Acute effects of starvation and treatment of rats with anti-insulin serum, glucagon and catecholamines on the state of phosphorylation of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase in vivo.
The fraction of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase in the dephosphorylated (active) form in rat liver in vivo was measured after various experimental treatments of animals. Intraperitoneal injection of glucose (to raise serum insulin concentrations) into rats 4 h into the light phase (L-4) resulted in a transient (30 min) increase in the expressed (E)/total (T) activity ratio of HMG-CoA reductase without any change in total activity (obtained after complete dephosphorylation of the enzyme). Conversely, intravenous injection of guinea-pig anti-insulin serum into rats 4 h into the dark phase (D-4) significantly depressed the E/T ratio within 20 min. Intravenous injection of glucagon into normal rats at this time point did not affect the degree of phosphorylation of the enzyme, in spite of a 10-fold increase in hepatic cyclic AMP concentration induced by the hormone treatment. A 3-fold increase in the concentration of the cyclic nucleotide induced by adrenaline infusion was similarly ineffective in inducing any change in expressed or total activities of hepatic HMG-CoA reductase. However, when insulin secretion was inhibited, either by the induction of streptozotocin-diabetes or by simultaneous infusion of somatostatin, glucagon treatment was able to depress the expressed activity of HMG-CoA reductase (i.e. it increased the phosphorylation of the enzyme). Therefore insulin appears to have a dominant role in the regulation of the phosphorylation state of hepatic HMG-CoA reductase. In apparent corroboration of this suggestion, short-term 4 h food deprivation of animals before D-4 resulted in a marked decrease in the E/T activity ratio of reductase, which was not affected further by an additional 8 h starvation. By contrast, the total activity of the enzyme was not significantly affected by 4 h starvation, but was markedly diminished after 12 or 24 h starvation. Longer-term starvation also produced a chronic increase in the degree of phosphorylation of the enzyme. These results are discussed in relation to the role of reversible phosphorylation in the control of hepatic HMG-CoA reductase activity in vivo.
D000818:Animals; D002395:Catecholamines / Q000494:pharmacology*; D005934:Glucagon / Q000494:pharmacology*; D005947:Glucose / Q000494:pharmacology; D006903:Hydroxymethylglutaryl CoA Reductases / Q000378:metabolism*; D025782:Hydroxymethylglutaryl-CoA-Reductases, NADP-dependent; D007330:Insulin Antibodies; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D010766:Phosphorylation; D051381:Rats; D011919:Rats, Inbred Strains; D013004:Somatostatin / Q000494:pharmacology; D013217:Starvation
D002395:Catecholamines; D007330:Insulin Antibodies; D013004:Somatostatin; D005934:Glucagon; D006903:Hydroxymethylglutaryl CoA Reductases; D025782:Hydroxymethylglutaryl-CoA-Reductases, NADP-dependent; D005947:Glucose