The long-term effect of glucagon on pyruvate kinase activity in primary cultures of hepatocytes.
Glucagon caused a marked decrease in the total L-pyruvate kinase activity of control hepatocytes maintained in monolayer culture (t1/2 = 54 h), while the addition of insulin to hepatocytes isolated from a fasted rat caused a four- to fivefold increase in the total enzyme activity. Maintenance of L-pyruvate kinase in control cultures of hepatocytes was shown to require insulin. However, when 1 microM glucagon was present in the medium, the total L-pyruvate kinase activity was not maintained even in the presence of 1 microM insulin, but rather the total L-pyruvate kinase activity of the cells steadily declined from 12.1 to 5.7 units/mg DNA by the 6th day in culture. The increase in the total L-pyruvate kinase activity of fasted hepatocytes cultured in the presence of insulin was shown to result from an increase in protein synthesis, since actinomycin D and cycloheximide blocked the insulin-induced increase in the enzyme activity. The addition of 1 microM glucagon to cultures of fasted hepatocytes also blocked the insulin-induced increase in total L-pyruvate kinase activity. Since glucagon decreased the total L-pyruvate kinase activity in control hepatocytes and blocked the increase in L-pyruvate kinase activity in fasted hepatocytes, it is suggested that, in addition to the phosphorylation of L-pyruvate kinase by a cAMP-dependent protein kinase, glucagon also acts to decrease the synthesis of L-pyruvate kinase in vitro.
D000818:Animals; D002478:Cells, Cultured; D004247:DNA / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology; D004790:Enzyme Induction / Q000187:drug effects; D005215:Fasting; D005934:Glucagon / Q000494:pharmacology*; D007328:Insulin / Q000494:pharmacology; D008099:Liver / Q000201:enzymology*; D008297:Male; D011770:Pyruvate Kinase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013997:Time Factors
D007328:Insulin; D003907:Dexamethasone; D004247:DNA; D005934:Glucagon; D011770:Pyruvate Kinase