Inhibition by insulin of the formation of autophagic vacuoles in rat liver. A morphometric approach to the kinetics of intracellular degradation by autophagy

Electron microscopic morphometry has demonstrated a rapid decrease in the fractional volume of autophagic vacuoles (AV) in hepatocytes of adult male rats after the intraperitoneal administration of insulin (5 U/kg of body weight). Except for a significant decrease in glycogen to about one-half its initial value, no major changes in the composition of the remaining cytoplasm, or in the average volume of the single hepatocyte, were seen. The decrease found in the AVs is attributed to an inhibition of the formation of new AVs-probably the morphologic counterpart of the well-known anticatabolic effects of insulin. The decay of the fractional volume of the AVs appeared to follow first- order kinetics. Thus, the termination of the "life" of an AV by destruction of its contents may not depend directly on the "age" of the AV. The average half-life of the AVs amounted to approximately 9 min. Similar values were found for the different types of AVs, except for those containing glycogen. The half-life of these AVs was approximately 18 min. From the half-life values and from the "segregated fractions" at time zero, which were different for the different cytoplasmic components, rates of removal from the cytoplasm by autophagy were calculated. Expressed as "percent per day", the following rates were found: whole cytoplasm, 2.3; mitochondria, 3.9; microbodies, 8.9; and glycogen, 1.1. The results indicate that autophagy, to some extent, is selective and plays an important, but not an exclusive, role in intracellular turnover.