TOLBUTAMIDE INHIBITS GLUCAGON-INDUCED PHOSPHORYLATION OF 6PF-2-K FRU-2,6-P(2)ASE IN RAT HEPATOCYTES/

In previous studies, we demonstrated that tolbutamide inhibits a phosp horylation of hepatic 6-phosphofructo-2-kinase (6PF-2-K)/fructose-2,6- bisphosphatase (Fru-2,6-P(2)ase) catalyzed by the adenosine 3',5'-cycl ic monophosphate-dependent protein kinase in a reconstruction system u sing the purified enzyme from the rat liver. In the current study, to assess a role of tolbutamide on hepatic 6PF-2-K/Fru-2,6-P(2)ase physio logically, we used intact rat hepatocytes and examined effects of tolb utamide on a phosphorylation of the bifunctional enzyme in the presenc e of glucagon. Glucagon induced a rapid phosphorylation of hepatic 6PF -2-K/ Fru-2,B-P(2)ase accompanied by an inhibition of 6PF-2-K activity and a stimulation of Fru-2,B-P(2)ase activity in a dose-dependent man ner. Tolbutamide inhibited glucagon-induced phosphorylation of the bif unctional enzyme protein in a dose-dependent manner. By adding 2 mM to lbutamide, reduced activity of 6PF-2-K and increased activity of Fru-2 ,6-P(2)ase in the presence of 10(-9) M glucagon were partially restore d. The present results suggest the possibility that tolbutamide modula tes the activity of hepatic 6PF-2-K/Fru-2,6-P(2)ase through inhibiting a phosphorylation of the enzyme protein. The counterregulatory influe nce of tolbutamide on the effect of glucagon suggests a possible mecha nism for the extrapancreatic effect of sulfonylurea drugs.