Peripheral circulatory and metabolic consequences of thyrotoxicosis. IX. Ultrastructural changes in skeletal muscle during experimental thyrotoxicosis in the dog.

Abstract

Biopsies were taken from skeletal muscle of two dogs before, during and after thyroxin feeding, which resulted in the production of a marked thyrotoxicosis.Electron microscopical examination of the tissue revealed the following findings.In one dog, no morphological changes were detected in the mitochondria. In the other, mitochondrial volume increased, but the increase came later in time than the elevation of oxygen consumption, and persisted after oxygen consumption had returned to normal. No other morphological changes were observed in the mitochondria of this dog.Intracellular lipid droplets were found to constitute 0.1-0.5 per cent of the normal skeletal muscle cell. A significant increase in the lipid content occurred during thyrotoxicosis.Intracellular glycogen diminished in the early stages of thyrotoxicosis, but later increased again in spite of continued thyroxin feeding.No morphological signs of loss of contractile protein or of cellular damage were detected.Increase in mitochondrial volume in skeletal muscle cells is not a prerequisite for the increased calorigenesis of thyrotoxicosis. The pathophysiological implications of the changes in lipid and glycogen are discussed.