Impaired glycogen synthesis in hepatocytes from Zucker fatty fa/fa rats: the role of increased phosphorylase activity.
OBJECTIVE
The Zucker fatty fa/fa rat develops hyperinsulinaemia, insulin-resistance and severe obesity as a result of a homozygous mutation in the leptin receptor gene. The aim was to characterise the metabolic defect(s) in hepatocytes from fa/fa rats.


METHODS
Glucose metabolism and key regulatory enzymes were investigated in hepatocytes from fa/fa and Fa/? rats after short-term culture in the absence of insulin.


RESULTS
Hepatocytes from fa/fa rats have higher glucokinase activity and expression of the glucokinase regulatory protein and higher rates of glycolysis and lipogenesis, but lower rates of glycogen synthesis than hepatocytes from Fa/? controls. Insulin caused a similar stimulation of glycogen synthesis in hepatocytes from fa/fa rats as in controls ( > twofold) but did not restore the impaired glycogen synthesis in cells from fa/fa rats. Adenovirus-mediated glucokinase overexpression stimulated glycogen synthesis and glycolysis but aggravated rather than abolished the relative impairment of glycogen synthesis in cells from fa/fa rats. Inhibition of glycolysis with 2,5-anhydromannitol, an inhibitor of glycolysis and gluconeogenesis, increased glucose 6-phosphate concentrations and glycogen synthesis in hepatocytes from Fa/? and fa/fa rats but did not restore the impaired glycogen synthesis in cells from fa/fa rats. Hepatocytes from fa/fa rats had a higher activity of phosphorylase a in the basal state and after incubation with insulin or glucagon and higher total phosphorylase.


CONCLUSIONS
The increased activity of phosphorylase is a major contributing factor to the impaired glycogen synthesis in hepatocytes from fa/fa rats and could contribute to the lipogenic state by a glycogenolytic-glycolytic-lipogenic pathway.
Aiston
S
S
Department of Diabetes and Metabolism, The Medical School, University of Newcastle upon Tyne, UK.
Peak
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M
Agius
L
L
D000818:Animals; D005260:Female; D005941:Glucokinase / Q000378:metabolism; D005943:Gluconeogenesis; D006003:Glycogen / Q000096:biosynthesis* / Q000378:metabolism; D006019:Glycolysis; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008353:Mannitol / Q000031:analogs & derivatives* / Q000494:pharmacology; D009765:Obesity / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D011924:Rats, Zucker
D008353:Mannitol; C031765:2,5-anhydromannitol; D006003:Glycogen; D005941:Glucokinase