Ultrastructural examinations of rat myocardium on low-magnesium diet during administration of methotrexate.

Methotrexate administration intraperitoneally in doses of 1 mg/kg body weight twice weekly for six weeks produced in the myocardial fibres of these animals slight focal lesions from the third week of the experiment on with the appearance of isolated giant mitochondria of bizarre shapes with electron-dense membranes containing granules, or groups of mitochondria with swollen matrix and disintegration or, partial breakdown of mitochondrial cristae, numerous lysosomes and less numerous autophagosomes, considerable lysis of myofibrils and myofilaments, widening of the tubules of the sarcoplasmic reticulum and T-tubules, presence of few fat droplets, widening of intercalated discs, rarely cytoplasm swelling. From the fifth week on foci of cardiomyocyte fibrosis, in some places swelling of vascular endothelial cells, and a very considerable rise in the number of collagen cells in extracellular spaces. Simultaneous low-magnesium diet and methotrexate caused earlier, from the second week on, and greater intensity of focal necrotic and degenerative changes of cardiomyocytes.