Ultrastructural observations in ponies after treatment with monensin.

: Ultrastructural studies were made of myocardium, diaphragm, appendicular muscle, liver, and kidney of 3 ponies acutely poisoned with a single oral dose of monensin (4 mg/kg of body weight). These ponies developed severe signs of toxicosis and were killed 28 to 72 hours after treatment. Severe mitochondrial damage (swelling) and lipoidosis in myocardial tissues were observed in 2 of the 3 ponies; similar, but less severe, changes were observed in the 3rd pony. The hepatocytes of the 3 ponies were characterized by increased amounts of smooth endoplasmic reticulum, large numbers of lipid droplets, vacuoles bounded by fibrous material, and a 2-fold increase in the number of peroxisomes per cell. Some hepatocytes also contained a membrane-bounded protein-like body. The observations indicate that heart mitochondria are primary targets of monensin poisoning in ponies.