Ultrastructural and autoradiographic observations of cultured chick hepatocytes under the following conditions are described: Induction of glycogen synthesis with glucose alone and glucose plus insulin, and glucagon-induced glycogen breakdown. Profiles of hepatocytes cultured in medium containing 10 mM glucose showed typical cellular organelles and occasionally a few glycogen granules. After incubation of hepatocytes with 3H-glucose, silver grains were found over these sparse glycogen granules, indicating a low level of glycogen synthesis by a few cells. After addition of 75 mM glucose for 1 hr about 3% of the profiles of cells showed glycogen, and by 24 hr half of the hepatocytes had glycogen. Addition of insulin plus glucose induced glycogen synthesis in 82% of the cells after 6 hr, and by 24 hr almost every cellular profile showed glyocgen particles. Morphologically, glycogen accumulation was similar whether the cells were stimulated by high glucose or by glucose plus insulin: glycogen granules appeared in restricted regions of the cytoplasm, which were rich in smooth endoplasmic reticulum (SER), and peroxisomes were found close to the newly deposited glycogen particles. At maximum glycogen accumulation the association of SER and peroxisomes with glycogen was less obvious. Glycogenolysis induced by incubation of glycogen-rich hepatocytes with glucagon resulted in proliferation of SER in the glycogen regions of the cells. These observations are compatible with the concept of regions in the hepatocyte cytoplasm specialized for glycogen metabolism. Possible roles for SER and peroxisomes found near glycogen particles and other organelles in hepatic glycogen metabolism are discussed.Recently the well-established embryonic chicken hepatocyte culture system (Grieninger and Granick, 1978;Grieninger, 1983) was used to study the regulation of glycogen metabolism under precisely defined conditions. By culturing hepatocytes in a chemically defined medium (without serum) it was possible to evaluate the role of glucose and insulin in hepatic glycogenesis, to study the effects of glucagon on glycogen breakdown, and to observe the influence of insulin-like growth factors (Parkes and Grieninger, 1985;Parkes et al., 1986). These studies showed that addition of high concentrations of glucose to the culture medium caused rapid glycogen deposition, but the levels obtained never equaled maximal in vivo fed hepatic glycogen levels. It was concluded that glycogen synthesis in response to glucose is driven primarily by mass action (i.e., the effect of increasing substrate concentration). In contrast, insulin added to the culture medium with glucose causes the restoration of physiological maximal levels of glycogen. Thus insulin was proposed as the major regulator of glycogen synthesis in the hepatocyte. No major differences were observed between insulin and insulin-like growth factors with respect to glycogen deposition. The addition of glucagon to the 0 1990 WILEY-LISS, INC. culture medium caused a rapid breakdown of glyco...
