SITES OF GLYCOGEN SYNTHESIS IN RAT LIVER CELLS AS SHOWN BY ELECTRON MICROSCOPE RADIOAUTOGRAPHY AFTER ADMINISTRATION OF GLUCOSE-H3

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Abstract
Glycogen synthesis was investigated by giving tritium (H3)-labeled glu-cose with carrier to fasted rats in vivo or incubating liver slices from fasted rats in vitro using a glucose[long dash]H3-containing medium. After 15 min. or 1 hr., pieces of liver were fixed and radioautographed for light and electron microscopy. In vivo and in vitro, radioautographic reactions appeared over "glycogen areas" and over zones transitional between these areas and ergastoplasm. Treatment of sections by alpha amylase removed all but about 5% of the radioactivity, so that about 95% of it consisted of glycogen (synthesized during the 15 min. or 1 hr. elapsing after administration of glucose-H3). Within glycogen areas and transitional zones, most Ag grains were over or very close to glycogen granules and smooth (or partly smooth) vesicles. That much of the label was added onto growing glycogen granules suggests that glycogen may serve as substrate for further glycogen synthesis. The few Ag grains located far from glycogen granules[long dash]15% at the 15 min. interval in vivo[long dash]approximated smooth (or partly smooth) vesicles of endoplasmic reticulum. Smooth membranes may play a role in glucose uptake at an early stage in de novo formation of glycogen granules.