Uptake and degradation of glycogen by Kupffer cells.

Rat liver glycogen was isotopically labeled with [¹⁴C] glucose and isolated. The isolated glycogen was injected intravenously into a series of rats and its vascular clearance, uptake and degradation in liver was analyzed by means of labeling and ultrastructural techniques. Injected glycogen was quickly removed from serum with a $\text{t}\text{1}\text{2}$ of less than 15 min. Glycogen particles, identified in the electron microscope, were never seen to attach to the surface of Kupffer cells or hepatocytes. These particles appeared to be taken up by Kupffer cells by nonspecific pinocytosis “fluid endocytosis” e.g., as a solute with engulfed liquid. By 10 min the particles were present within single membrane bound vacuoles of Kupffer cells. At this early time point, the vacuoles did not seem to have fused with preexisting prelabeled secondary lysosomes containing ferritin. At later time points, glycogen particles were seen intermingled with ferritin. By 1, 2, and 4 hr, increasing numbers of vacuoles containing granular material believed to represent glycogen were observed. These vacuoles often showed extensive enlargement (“swelling”). By 24 hr, most glycogen particles had disappeared and granular material was prresent only in occasional lysosomes which no longer appeared swollen. The estimated half-life for glycogen in Kupffer cell lysosomes was in the range of 12 to 16 hr. This is considerably longer than for membrane proteins and lipids introduced into Kupffer cell lysosomes by means of heterophagy. Because of possible reutilization of isotope it was difficult to define the half-life of glycogen more exactly. It is concluded that glycogen is degraded in Kupffer cell lysosomes, although at a relative slow rate, in comparison with the capability of lysosomal hydrolases to digest proteins and lipids. This conclusion is in line with the general notion that glycogen degradation takes place in the cell sap and is not primarily associated with any particular organelle.