factor in rhabdomyolysis.