EFFECTS OF GLYCOGEN STORES AND NONESTERIFIED FATTY-ACID AVAILABILITY ON INSULIN-STIMULATED GLUCOSE-METABOLISM AND TISSUE PYRUVATE-DEHYDROGENASE ACTIVITY IN THE RAT

The effects of increased tissue glycogen stores on insulin sensitivity, and on the response of insulin-stimulated glucose utilisation to an acute elevation in plasma fatty acid levels (approximately 1.5 mmol/l), were investigated in conscious rats using the hyperinsulinaemic euglycaemic clamp. Studies were performed in two groups of rats: (a) fasted 24 h; (b) fasted 4.5 h, but infused with glucose for 4 h (0.5 g/h) of this period before the clamp (fed, glucose infused rats). Clamp glucose requirement and 3- H-glucose turnover were 20-25% lower in the fed, glucose-infused rats. In these rats, elevation of plasma fatty acid levels resulted in impaired suppression of hepatic glucose output (residual hepatic glucose output: 41 +/- 4 vs 8 +/- 6-mu-mol.min-1.kg-1.p < 0.001) but did not further decrease 3- H-glucose turnover. Elevated non-esterified fatty acid levels had no significant effect on glucose kinetics in 24 h fasted rats. In the fed glucose-infused rats, at low plasma fatty acid levels, there was no deposition of glycogen in muscle during the clamp and liver glycogen levels fell. With elevation of non-esterified fatty acid levels muscle glycogen deposition was stimulated in both groups, and there was no fall in liver glycogen during the clamps in the fed glucose-infused rats. Increased non-esterified fatty acid availability during the clamps decreased pyruvate dehydrogenase activity in liver, heart, adipose tissue and quadriceps muscle, in both groups of rats. The findings are consistent with an inhibition of glycolysis in liver, skeletal muscle and heart by increased fatty acid availability. Increased glycogen synthesis, however, compensates for decreased glycolytic flux so that glucose turnover is not decreased. When liver glycogen stores are high, an acute increase in non-esterified fatty acid availability impairs suppression of hepatic glucose output. A chronic increase in non-esterified fatty acid availability may lead to insulin resistance by increasing glycogen stores.